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KH561
Wideband, Low Distortion Driver Amplifier
Features
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General Description
The KH561 is a wideband DC coupled, amplifier that combines high output drive and low distortion. At an output of +24dBm (10Vpp into 50), the -3dB bandwidth is 150MHz. As illustrated in the table below, distortion performance remains excellent even when amplifying high-frequency signals to high output power levels. With the output current internally limited to 250mA, the KH561 is fully protected against shorts to ground and can, with the addition of a series limiting resistor at the output, withstand shorts to the 15V supplies. The KH561 has been designed for maximum flexibility in a wide variety of demanding applications. The two resistors comprising the feedback network set both the gain and the output impedance, without requiring the series backmatch resistor needed by most op amps. This allows driving into a matched load without dropping half the voltage swing through a series matching resistor. External compensation allows user adjustment of the frequency response. The KH561 is specified for both maximally flat frequency response and 0% pulse overshoot compensations. The combination of wide bandwidth, high output power, and low distortion, coupled with gain, output impedance and frequency response flexibility, makes the KH561 ideal for waveform generator applications. Excellent stability driving capacitive loads yields superior performance driving ADC's, long transmission lines, and SAW devices. A companion part, the KH560, offers superior pulse fidelity for high accuracy DC coupled applications. The KH561 is constructed using thin film resistor/bipolar transistor technology, and is available in the following versions:
KH561AI KH561AK -25C to +85C -55C to +125C 24-pin Ceramic DIP 24-pin Ceramic DIP, features burn-in and hermetic testing 24-pin Ceramic DIP, environmentally screened and electronically tested to MIL-STD-883
150MHz bandwidth at +24dBm output Low distortion (2nd/3rd: -59/-62dBc @ 20MHz and 10dBm) Output short circuit protection User-definable output impedance, gain, and compensation Internal current limiting Direct replacement for CLC561
Applications
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Output amplification Arbitrary waveform generation ATE systems Cable/line driving Function generators SAW drivers Flash A/D driving and testing
Frequency Response vs. Output Power
16 14
Po = 10dBm Vo = 2Vpp
Gain (dB)
12 10 8 6 0
Po = 24dBm Vo = 10Vpp Po = 27.5dBm Vo = 15Vpp Po = 18dBm Vo = 5Vpp
40
80
120
160
200
Frequency (MHz)
4
+VCC Compensation Vo
V+
8
+ -
19
23
V-
18
5 21 10 15 20
-VCC
All undesignated pins are internally unconnected. May be grounded if desired.
Typical Distortion Performance
Output Power 10dBm 18dBm 24dBm 20MHz 2nd 3rd -59 -52 -50 -62 -48 -41 50MHz 2nd 3rd -52 -45 -36 -60 -46 -32 100MHz 2nd 3rd -35 -30 -40 -49 -36 -30
KH561AM
-55C to +125C
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DATA SHEET
KH561
KH561 Electrical Characteristics (Av = +10V, VCC = 15V, RL = 50, Rf = 410, Rg = 40, Ro = 50; unless specified)
NOTES TO THE ELECTRICAL SPECIFICATIONS The electrical characteristics shown here apply to the specific test conditions shown above (see also Figure 1 in description of the operation). The KH561 provides an equivalent, non-zero, output impedance determined by the external resistors. The signal gain to the load is therefore load dependent. The signal gain shown above (Av = +10) is the no load gain. The actual gain to the matching 50 load used in these specifications is half of this (+5). The KH561 requires an external compensation capacitor. Unless otherwise noted, this has been set to 10.5pF for the frequency domain specifications (yielding a maximally flat frequency response) and 12.5pF for the time domain specifications (yielding a 0% small signal pulse overshoot response).
PARAMETERS Case Temperature Case Temperature CONDITIONS KH561AI KH561AK/AM TYP +25C +25C MIN & MAX RATINGS -25C -55C +25C +25C +85C +125C UNITS SYM
FREQUENCY DOMAIN RESPONSE (Max. Flat Compensation) -3dB bandwidth maximally flat compensation Vo <2Vpp (+10dBm) 0% overshoot compensation Vo <2Vpp (+10dBm) large signal bandwidth Vo <10Vpp (+24dBm) (see Frequency Response vs. Output Power plot) gain flatness Vo <2Vpp (+10dBm) peaking 0.1 -50MHz peaking >50MHz rolloff at 100MHz group delay to 100MHz linear phase deviation to 100MHz return loss (see discussion of Rx) to 100MHz DISTORTION (Max. Flat Compensation) 2nd harmonic distortion 24dBm (10Vpp): 20MHz 50MHz 100MHz 18dBm (5Vpp): 20MHz 50MHz 100MHz 10dBm (2Vpp): 20MHz 50MHz 100MHz 3rd harmonic distortion 24dBm (10Vpp): 20MHz 50MHz 100MHz 18dBm (5Vpp): 20MHz 50MHz 100MHz 10dBm (2Vpp): 20MHz 50MHz 100MHz 2-tone 3rd order intermod intercept2 20MHz 50MHz 100MHZ
215 210 150 0 0 0.1 2.9 0.6 -15
>175 >170 >145 <0.50 <1.75 <1.00 - <1.7 <-11
>185 >180 >135 <0.40 <0.75 <0.75 - <1.2 <-11
>175 >170 >120 <0.50 <1.00 <1.00 - <1.7 <-11
MHz MHz MHz dB dB dB ns dB
SSBW FPBW GFPL GFPH GFR GD LPD RL
-50 -36 -40 -52 -45 -30 -59 -52 -35 -41 -32 -30 -48 -46 -36 -62 -60 -49 38 35 29
<-38 <-29 <-25 <-42 <-30 <-22 <-48 <-36 <-27 <-34 <-26 <-24 <-40 <-37 <-30 <-54 <-49 <-45 >36 >32 >27
<-40 <-29 <-25 <-44 <-35 <-25 <-52 <-40 <-28 <-34 <-26 <-24 <-44 <-37 <-30 <-57 <-52 <-45 >36 >32 >27
<-38 <-22 <-25 <-42 <-30 <-25 <-48 <-40 <-28 <-30 <-21 <-24 <-44 <-35 <-30 <-57 <-49 <-45 >36 >32 >23
dBc dBc dBc dBc dBc dBc dBc dBc dBc dBc dBc dBc dBc dBc dBc dBc dBc dBc dBm dBm dBm
HD2HL HD2HM HD2HH HD2ML HD2MM HD2MH HD2LL HD2LM HD2LH HD3HL HD3HM HD3HH HD3ML HD3MM HD3MH HD3LL HD3LM HD3LH IM3L IM3M IM3H
Min/max ratings are based on product characterization and simulation. Individual parameters are tested as noted. Outgoing quality levels are determined from tested parameters.
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KH561
DATA SHEET
KH561 Electrical Characteristics (Av = +10V, VCC = 15V, RL = 50, Rf = 410, Rg = 40, Ro = 50; unless specified)
PARAMETERS Case Temperature Case Temperature CONDITIONS KH561AI KH561AK/AM TYP +25C +25C MIN & MAX RATINGS -25C -55C +25C +25C +85C +125C UNITS SYM
TIME DOMAIN RESPONSE (0% Overshoot Compensation) rise and fall time 2V step 10V step settling time to 0.5% (time <1s) 5V step long term thermal tail (time >1s) 5V step slew rate 10Vpp, 175MHz overshoot 2V step maximally flat compensation 0% overshoot compensation EQUIVALENT INPUT NOISE voltage inverting current non-inverting current noise floor integrated noise noise figure STATIC, DC PERFORMANCE * input offset voltage average temperature coefficient * non-inverting bias current average temperature coefficient * inverting bias current average temperature coefficient * power supply rejection ratio (DC) * supply current MISCELLANEOUS PERFORMANCE open loop current gain average temperature coefficient inverting input resistance average temperature coefficient non-inverting input resistance non-inverting input capacitance output voltage range output current limit >100KHz >100KHz >100KHz >100KHz 1kHz to 200MHz >100KHz
1.5 2.4 7 1.5 3300 5 0 2.1 34 2.8 -159 35 15 2.0 35 5.0 20 10.0 100 57 50 10.0 +0.02 14.0 +.02 700 2.7 10.5 210
<2.0 <2.8 <12 <2.0 >3000 <13 <5 <2.5 <40 <4.5 <-157 <45 <17 <14.0 <100 <35 <175 <50 <200 >54 <60 - <+.03 - <+.025 >200 <3.5 - <250
<1.9 <2.8 <12 <2.0 >2900 <10 <3 <2.5 <40 <4.5 <-157 <45 <17 <5.0 - <20 - <30 - >54 <60 - - - - >400 <3.5 >10.0 <250
<2.0 <3.4 <15 <2.0 >2500 <13 <5 <2.5 <45 <5.0 <-157 <45 <17 <15.0 <100 <20 <100 <50 <200 >52 <65 - <+.02 - <+.025 >400 <3.5 - <250
ns ns ns % V/s % % nV/Hz pA/Hz pA/Hz
dBm/(1Hz)
TRS TRL TS SE SR OSMF OSZO VN ICN NCN SNF INV NF VIO DVIO IBN DIBN IBI DIBI PSRR ICC G DG RIN DRIN RNI CNI VO OCL
V dB
no load (2% tolerance) (5% tolerance) to 100MHz 150mA load current
mV V/C A nA/C A nA/C dB mA mA/mA %/C /C K pF V mA
Min/max ratings are based on product characterization and simulation. Individual parameters are tested as noted. Outgoing quality levels are determined from tested parameters.
Absolute Maximum Ratings
VCC (reversed supplies will destroy part) differential input voltage common mode input voltage junction temperature (see thermal model) storage temperature lead temperature (soldering 10s) output current (internally limited) 20V 3V VCC +175C -65C to +150C +300C 250mA
Recommended Operating Conditions
VCC Io common mode input voltage output impedance gain range (no-load voltage gain) case temperature: AI AK/AM 10V to 15V 200mA < (|VCC| -6)V 25 to 200 +5 to +80 -25C to +85C -55C to +125C
Notes
AI, AK, AM 100% tested at +25C AK, AM 100% tested at at +25C and sample tested at -55C and +125C AI sample tested at +25C 2) Test Tones are set 100kHz of indicated frequency. 1) *
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DATA SHEET
KH561
KH561 Typical Performance Characteristics (TA = +25C, Circuit in Figure 1; unless specified)
Small Signal Gain and Phase Normalized Magnitude (1dB/div)
16
Po = 10dBm
Frequency Response vs. Gain
Av = 10 Po = 10dBm
Frequency Response vs. Output Power
16 14
Av = 5 Po = 10dBm Vo = 2Vpp
14
Maximally Flat 0% Overshoot
Phase (degrees)
Gain (dB)
Gain (dB)
12 0 10 8 6 0 50 100 150 200 250
Phase Gain
12 10 8
Po = 24dBm Vo = 10Vpp Po = 27.5dBm Vo = 15Vpp Po = 18dBm Vo = 5Vpp
-90 -180 -270 -360
Av = 15
Re-compensated at each gain (see text)
Av = 20
6 0 50 100 150 200 250 0 40 80 120 160 200
Frequency (MHz) Frequency Response vs. RL Normalized Magnitude (1dB/div)
Pi = -4dBm
Frequency (MHz) Frequency Response vs. Power Supply
Po = 10dBm
Frequency (MHz) Frequency Response vs. Ro Normalized Magnitude (1dB/div)
Pi = -4dBm
16 14
RL = 25
VCC = 18
Gain (dB)
RL = 50
12
VCC = 15
Ro = 25 Ro = 75 Ro = 100
Ro = 50
RL = 75
10 8
Re-compensated at each supply voltage
VCC = 12 VCC = 10
RL = 100 Fixed gain and compensated vs. load
Response measured with matched load Re-compensated at each Ro
6 0 50 100 150 200 250 0 50 100 150 200 250
0
50
100
150
200
250
Frequency (MHz) Frequency Response vs. Gain (Ro, RL = 75) Normalized Magnitude (1dB/div)
Vo = 2Vpp Gain
Frequency (MHz) Gain Flatness/Deviation from Linear Phase
Po = 10dBm
Frequency (MHz) Internal Current Gain and Phase
Cx = 0 RL = 0
Gain (0.1dB/div)
Gain (10dB/div)
Gain
Phase (0.5/div)
Phase (90/div)
Av = 5 Av = 10 Av = 15 Av = 20
30 20 10 0 -10 -20 -30
Phase consistant with current polarity connection of Figure 3 Phase
180 90 0 -90 -180
Phase
Re-compensated at each gain
0
50
100
150
200
250
0
20
40
60
80
100
0
100
200
300
400
500
Frequency (MHz) Two Tone, 3rd-Order Intermodulation
45 40 35 30 25
Re-compensated at each gain Av = 15 Av = 20 Av = 5
Frequency (MHz) 2nd Harmonic Distortion vs. Frequency
-25 -35
100MHz
Frequency (MHz) 3rd Harmonic Distortion vs. Frequency
-25
50MHz
Intercept (2.5dB/div)
-35
Distortion (dBc)
Distortion (dBc)
Av = 10
100MHz
-45
50MHz
-45 -55 -65 -75
10MHz
-55 -65 -75
20MHz
20MHz
10MHz
20 0 20 40 60 80 100
4
8
12
16
20
24
4
8
12
16
20
24
Frequency (MHz) Frequency Response Driving CL
-30
Av = +5 Ro = 25 Vo = 2Vpp
Output Power (dB) 2nd Harmonic Distortion Driving CL
-30
Compensation as shown in Frequency Response plot
Output Power (dB) 3rd Harmonic Distortion Driving CL
Av = +5 Ro = 25 Vo = 2Vpp
CL = 20pF
Distortion (5dBc/div)
-50 -60 -70
CL = 20pF
Distortion (5dBc/div)
-40
-40 -50 -60 -70 -80 10
Gain (1dB/div)
CL = 100pF
CL = 50pF Av = +5 Ro = 25 Vo = 2Vpp
CL = 100pF CL = 20pF
CL = 50pF
CL = 50pF Re-compensated at each CL
CL = 100pF
-80 0 50 100 150 200 250 10 20 30 40 50
70
100
20
30
40
50
70
100
Frequency (MHz)
Frequency (MHz)
Frequency (MHz)
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KH561
DATA SHEET
KH561 Typical Performance Characteristics (TA = +25C, Circuit in Figure 1; unless specified)
Small Signal Pulse Response
1.2
Maximally Flat Compensation
Large Signal Pulse Response
6
Maximally Flat Compensation
Uni-Polar Pulse Response
6
Maximally Flat Compensation
Output Voltage (V)
Output Voltage (V)
0.8 0.4 0 -0.4 -0.8 -1.2
0% Overshoot Compensation
4 2 0 -2 -4 -6
Output Voltage (V)
0% Overshoot Compensation
4 2 0 -2 -4 -6
Time (2ns/div)
Time (5ns/div)
Time (5ns/div)
Settling Time into 50 Load
2.0
5V Output Step
Settling Time into 500 Load
2.0
5V Output Step
Reverse Transmission Gain & Phase (S12)
0 -20
1.5
1.5
Reverse Phase (degrees)
Settling Error (%)
Settling Error (%)
1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 10-9 10-7 10-5 10-3 10-1 101
1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 10-9 10-7 10-5 10-3 10-1 101
Reverse Gain (dB)
-40 -60 -80 -100
Phase Gain
0 -45 -90 -135 -180
0
50
100
150
200
250
Time (sec) Settling Time into 50pF Load
2.0
5V Output Step
Time (sec) Output Return Loss (S22)
0 -5 -10
Ro = 50 Rx = 0
Frequency (MHz) Input Return Loss (S11)
0 -10 -20
1.5
Settling Error (%)
Magnitude (dB)
Magnitude (dB)
1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 10-9 10-7 10-5 10-3 10-1 101
Phase (degrees)
-15 -20 -25 -30 -35 -40 -45 -50 0 50 100 150 200 250
Re-compensated at each Rx Ro = 40 Rx = 10
-30 -40 -50
Magnitude
0
Phase
-45 -90 -135
Re-compensated at each Rx
-180 100 150 200 250
0
50
Time (sec) -1dB Compensation Point
34 22 21
Frequency (MHz) Noise Figure
100 60
Frequency (MHz) Equivalent Input Noise
100 60
-1dB Compensation (dBm)
33
Noise Voltage (nV/Hz)
Noise Current (pA/Hz)
Noise Figure (dBm)
32 31 30 29 28 27 26 25 24 0 20
Ro = 50 Ro = 75
20 19 18 17 16 15 14 13 12
Ro = 25
Ro = 100 Ro = 75 Ro = 50
40 20 10 6 4 2 1
Inverting Current 34pA/Hz
40 20 10 6 4 2
Non-Inverting Current 2.8pA/Hz
Match Load Re-compensated at each load
Non-inverting input impedance matched to source impedance
Non-Inverting Voltage 2.1nV/Hz
40
60
80
100
5
10
15
20
25
30
100
1k
10k
100k
1M
10M
1 100M
Frequency (MHz) Group Delay
4.0 3.8 5 4
No Load Gain Gain Error Band (Worst Case, DC)
100
Ro (nominal) = 50 RL = 50 0%
Frequency (Hz) PSRR
90 80 70 60 50 40 30 20
Gain Error at Load (%)
3.6
3 1 0 -1 -2 -3 -4 -5
Rf and Rg tolerance = 1%
Group Delay (ns)
3.2 3.0 2.8 2.6 2.4 2.2 2.0 0 50 100 150 200 250
Aperture set to 5% of span (12.8MHz)
Rf and Rg tolerance = 0.1%
PSRR (dB)
17 21 25
3.4
2
10 0 100 1k 10k 100k 1M 10M 100M
5
9
13
Frequency (MHz)
No Load Gain
Frequency (Hz)
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DATA SHEET
KH561
+VCC (+15)
SUMMARY DESIGN EQUATIONS AND DEFINITIONS
R f = (G + 1) Ro - A vRi Rg = R f - Ro Av - 1 1 Ro - 0.08 2 300 1 - Rg
Rf - Feedback resistor from output to inverting input Rg - Gain setting resistor from inverting input to ground Cx - External compensation capacitor from output to pin 19 (in pF)
6.8F Vi (Pi)
+
.1F 4 8 19
Cx 10.5pF 23 Ro RL 50 Vo (Po)
Rs 50
18
+ KH561 21 Rf
Cx =
5,10,15, 20
Rg 40
410
Resistor Values shown result in:
Ro = 50 + 6.8F Av = +10 (no-load gain) AL = +5 [14dB] (gain to 50 load)
.1F
Where: Ro - Desired equivalent output impedance Av - Non-inverting input to output voltage gain with no load G - Internal current gain from inverting input to output = 10 1% Ri - Internal inverting input impedance = 14 %5 Rs - Non-inverting input termination resistor RL - Load resistor AL - Voltage gain from non-inverting input to load resistor
KH561 Description of Operation Looking at the circuit of Figure 1 (the topology and resistor values used in setting the data sheet specifications), the KH561 appears to bear a strong external resemblance to a classical op amp. As shown in the simplified block diagram of Figure 2, however, it differs in several key areas. Principally, the error signal is a current into the inverting input (current feedback) and the forward gain from this current to the output is relatively low, but very well controlled, current gain. The KH561 has been intentionally designed to have a low internal gain and a current mode output in order that an equivalent output impedance can be achieved without the series matching resistor more commonly required of low output impedance op amps. Many of the benefits of a high loop gain have, however, been retained through a very careful control of the KH561's internal characteristics. The feedback and gain setting resistors determine both the output impedance and the gain. Rf predominately sets the output impedance (Ro), while Rg predominately determines the no load gain (Av). solving for the required Rf and Rg, given a desired Ro and Av, yields the design equations shown below. Conversely, given an Rf and Rg, the performance equations show that both Rf and Rg play a part in setting Ro and Av. Independent Ro and Av adjustment would be possible if the inverting input impedance (Ri) were 0 but, with Ri = 14 as shown in the specification listing, independent gain and output impedance setting is not directly possible.
-VCC (-15)
Figure 1: Test Circuit Design Equations
R f = (G + 1) Ro - A vRi Rg = R f - Ro Av - 1 Where: G forward current gain (=10) Ri inverting node input resistance (=14) Ro desired output impedance Av desired noninverting voltage gain with no load
R R f + Ri 1 + f Rg Ro = R G + 1+ i Rg Ri G- R Rf A v = 1+ f R g G + 1 + Ri Rg
Performance Equations Simplified Circuit Description Looking at the KH561's simplified schematic in Figure 2, the amplifier's operation may be described. Going from the non-inverting input at pin 8 to the inverting input at pin 18, transistors Q1 - Q4 act as an open loop unity gain buffer forcing the inverting node voltage to follow the noninverting voltage input. Transistors Q3 and Q4 also act as a low impedance (14 looking into pin 18) path for the feedback error current. This current, (ierr), flows through those transistors into a very well defined current mirror having a gain of 10 from this error current to the output. The current mirror outputs act as the amplifier output. The input stage bias currents are supply voltage independent. Since these set the bias level for the whole
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KH561
DATA SHEET
part, relatively constant performance over supply voltage is achieved. A current sense in the error current leg of the 10X current mirror feeds back to the bias current setup providing a current shutdown feature when the output current approaches 250mA.
+VCC
4
Get both Vo and Io into terms of just the error current, ierr, using:
V - = ierr Ri and if = ierr + V- R = ierr 1 + i Rg Rg R 1+ i R g
Ibias
Current Limit
10X Current Mirror
5pF
19
Io Cx Ro Vo
Vo = V - + if R f = ierr Ri + R f R Vo = ierr R f + Ri 1 + f Rg and
Q3 Q1 Vi
8
-VCC +VCC Q2
ierr Rg Q4 Rf 5pF
23
Io
Ibias Current Limit
10X Current Mirror
21
R Io = Gierr + if = ierr G + 1 + i Rg then R R f + Ri 1 + f Rg V Ro o = R Io G + 1+ i Rg note that Ro = Rf G+1 Ri = 0
-VCC
Figure 2: Simplified Circuit Diagram Developing the Performance Equations The KH561 is intended to provide both a controllable voltage gain from input to output as well as a controllable output impedance. It is best to treat these two operations separately with no load in place. Then, with the no-load gain and output impedance determined, the gain to the load will simply be the no-load gain attenuated by the voltage divider formed by the load and the equivalent output impedance. Figure 3 steps through the output impedance development using an equivalent model of Figure 2. Offering an equivalent, non-zero, output impedance into a matched load allows the KH561 to operate at lower internal voltage swings for a given desired swing at the load. This allows higher voltage swings to be delivered at the load for a given power supply voltage at lower distortion levels than an equivalent op amp needing to generate twice the voltage swing actually desired at the matched load. This improved distortion is specified and tested over a wide range as shown in the specification listing.
Figure 3: Output Impedance Derivation Note that the Ro expression simplifies considerably if Ri = 0. Also note that if the forward current gain were to go to infinity, the output impedance would go to 0. This would be the normal op amp topology with a very high internal gain. The KH561 achieves a non-zero Ro by setting the internal forward gain to be a low, well controlled, value. Developing the No-Load Gain Expression Taking the output impedance expression as one constraint setting the external resistor values, we now need to develop the no-load voltage gain expression from the non-inverting input to the output as the other constraint. Figure 4 shows the derivation of the no load gain.
+ X1 ierr VRg Rf Ri Gierr if Ro Vo lo
Vi
+ X1 Ri Gierr Vo
ierr
VRg Rf
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DATA SHEET
KH561
recognize that [taking Vi positive] Vo = V - + Gierr R f solving for V - from two directions V - = Vi - ierr Ri = (G + 1) ierr R g solving for ierr from this ierr = then Vi Ri V - = Vi - (G + 1) Rg + Ri and, substituting for V - and ierr in the original Vo expression GR f - Ri Vo = Vi 1 + (G + 1) R g + Ri pulling an Rf out of the fraction Rg Vi (G + 1) Rg + Ri
Equivalent Model Given that the physical feedback and gain setting resistors have been determined in accordance with the design equations shown above, an equivalent model may be created for the gain to the load where the amplifier block is taken as a standard op amp. Figure 5 shows this analysis model and the resulting gain equation to the load.
Vi + Classical op-amp Rf - R o Rg Ro Vo RL
Vo R f - Ro RL = 1+ Vi R g RL + Ro substituting in for R f and R g with their design equation yields Vo RL = Av = AL (gain to load) Vi RL + Ro
Figure 5: Equivalent Model This model is used to generate the DC error and noise performance equations. As with any equivalent model, the primary intent is to match the external terminal characteristics recognizing that the model distorts the internal currents and voltages. In this case, the model would incorrectly predict the output pin voltage swing for a given swing at the load. But it does provide a simplified means of getting to the external terminal characteristics. External Compensation Capacitor (Cx) As shown in the test circuit of Figure 1, the KH561 requires an external compensation capacitor from the output to pin 19. The recommended values described here assume that a maximally flat frequency response into a matched load is desired. The required Cx varies widely with the desired value of output impedance and to a lesser degree on the desired gain. Note from Figure 2, the simplified internal schematic, that the actual total compensation (Ct) is the series combination of Cx and the internal 10pF from pin 19 to the compensation nodes. The total compensation (Ct) is developed in two steps as shown below.
Ri G- V Rf R A v o = 1+ f Vi R g G + 1 + Ri Rg note that A v = 1 + Rf Rg G G + 1
Ri = 0
Figure 4: Voltage Gain Derivation Note again that if Ri = 0 this expression would simplify considerably. Also, if G were very large the voltage gain expression would reduce to the familiar non-inverting op amp gain equation. These two performance equations, shown below, provide a means to derive the design equations for Rf and Rg given a desired no load gain and output impedance. Performance Equations Design Equations
R R f + Ri 1 + f Rg Ro = R G + 1+ i Rg Ri G- Rf R A v = 1+ f R g G + 1 + Ri Rg
R f = (G + 1) Ro - A v Ri Rg = R f - Ro Av - 1
C1 = Ct =
300 2.0 1- pF intermediate equation Ro Rg C1 pF total compensation 1 + (0.02) C1
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KH561
DATA SHEET
With this total value derived, the required external Cx is developed by backing out the effect of the internal 10pF. This, and an expression for the external Cx without the intermediate steps are shown below. Cx = or Cx = 1 pF Ro - 0.08 2 300 1 - Rg 10 C t 10 - C t
Gain and Output Impedance Range Figure 7 shows a plot of the recommended gain and output impedances for the KH561. Operation outside of this region is certainly possible with some degradation in performance. Several factors contribute to set this range. At very low output impedances, the required value of feedback resistor becomes so low as to excessively load the output causing a rapid degradation in distortion. The maximum Ro was set somewhat arbitrarily at 200. This allows the KH561 to drive into a 2:1 step down transformer matching to a 50 load. (This offers some advantages from a distortion standpoint. See Kota Application Note KAN-01 for details.)
100 90 80 70 60 50 40 30 20 10
High Noise Region Recommended Region Low Rf or Rg Region
The plot in Figure 6 shows the required Cx vs. gain for several desired output impedances using the equations shown above. Note that for lower Ro's, Cx can get very large. But, since the total compensation is actually the series combination of Cx and 10pF, going to very high Cx's is increasingly ineffective as the total compensation is only slightly changed. This, in part, sets the lower limits on allowable Ro.
20 18 16 14 12 10 8 6 4 2 0 5 10 15 20 25 30 35 40 45 50 55
Ro = 100 Ro = 75 Ro = 50 Maximally Flat Response into a Matched Load
No Load Gain
0 0 20 40 60 80 100 120 140 160 180 200
Output Impedance ()
Cx (pF)
Figure 7: Recommended Gain and Output Impedance Range For a given Ro, the minimum gain shown in Figure 7 has been set to keep the equivalent input noise voltage less than 4nV/Hz. Generally, the equivalent input noise voltage decreases with higher signal gains. The high gain limit has been set by targeting a minimum Rg of 10 or a minimum Rf of 100. Amplifier Configurations The KH561 is intended for a fixed, non-inverting, gain configuration as shown in Figure 1. The KH560 offers the better pulse fidelity with its improved thermal tail in the pulse response (vs. the KH561). Due to its low internal forward gain, the inverting node does not present a low impedance, or virtual ground, node. Hence, in an inverting configuration, the signal's source impedance will see a finite load whose value depends on the output loading. Inverting mode operation can be best achieved using a wideband, unity gain buffer with low output impedance, to isolate the source from this varying load. A DC level can, however, be summed into the inverting node to offset the output either for offset correction or signal conditioning. Accuracy Calculations Several factors contribute to limit the achievable KH561 accuracy. These include the DC errors, noise effects, and the impact internal amplifier characteristics have on the signal gain. Both the output DC error and noise model may be developed using the equivalent model of Figure 5. Generally, non-inverting input errors show up at the
No Load Voltage Gain
Figure 6: External Compensation Capacitance (Cx) A 0% small signal overshoot response can be achieved by increasing Cx slightly from the maximally flat value. Note that this applies only for small signals due to slew rate effects coming into play for large, fast edge rates. Beyond the nominal compensation values developed thus far, this external Cx provides a very flexible means for tailoring the frequency response under a wide variety of gain and loading conditions. It is oftentimes useful to use a small adjustable cap in development to determine a Cx suitable to the application, then fixing that value for production. An excellent 5pF to 20pF trimmer cap for this is a Sprague-Goodman part #GKX20000. When the KH561 is used to drive a capacitive load, such as an ADC or SAW device, the load will act to compensate the response along with Cx. Generally, considerably lower Cx values are required than the earlier development would indicate. This is advantageous in that a low Ro would be desired to drive a capacitive load which, without the compensating effect of load itself, would otherwise require very large Cx values.
REV. 1A February 2001
9
DATA SHEET
KH561
Rs 4kTRs
output with the same gain as the input signal, while the inverting current errors have a gain of simply (Rf - Ro) to the output voltage (neglecting the Ro to RL attenuation). Output DC Offset: The DC error terms shown in the specification listing along with the model of Figure 5 may be used to estimate the output DC offset voltage and drift. Each term shown in the specification listing can be of either polarity. While the equations shown below are for output offset voltage, the same equation may be used for the drift with each term replaced by its temperature drift value shown in the specification listing.
* eni * * ini
+ Classical op-amp 4kT(Rf - Ro) 4kTRVo
* *
eo
Ro
4kT * * ii Rg
where:
Rg
Rf - R o
Gain to eo Av AvRs Rf - Ro Av
eni - non-inverting input voltage noise ini - non-inverting input current noise ii - inverting input current noise
R - Ro Vos = (Ibn R s Vio ) 1 + f Ibi (R f - Ro ) Rg where: Ibn non - inverting bias current Ibi inverting bias current Vio input offset voltage An example calculation for the circuit in Figure 1 using typical 25C DC error terms and Rs = 25, RL = 50 yields: Vo = (5A 25 2.0mV) 10 10A (360) L 1/ 2 = 12.4mV DC
4kTRs - source resis tan ce voltage noise 4kT / R g - gain settling resistor noise current 4kT(R f - Ro ) - feedback resistor voltage noise 4kTRo - output resistor voltage noise
Figure 8: Equivalent Noise Model
Rf - Ro
1
1
[
]
attentuation between Ro and RL
Recall that the source impedance, Rs, includes both the terminating and signal source impedance and that the actual DC level to the load includes the voltage divider between Ro and RL. Also note that for the KH561, as well as for all current feedback amplifiers, the non-inverting and inverting bias currents do not track each other in either magnitude or polarity. Hence, there is no meaning in an offset current specification, and source impedance matching to cancel bias currents is ineffective. Noise Analysis: Although the DC error terms are in fact random, the calculation shown above assumes they are all additive in a worst case sense. The effect of all the various noise sources are combined as a root sum of squared terms to get an overall expression for the spot noise voltage. The circuit of Figure 8 shows the equivalent circuit with all the various noise voltages and currents included along with their gains to the output.
To get an expression for the equivalent output noise voltage, each of these noise voltage and current terms must be taken to the output through their appropriate gains and combined as the root sum of squares.
eo =
(e
ni
2
+ (iniR s ) + 4kTRs A v 2 + ii2 (R f - Ro ) L
2 2
)
+ 4kT (R f - Ro ) A v + 4kTRo
Where the 4kT(Rf - Ro) Av term is the combined noise power of Rg and Rf - Ro. It is often more useful to show the noise as an equivalent input spot noise voltage where every term shown above is reflected to the input. This allows a direct measure of the input signal to noise ratio. This is done by dividing every term inside the radical by the signal voltage gain squared. This, and an example calculation for the circuit of Figure 1, are shown below. Note that RL may be neglected in this calculation.
en =
eni2 + (iniR s ) + 4kTRs +
2
ii2 (R f - Ro ) Av2 +
2
+L
4kT (R f - Ro ) Av
4kTRo Av2
10
REV. 1A February 2001
KH561
DATA SHEET
For the circuit of Figure 1, the equivalent input noise voltage may be calculated using the data sheet spot noises and Rs = 25, RL = . Recall that 4kT = 16E-21J. All terms cast as (nV/Hz)2
Vi Rs
+
Cx
R'o = Rx + Ro Rx Vo RL Ro = R'o - Rx
KH561
Rf Rg
en =
(2.1)
2
+ (.07) + (.632) + (1.22) + (.759) + (.089)
2 2 2 2
2
= 2.62nV/ Hz
Gain Accuracy (DC): A classical op amp's gain accuracy is principally set by the accuracy of the external resistors. The KH561 also depends on the internal characteristics of the forward current gain and inverting input impedance. The performance equations for Av and Ro along with the Thevinin model of Figure 5 are the most direct way of assessing the absolute gain accuracy. Note that internal temperature drifts will decrease the absolute gain slightly as the part warms up. Also note that the parameter tolerances affect both the signal gain and output impedance. The gain tolerance to the load must include both of these effects as well as any variation in the load. The impact of each parameter shown in the performance equations on the gain to the load (AL) is shown below. Increasing Increasing Increasing Increasing current gain G inverting input Ri Rf Rg Increases AL Decreases AL lncreases AL Decreases AL
With: Ro = KH561 output impedance and Ro + Rx = RL generally
Figure 9: Improving Output Impedance Match vs. Frequency Increasing Rx will decrease the achievable voltage swing at the load. A minimum Rx should be used consistent with the desired output match. As discussed in the thermal analysis discussion, Rx is also very useful in limiting the internal power under an output shorted condition. Interpreting the Slew Rate: The slew rate shown in the data sheet applies to the voltage swing at the load for the circuit of Figure 1. Twice this value would be required of a low output impedance amplifier using an external matching resistor to achieve the same slew rate at the load. Layout Suggestions: The fastest fine scale pulse response settling requires careful attention to the power supply decoupling. Generally, the larger electrolytic capacitor ground connections should be as near the load ground (or cable shield connection) as is reasonable, while the higher frequency ceramic de-coupling caps should be as near the KH561's supply pins as possible to a low inductance ground plane. Evaluation Boards: An evaluation board (showing a good high frequency layout) for the KH561 is available. This board may be ordered as part #730019. Thermal Analysis and Protection A thermal analysis of a chip and wire hybrid is directed at determining the maximum junction temperature of all the internal transistors. From the total internal power dissipation, a case temperature may be developed using the ambient temperature and the case to ambient thermal impedance. Then, each of the dominant power dissipating paths are considered to determine which has the maximum rise above case temperature. The thermal model and analysis steps are shown below. As is typical, the model is cast as an electrical model where the temperatures are voltages, the power dissipators are current sources, and the thermal impedances are resistances. Refer to the summary design equations and Figure 1 for a description of terms.
Applications Suggestions Driving a Capacitive Load: The KH561 is particularly suitable for driving a capacitive load. Unlike a classical op amp (with an inductive output impedance), the KH561's output impedance, while starting out real at the programmed value, goes somewhat capacitive at higher frequencies. This yields a very stable performance driving a capacitive load. The overall response is limited by the (1/RC) bandwidth set by the KH561's output impedance and the load capacitance. It is therefore advantageous to set a low Ro with the constraint that extremely low Rf values will degrade the distortion performance. Ro = 25 was selected for the data sheet plots. Note from distortion plots into a capacitive load that the KH561 achieves better than 60dBc THD (10-bits) driving 2Vpp into a 50pF load through 30MHz. Improving the Output Impedance Match vs. Frequency - Using Rx: Using the loop gain to provide a non-zero output impedance provides a very good impedance match at low frequencies. As shown on the Output Return Loss plot, however, this match degrades at higher frequencies. Adding a small external resistor in series with the output, Rx, as part of the output impedance (and adjusting the programmed Ro accordingly) provides a much better match over frequency. Figure 9 shows this approach.
REV. 1A February 2001
11
DATA SHEET
Case Temperature Tc 20C/W Tj(t) Pt 200C/W Tj(q) Pq Pcircuit ca Case to Ambient Termal Impedance TA Ambient Temperature
KH561
As an example of calculating the maximum internal junction temperatures, consider the circuit of Figure 1 driving 2.5V, 50% duty cycle, square wave into a 50 load. Note that 1/2 of the total PT and Pa powers were used
410 5 Req = 50 = 45.6 5 -1 Io = 2.5V / (45.6) = 54.9mA IT =
1 54.9mA + 2
Figure 10: Thermal Model
Io = Vo / Req total output current R A with Req = RL f L total load AL - 1 It =
2 Io 2 + (.06) total internal output stage current 1 I 2 o
(54.9mA)2 + (.06)2 = 68.1mA
PT = 68.1mA [15 - 2.5 - 0.7 - 15.3 68.1mA] = 733mW total power in both sides of the output stage Pq = 0.2 68.1mA [15 - 1.4 - 17.3 68.1mA] = 169mW total power in both sides of hottest junctions prior to output stage Pcircuit = 1.3 (15) [2 68.1mA - 54.9mA + 19.2mA] - 733mW - 169mW = 1.058W power in the remainder of circuit With these powers and TA = 25C and ca = 35C / W Tc = 25C + (.733 + .169 + 1.058) 35 = 94C case temperature From this, the hottest internal junctions may be found as
+
Pt = It (VCC - 1.4 - 17.3 It ) output stage power Pq = 0.2 It (VCC - Vo - 0.7 - 15.3 It ) power in hottest internal junction prior to output stage Pcircuit = 1.3 VCC (2 It - Io + 19.2mA) - Pt - Pq power in remainder of circuit [note VCC = | - VCC |]
Note that the Pt and Pq equations are written for positive Vo. Absolute values of -VCC, Vo, and Io, should be used for a negative going Vo. since we are only interested in delta V's. For bipolar swings, the two powers for each output polarity are developed as shown above then ratioed by the duty cycle. Having the total internal power, as well as its component parts, the maximum junction temperature may be computed as follows. Tc = TA + (Pq + PT + Pcircult) * ca Case Temperature ca = 35C/W for the KH561 with no heatsink in still air Tj(t) = Tc + Pt * 20C/W output transistor junction temperature Tj(q) = Tc + Pq * 200C/W hottest internal junction temperature The Limiting Factor for Output Power is Maximum Junction Temperature Reducing ca through either heatsinking and/or airflow can greatly reduce the junction temperatures. One effective means of heatsinking the KH561 is to use a thermally conductive pad under the part from the package bottom to a top surface ground plane on the component side. Tests have shown a ca of 24C in still air using a "Sil Pad" available from Bergquist (800-347-4572).
Tj ( t) = 94C +
1 2
(.733) 20 = 101C output stage
Tj (q) = 94C + 12 (.169) 200 = 111C hottest internal junction
here since the 50% duty cycle output splits the power evenly between the two halves of the circuit whereas the total powers were used to get case temperature. Even with the output current internally limited to 250mA, the KH561's short circuiting capability is principally a thermal issue. Generally, the KH561 can survive short duration shorts to ground without any special effort. For protection against shorts to the 15 volt supply voltages, it is very useful to reduce some of the voltage across the output stage transistors by using some external output resistance, Rx, as shown in Figure 9. Evaluation Board An evaluation board (part number 730019) for the KH561 is available.
12
REV. 1A February 2001
DATA SHEET
KH561
KH561 Package Dimensions
C
Pin #1 Index Q L
b1 A
b e D1
E
E1
D
A1
Symbol
A A1 b b1 c D D1 E E1 e L Q
Inches
Minimun Maximum 0.225 0.139 0.014 0.192 0.026
Milimeters
Minimum Maximum 5.72 3.53 0.36 4.88 0.66
NOTES: Seal: seam weld (AM, AK), epoxy (AI) Lead finish: gold finish Package composition: Package: ceramic Lid: kovar/nickel (AM, AK), ceramic (AI) Leadframe: alloy 42 Die attach: epoxy
0.050 BSC 0.008 1.190 1.095 0.500 0.018 1.290 1.105 0.610
1.27 BSC 0.20 30.23 27.81 12.70 0.46 32.77 28.07 15.49
0.600 BSC 0.100 BSC 0.165 BSC 0.015 0.075
15.24 BSC 2.54 BSC 4.19 BSC 0.38 1.91
DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICES TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, and (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in a significant injury of the user. 2. A critical component in any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.
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(c) 2001 Fairchild Semiconductor Corporation

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