5 6 3 4 2 1 figure 1. basic structure of a symmetrical transmission path. in radio frequency (rf) technology, signals from oscillo- scopes, monitors, transient recorders, and many other de- vices are usually connected to sensors and generators via coaxial lines. in any transmission, however, interference voltages caused by differences in potential between the sender and receiver and by electromagnetic interference distort the results, particularly when the signals being trans- mitted are sensitive and wide-band. designers of this type of transmission system need shielded, symmetrical transmis- sion lines and input differential amplifiers with high com- mon-mode rejection, which bring home the signals without humming or radio interference. designing this type of differential amplifier used to be quite a chore, involving extensive and complicated hardware. but the development of new, monolithic ics such as the dia- mond transistor opa660 has changed all that. the opa660 makes it easy to design a 400mhz differential amplifier offering C60db common-mode rejection at a 1mhz fre- quency. this amplifier uses an open-loop amplifier structure with two identical high-impedance inputs and no feedback. the parameters such as wide bandwidth, stable operation, and excellent pulse processing, common-mode rejection, and harmonic distortion let the performance speak for itself. basic transmission structures figure 1 shows a symmetrical transmission path with signal voltage v s and cable termination resistors r in and r t . a symmetrical voltage source normally uses amplifiers with complementary outputs or transformers to balance or adapt the circuits. the relatively high-impedance input resistor r b limits the input potential drift through the input bias currents (i bias ), and the symmetrical differential amplifier input re- jects interference voltages superimposed upon the input signal and its reference potential. the voltage-controlled current source converts the symmetrical input voltage v in either into an output current or into the asymmetrical output voltage v out when a voltage drop is present at the external resistor r out . v in and v out are related as follows: v out = v in ? gm ? r out , where gm is the transconductance of the operational transconductance amplifier (ota). the buffer following the input amplifier decouples the low-impedance load resistor from the high-impedance ota output. instead of symmetrical signal excitation, many applications use the type of transmission path shown in figure 2. a single-ended signal voltage v in drives an asymmetrical coaxial cable terminated on both sides. in this structure as well, the symmetrical differential amplifier input rejects interference voltages superimposed on the signal. instrumentation amplifier with feedback otas and buffers have conventionally been designed using differential amplifiers as shown in figure 3. the feedback path from the op amp output over r 4 generates a relatively low-impedance inverting input, which is equal to the r 3 resistor value. inserting the buffer amplifier, buf2, converts the low-impedance input to high impedance, while inserting the buffer amplifier, buf1, optimizes the input symmetry and thus the common-mode rejection at dc and vs fre- quency. the gain is r 4 /r 3 during signal excitation at the inverting input and 1 + r 4 /r 3 during signal excitation at the noninverting input. a divider is inserted between r 1 and r 2 to compensate for these differing gains. buffer 1 also synchronizes the signal delay times of the two inputs, which is important for good common-mode rejection at high frequencies. to achieve high common-mode rejection over frequency, it is important that the gain curve of the two input buffers be as identical as possible. buffer
r load
v out
ota
+v cc
? cc
r out
v out
i out
r b
i bias
i bias
v in
r t
r in
v in
gm
application bulletin mailing address: po box 11400 ?tucson, az 85734 ?street address: 6730 s. tucson blvd. ?tucson, az 85706 tel: (602) 746-1111 ?twx: 910-952-111 ?telex: 066-6491 ?fax (602) 889-1510 ?immediate product info: (800) 548-6132 building a 400mhz wide-band differential amplifier: it's a breeze with the diamond transistor opa660. by christian henn and ernst rau, burr-brown international gmbh ? 1993 burr-brown corporation an-188 printed in u.s.a. november, 1993
2 v out = v in r out r e + 2 /gm in addition to requiring more hardware, this type of system also has smaller bandwidth than the open-loop amplifier shown in figure 2 due to the delay time in its amplifier feedback loop (phase shift). a synthesis: open-loop amplifier using the diamond transistor the open-loop amplifier using the diamond transistor opa660 and buffer amplifier buf601 combines the best of both worlds, offering better bandwidth than a normal open- loop amplifier, excellent pulse responses down to rise/fall times of 1ns, and reduced hardware. the basic concept is shown in figure 4. the gain can be determined according to the following equation: figure 3. instrumentation amplifier with feedback. figure 4. wide-band open-loop amplifier. dt
r t
r in
v in
buf 2
buf 1
r load
v out
v in
r e
r out
v out
gm
i out
buf601
opa660
opa
r load
v out
r 1
r t
r in
v in
buf 1
buf 2
buf600
buf600
r 3
opa622
r 2
r 4
v in
figure 2. signal transmission using an asymmetrical coaxial cable and a signal voltage referred to ground. buffer
r load
v out
ota
+v cc
? cc
r out
v out
i out
i bias
i bias
v in
r t
r in
v in
gm
3 since the actual symmetrical structure of the circuit layout greatly effects the bandwidth and common-mode rejection, a demo board was used to determine the characteristic transmission parameters that this configuration shows in practice. figure 5 illustrates the demo board in detail. the silkscreen and layout tips can be extremely useful in design- ing your own layouts. the opa660 contains a transconductance amplifier nick- named the diamond transistor and a buffer called the diamond buffer in an 8-pin package. the diamond transis- tor itself consists of a buffer identical to the diamond buffer, followed by a current mirror. on the output side, the buffers are connected to each other via the resistor r 8 , forming the differential input stage. when the input voltage is differential, a current flows through r 8 , is reflected in high-impedance form to pin 8, and produces the output voltage at r 9 . to drive low-impedance transmission lines or input resistors, the buffer amplifier buf601 decouples the relatively high-impedance output of the differential ampli- fier. both inputs and the output are laid out for 50 w systems, but they can also be adapted to other characteristic imped- ances by replacing the resistors r 3 , r 7 , and r 11 . capacitor c 5 parallel to r 8 compensates the parasitic capacitor at pin 8 of the opa660, thus expanding the achievable bandwidth. the resistors, r 4 , r 6 , and r 10 , located at the front of the circuit in series to the high-impedance inputs, make it possible to set the frequency response at the end of the bandwidth for a flat response. the quiescent current of the opa660 is 20ma at an r 16 of 560 w . test results the amplifier stage is set to a gain of +4 at an r 9 of 240 w and r 8 of 43 w . the total gain from input to output, including the output divider r 11 /r l , is +2. figure 6 illustrates the frequency response of the two inputs in+ and inC. the C3db frequency (fg) is 400mhz. figure 7 shows the impact of the capacitor c 5 on the bandwidth. the common-mode gain over frequency curve shown in figure 8 demonstrates the rejection of interference voltages on both input voltages. the interference remains less than C18db over the entire bandwidth, starting at a common- mode gain of C68db. while the 400mhz differential ampli- fier amplifies differential signals by 4, the common-mode noise of the same frequency that appears at the output is multiplied only by 0.125. table i lists the common-mode gain for several frequency levels. f in cg 1mhz C60db 10mhz C45db 100mhz C23db table i. several common-mode gains. the harmonic distortions shown in figure 9 and table ii for two different output voltages over frequency are outstanding parameters for a 400mhz differential amplifier and prove that the opa660 and buf601 provide excellent reproduc- tion of wide-band input signals even without feedback. furthermore, the low noise voltage density of 7.7nv/ ? hz makes it possible to process even very small signals. figure 5. circuit diagram of the demo board. dt
buf601
db
r 16
560 w
out
r 9
240 w
8
opa660
r 10
150 w
r 11
51 w
8
c b
+5v
1
c b
?v
5
r 8
43 w
c 5
18pf
c b
?v
4
1
6
2
c b
+5v
7
r 4
150 w
r 6
150 w
r 3
51 w
r 7
51 w
in+
in? 3
5
c b = 2.2? || 10nf
i q = ?0ma (opa660)
i q = ?ma (buf601)
4
4 high processing power, low power requirements the most important job of a differential amplifier is to reject common-mode interference arising during the transmission of analog signals. the 400mhz differential amplifier using the opa660 impressively demonstrates how easy it now is to design wide-band input amplifiers for test devices, moni- tors, transient recorders, and other rf devices. while achiev- ing excellent parameters for bandwidth, common-mode re- jection, and frequency response, the opa660 and buf601 also offer such low power consumption that the entire differential amplifier can be powered from a separate battery supplya truly compact, high-performance alternative. figure 6. frequency responses of the inputs in+ and inC. figure 7. impact of capacitor c s on the bandwidth. figure 8. common-mode gain. figure 9. harmonic distortion. harmonic fv out distortion 10mhz 1vp-p 1st harmonic C61db 10mhz 1vp-p 2nd harmonic C64db 10mhz 2vp-p 1st harmonic C57db 10mhz 2vp-p 2nd harmonic C55db table ii. harmonic distortion. gain (db)
10
0
?0
?0
?0
frequency (hz)
1m
10m
100m
1g
300k
c 5 = 18pf
c 5
10
0
?0
?0
?0
frequency (hz)
gain (db)
1m
10m
100m
1g
300k
? in (pin 3)
+v in (pin 5)
?0
?0
?0
?0
?0
?0
frequency (hz)
common mode gain (db)
1m
10m
100m
1g
300k
?0
?5
?5
?5
?5
?5
?5
frequency (hz)
1m
10m
100m
harmonic distortion (dbc)
2f 1vp-p
1f 1vp-p
2f 2vp-p
1f 2vp-p
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