廬
APPLICATION BULLETIN
Mailing Address: PO Box 11400 鈥?Tucson, AZ 85734 鈥?Street Address: 6730 S. Tucson Blvd. 鈥?Tucson, AZ 85706
Tel: (602) 746-1111 鈥?Twx: 910-952-111 鈥?Telex: 066-6491 鈥?FAX (602) 889-1510 鈥?Immediate Product Info: (800) 548-6132
CLASSICAL OP AMP OR CURRENT-FEEDBACK OP AMP?
THIS COMPOSITE OP AMP GIVES YOU
THE BEST OF BOTH WORLDS
BY TIM KALTHOFF, TONY WANG, AND R. MARK STITT (602) 746-7445
Classical op amps such as the OPA627 have excellent
performance in applications where the required gain band-
width is low compared to the gain-bandwidth product of the
op amp. However, increasing closed-loop gain decreases the
error-reducing loop gain. Furthermore, starting at relatively
low frequencies, the loop gain rolls-off at 20dB/decade of
signal frequency increase. In combination these effects can
produce significant errors, especially at higher frequencies
where the loop gain can be very low.
Current-feedback op amps, such as the OPA603, have good
dynamic performance at both low and high gains. This is
because the feedback components set both closed-loop gain
and open-loop gain, making loop gain and dynamic per-
formance relatively independent of closed-loop gain. Unfor-
tunately, the DC performance (V
OS
, dV
OS
/dT, CMR, etc) of
current feedback amplifiers is poor compared to classical op
amps.
A composite amplifier using a classical amplifier and the
OPA603 current-feedback amplifier can combine the best
qualities of both amplifiers.
Figures 1 and 2 show noninverting and inverting composite
amplifiers. Table I shows suggested component values for
selected gains and measured performance results.
DC performance of the composite amplifier is excellent.
Since the OPA603 is in the feedback of the OPA627, the
composite amplifier retains the excellent DC characteristics
of the OPA627. In fact, since the OPA627 does not drive the
load directly, its DC accuracy can be better than the OPA627
alone. Thermal feedback within an amplifier driving large
loads will cause errors due to internal thermal gradients and
package self-heating. The composite amplifier with an
OPA603 can drive 150鈩?loads to
鹵10V
with no thermal
feedback to the OPA627.
1
3
4
5
6
7
V
IN
R
1
A
1
OPA627
V
IN
R
2
A
2
OPA603
R
1
A
1
OPA627
V
OUT
V
OUT
= V (1 + R /R )
IN
2 1
R
2
A
2
OPA603
V
OUT
V
OUT
= 鈥揤 (R /R )
IN
2 1
8
9
10
11
12
13
14
15
16
R
4
R
3
R
3
R
4
FIGURE 1. Composite Noninverting Amplifier with Preci-
sion of OPA627 and Speed of OPA603.
OVERALL
GAIN
[V/V]
5
10
20
50
100
200
500
1000
OPA603
GAIN
[V/V]
3
6
12
26
52
18
42
85
FIGURE 2. Composite Inverting Amplifier with Precision of
OPA627 and Speed of OPA603.
SLEW
RATE
[V/
碌
s]
100
240
620
730
730
580
590
510
SETTLING
(0.1%)
(2)
[ns]
265
240
200
320
330
350
580
640
SETTLING
(0.01%)
(2)
[ns]
520
500
520
530
(3)
(3)
(3)
(3)
GBW
[Hz]
90M
180M
330M
750M
1.5G
2.5G
6.0G
10.0G
A
1
OPA627
OPA627
OPA627
OPA627
OPA627
OPA637
OPA637
OPA637
R
1(1)
[
鈩?/div>
]
255
110
52.3
49.9
49.9
49.9
49.9
49.9
R
2
[
鈩?/div>
]
1020
1000
1000
2430
4990
10k
25k
50k
R
3(4)
[
鈩?/div>
]
499
200
93.1
40.2
20
60.4
24.3
12.1
R
4
[
鈩?/div>
]
1020
1020
1020
1020
1020
1020
1020
1020
NOTES: (1) R
1
shown is for noninverting composite amplifier. For inverting amplifier, R
1
= Gain/R
2
. (2) Settling time for 10V output step. (3) Output noise exceeds
0.01% at this gain. (4) For intermediate gains, use the higher value R
3
.
TABLE I. Measured Results for Selected Composite-Amplifier Examples.
漏
1990 Burr-Brown Corporation
AB-007A
Printed in U.S.A. March, 1991
1
2
