10
LT1010
1010fc
APPLICATIO S I FOR ATIO
W
U
U
U
This is a good example of how fast op amps can be made
quite easy to use by employing an output buffer.
Integrator
A lowpass amplifier can be formed just by using large C
F
in the inverter described earlier, as long as the increasing
closed-loop output impedance above the cutoff frequency
is not a problem and the op amp is capable of supplying
the required current at the summing junction.
A2
LT1010
R
F
20k
C
F
500pF
C
I
I
IN
1010 AI07
V
OUT
–
+
A1
LT1012
BIAS
+
LT1010
22
µ
F
+
22
µ
F
TYPICAL SPECIFICATIONS
1V
P-P
INTO 75
Ω
AT A = 2
0.5dB TO 10MHz
3dB DOWN AT 16MHz
AT A = 10
0.5dB TO 4MHz
–3dB = 8MHz
+
68
µ
F
1010 AI08
PEAKING
5pF to 25pF
25
Ω
900
Ω
OUTPUT
(75
Ω
)
0.01
µ
F
1k
GAIN SET
5.1k
Q1, Q2: 2N3866
–15V
8.2k
15V
–15V
Q2
Q1
INPUT
stage. Feedback is arranged in the conventional manner,
although the 68
µ
F-0.01
µ
F combination limits DC gain to
unity for all gain settings. For applications sensitive to
NTSC requirements, dropping the 25
Ω
output stage bias
value will aid performance.
If the integrating capacitor must be driven from the buffer
output, the circuit above can be used to provide capacitive
load isolation. As before, the stability with large capacitive
loads is determined by R
F
C
F
.
Wideband Amplifiers
This simple circuit provides an adjustable gain video
amplifier that will drive 1V
P-P
into 75
Ω
. The differential
pair provides gain with the LT1010 serving as an output
A2
LT1010
R1
100
Ω
R2
800
Ω
C1
15pF
V
IN
1010 AI09
V
OUT
–
+
A1
HA2625
A2
LT1010
R1
400
Ω
R2
1.6k
INPUT
1010 AI10
OUTPUT
–
+
A1
HA2625
This shows the buffer being used with a wideband ampli-
fier that is not unity-gain stable. In this case, C1 cannot be
used to isolate large capacitive loads. Instead, it has an
optimum value for a limited range of load capacitances.
The buffer can cause stability problems in circuits like
this. With the TO-220 packages, behavior can be im-
proved by raising the quiescent current with a 20
Ω
resistor from the bias terminal to V
+
. Alternately, devices
in the miniDIP can be operated in parallel.
It is possible to improve capacitive load stability by
operating the buffer class A at high frequencies. This is
done by using quiescent current boost and bypassing the
bias terminal to V
–
with more than 0.02
µ
F.
Putting the buffer outside the feedback loop as shown
here will give capacitive load isolation, with large output
capacitors only reducing bandwidth. Buffer offset, re-
ferred to the op amp input, is divided by the gain. If the
load resistance is known, gain error is determined by the
output resistance tolerance. Distortion is low.
