廬
APPLICATION BULLETIN
VIDEO OPERATIONAL AMPLIFIER
By Klaus Lehmann, Burr-Brown International GmbH
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
CURRENT OR VOLTAGE FEEDBACK?
THAT鈥橲 THE QUESTION HERE.
In analog technology, important advances are being made in
video operational amplifiers, both in processing and in
circuitry. The complementary bipolar processes used by
鈥淏ell Laboratories鈥?should be emphasized, which have ver-
tically structured and approximately equal electric NPN and
PNP transistors on one substrate. These processes help to
create the basis for an effective complementary-symmetric
circuit technique, which is currently obtaining its best results
with the so-called 鈥淒iamond鈥?structure. This structure plays
a key role in transimpedance or current-feedback amplifiers.
The following article is concerned exclusively with explain-
ing these key topics and the new circuit designs.
The following discussion will deal first with practical cir-
cuits and then with theoretical models. It is based on publi-
cations [5] and [6], and essays [15] and [16], written at the
Institute for Semiconductor Technology of the Darmstadt
Technical College, with the assistance of Diplom Engineer
R.B. Steck.
CONVENTIONAL CIRCUIT TECHNIQUES
Figure 1 illustrates the conventional structure of a feedback
amplifier, including the wide-band operational amplifier
OPA discussed here. As can be seen in Figure 2, this kind of
OPA consists of a differential amplifier TA with high-
impedance output (7) and an impedance converter B inserted
afterwards. The elements R and C function between them,
determining, among other things, the open-loop gain G
OL
,
slew rate, and bandwidth f
鈥?dB
. The circuit techniques of B
are not discussed here, but it could have a structure such as
the push-pull one shown in Figure 3.
Figure 4 illustrates what is probably the simplest and most
well-known structure of the differential amplifier TA. It can
be seen in Figure 4 that the capacitor C can be charged with
R
+V
IN
1
2
R
2
TA 鈭?/div>
7
R
23
C
3
V
OUT
B
FIGURE 2. OPA as Series Connection between TA and B.
V
CC
I
+V
IN
V
OUT
I
V
EE
FIGURE 3. Buffer with Push-pull Structure.
V
CC
I
C
7
+SR
MAX
= I/C
鈥揝R
MAX
= I/C
B
V
IN
1
2
R
2
OPA
3
V
OUT
R
23
+V
IN
1
1
2
2
R
23
3
V
OUT
R
2
2I
V
EE
FIGURE 1. Voltage-Feedback Operational Amplifier.
漏
FIGURE 4. Conventional Structure of a Differential
Amplifier.
AB-179
Printed in U.S.A. May, 1993
1993 Burr-Brown Corporation
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