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
VOLTAGE-FEEDBACK AMPLIFIERS
vs CURRENT-FEEDBACK AMPLIFIERS:
BANDWIDTH AND DISTORTION CONSIDERATIONS
by Tony Wang, (602) 746-7650
Designers specify amplifiers based on certain key param-
eters, one of which is bandwidth. Traditionally, the gain-
bandwidth product of an amplifier told the user everything
he needed to know about its small-signal AC performance.
The useful bandwidth of an amplifier was determined by
dividing the gain-bandwidth product (GBW) by the desired
closed-loop gain. However, this simple formula cannot be
used with current-feedback amplifiers.
Current-feedback amplifiers have nearly constant bandwidth
for varying closed-loop gains. The reason is that the user can
adjust the open-loop gain of the current-feedback amplifier
by changing the feedback network without affecting the
open-loop pole. The concept can be more readily understood
with the aid of Figure 1, which shows a simplified AC model
for the current-feedback amplifier.
The primary determinants of AC performance for the cur-
rent-feedback amplifier are the transresistance, R
T
, and the
transcapacitance, C
T
. A well specified current-feedback
amplifier鈥檚 data sheet will list these parameters. Figure 1
shows the current-feedback amplifier with a bipolar transis-
tor as the input device. This is convenient because it has a
low impedance inverting input (the emitter) and a high
impedance noninverting input (the base). For this analysis,
the bipolar transistor is considered ideal (i.e., infinite beta,
zero base-emitter voltage, no base-collector capacitance).
The collector terminates in R
T
, C
T
and an inverting buffer.
The feedback network consists of R
F
and R
G
.
Figure 2 shows the same circuit reconfigured for analysis.
The feedback network is now the emitter load for the input
transistor. Open-loop voltage gain can be determined by
inspection to be:
91
A
OL
R ||
(
j2蟺f C
T
)
=
T
R
F
|| R
G
=
鈥?
V+
R
F
+
R
G
R
T
1
R
G
R
F
1
+
j2蟺f R
T
C
T
+In
R
T
C
T
鈥?
V
OUT
=
A
CL
R
T
1
R
F
1
+
j2蟺f R
T
C
T
Q
1
鈥揑n
V+
R
T
R
F
C
T
鈥?
V
OUT
R
L
= R
F
+ R
G
+In
鈥揑n
Q
1
R
G
R
E
= R
F
|| R
G
FIGURE 1. Current-Feedback Amplifier Simplified for AC
Analysis.
FIGURE 2. Current-feedback Amplifier Simplified and
Reconfigured for Analysis.
漏
1994 Burr-Brown Corporation
AB-091
Printed in U.S.A. January, 1994
Application Bulletin Number 91
1
2
