Datasheet

ADA4311-1
Rev. 0 | Page 10 of 16
THEORY OF OPERATION
The ADA4311-1 is a dual-current feedback amplifier with high
output current capability. With a current feedback amplifier, the
current into the inverting input is the feedback signal, and the
open-loop behavior is that of a transimpedance, dV
O
/dI
IN
or T
Z
.
The open-loop transimpedance is analogous to the open-loop
voltage gain of a voltage feedback amplifier.
Figure 21 shows a
simplified model of a current feedback amplifier. Because R
IN
is
proportional to 1/g
m
, the equivalent voltage gain is T
Z
× g
m
,
where g
m
is the transconductance of the input stage. Basic
analysis of the follower with gain circuit yields
()
()
FIN
Z
Z
IN
OUT
RRGsT
sT
G
V
V
+×+
×=
where:
G
F
R
R
G +=1
50
1
≈=
m
IN
g
R
Because G × R
IN
<< R
F
for low gains, a current feedback amplifier
has relatively constant bandwidth vs. gain, the 3 dB point being
set when |T
Z
| = R
F
.
For a real amplifier, there are additional poles that contribute
excess phase, and there is a value for R
F
below which the amplifier
is unstable. Tolerance for peaking and desired flatness determines
the optimum R
F
in each application.
R
F
V
OUT
R
G
R
N
V
IN
R
IN
I
IN
T
Z
0
6940-018
Figure 21. Simplified Block Diagram