Datasheet

ADA4310-1
Rev. 0 | Page 10 of 16
THEORY OF OPERATION
The ADA4310-1 is a 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 20 shows a
simplified model of a current feedback amplifier. Because R
IN
is
proportional to 1/g
m
, the equivalent voltage gain is just 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
O
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
.
Of course, 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.
06027-017
R
F
V
OUT
R
G
R
N
V
IN
R
IN
I
IN
T
Z
Figure 20. Simplified Block Diagram