Datasheet

AD8392A
Rev. 0 | Page 7 of 12
THEORY OF OPERATION
The AD8392A is a current feedback amplifier with high
(500 mA) 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 16 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
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.
0
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R
F
V
OUT
R
G
R
N
V
IN
R
IN
I
IN
T
Z
Figure 16. Simplified Block Diagram
The AD8392A is capable of delivering 500 mA of output
current while swinging to within 2 V of either power supply
rail. The AD8392A also has a power management system
included on-chip. It features four user-programmable power
levels (three active power modes as well as the provision for
complete shutdown).