Data Sheet

AD8392A

Rev. A | 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,

dVO/dIIN or TZ.

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 RIN is

proportional to 1/gm, the equivalent voltage gain is TZ × gm,

where gm is the transconductance of the input stage. Basic

analysis of the follower with gain circuit yields

( )

( )

F

IN

Z

Z

IN

O

R

R

G

S

T

S

T

G

V

V

+

×

+

×

=

where:

G

F

R

R

G

+

=1

Ω

50

1 ≈

=

m

IN

g

R

Because G × 

RIN << RF for low gains, a current feedback

amplifier has relatively constant bandwidth vs. gain, the 3 dB

point being set when |TZ| = RF.

Of course, for a real amplifier there are additional poles that

contribute excess phase, and there is a value for RF below which

the amplifier is unstable. Tolerance for peaking and desired

flatness determines the optimum RF in each application.

RF

VOUT

RG

RN

VIN

RIN

IIN

TZ

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).