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, dVO/dIIN or TZ.

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 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

OUT

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.

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 21. Simplified Block Diagram