REV. C

AD8004

–9–

THEORY OF OPERATION

The AD8004 is a member of a new family of high speed current-

feedback (CF) amplifiers offering new levels of bandwidth,

distortion, and signal-swing capability vs. power. Its wide dynamic

range capabilities are due to both a complementary high speed

bipolar process and a new design architecture. The AD8004 is

basically a two stage (Figure 30) rather than the conventional

one stage design. Both stages feature the current-on-demand

property associated with current feedback amplifiers. This

gives an unprecedented ratio of quiescent current to dynamic

performance. The important properties of slew rate and full

power bandwidth benefit from this performance. In addition

the second gain stage buffers the effects of load impedance,

significantly reducing distortion.

A full discussion of this new amplifier architecture is available on

the data sheet for the AD8011. This discussion only covers the

basic principles of operation.

DC AND AC CHARACTERISTICS

As with traditional op amp circuits the dc closed-loop gain is

defined as:

A

R

F

R

N

noninverting operation

A

R

F

R

N

inverting operation

The more exact relationships that take into account open-loop

gain errors are:

A

G

1

+

1

− G

A

+

R

F

T

for inverting (G is negative)

A

G

1

+

G

A

+

R

F

T

for noninverting (G is positive)

to both voltage and current-feedback amplifiers and is the ratio

of output voltage to differential input voltage. The open-loop

inverting input current and is applicable to current-feedback

amplifiers. The open-loop voltage gain and open-loop transim-

in TPCs 15 and 18. These plots and the basic relationships can

be used to predict the first order performance of the AD8004 over

the frequency response characteristics. This gives the result that

bandwidth is constant with gain, a familiar property of current

feedback amplifiers.

has been chosen as the nominal value to give

optimum frequency response with acceptable peaking at gains of

+2/–1. As can be seen from the above relationships, at higher

for a variety of gains.

Q1

Q2

IPP

IPN

INP

IPN

IQ1

Q3

Q4

IE

A2

IQ1

AD8004

A2

A3

A1

A1

Z2

Figure 5. Simplified Block Diagram