AD8004
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 perfor-
mance. 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 sig-
nificantly 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:
AV
=G
=1+
RF
RN
noninverting operation
AV
=G
=
−
RF
RN
inverting operation
The more exact relationships that take into account open-loop
gain errors are:
AV
=
1+
G
1−G +
RF
AO(s) TO(s)
for inverting (G is negative)
AV =
1+
G
G+
RF
AO(s) TO(s)
for noninverting (G is positive)
In these equations the open-loop voltage gain (AO(s)) is com-
mon to both voltage and current-feedback amplifiers and is the
ratio of output voltage to differential input voltage. The open-
loop transimpedance gain (TO(s)) is the ratio of output voltage
to inverting input current and is applicable to current-feedback
amplifiers. The open-loop voltage gain and open-loop transim-
pedance gain (TO(s)) of the AD8004 are plotted vs. frequency
in Figures 20 and 23. These plots and the basic relationships
can be used to predict the first order performance of the AD8004
over frequency. At low closed-loop gains the term (RF /TO(s))
dominates the frequency response characteristics. This gives the
result that bandwidth is constant with gain, a familiar property
of current feedback amplifiers.
An RF of 1 kΩ 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
closed-loop gains reducing RF has the effect of increasing closed-
loop bandwidth. Table I gives optimum values for RF and RG
for a variety of gains.
IPP
VP
A1
IPN
IQ1
Q3
C P1
Q1
VN
ZI
Q2
INP
IE
Q4
IQ1
IPN
C P1
A1
CD
A2
CP2
ICQ + IO
V O´
A3
Z2
A2
CD
AD8004
RF
RG
Figure 30. Simplified Block Diagram
VO
RL
CL
REV. B
–9–