AD825
1k
10 F
DRIVING CAPACITIVE LOADS
+V
S
The internal compensation of the AD825, together with its high
output current drive, permits excellent large signal performance
while driving extremely high capacitive loads.
THEORY OF OPERATION
0.01 F
HP
PULSE
GENERATOR
R
IN
V
IN
1k
50
V
OUT
AD825
0.01 F
TEKTRONIX
P6204 FET
PROBE
TEKTRONIX
7A24
PREAMP
C
L
10 F
–V
S
The AD825 is a low cost, wide band, high performance FET
input operational amplifier. With its unique input stage design,
the AD825 assures no phase reversal even for inputs that exceed
the power supply voltages, and its output stage is designed to
drive heavy capacitive or resistive load with small changes rela-
tive to no load condition.
The AD825 (Figure 27) consists of common-drain common-
base FET input stage driving a cascoded, common base matched
NPN gain stage. The output buffer stage uses emitter followers
in a class AB amplifier that can deliver large current to the load
while maintaining low levels of distortion.
The capacitor, C
F
, in the output stage, enables the AD825 to
drive heavy capacitive load. For light load, the gain of the out-
put buffer is close to unity, C
F
is bootstrapped and not much
happens. As the capacitive load is increased, the gain of the
output buffer is decreased and the bandwidth of the amplifier is
reduced through a portion of C
F
adding to the dominant pole.
As the capacitive load is further increased, the amplifier’s band-
width continues to drop, maintaining the stability of the AD825.
Input Consideration
Figure 26a. Inverting Amplifier Driving a Capacitive Load
INPUT
OUTPUT
Figure 26b. Inverting Amplifier Pulse Response
While Driving a 400 pF Capacitive Loads
VPOS
The AD825 with its unique input stage assures no phase rever-
sal for signals as large or even larger than the supply voltages.
Also, layout considerations of the input transistors assure func-
tionality even with a large differential signal.
The need for a low noise input stage calls for a larger FET transis-
tor. One should consider the additional capacitance that is added
to assure stability. When filters are designed with the AD825,
one needs to consider the input capacitance (5 pF–6 pF) of the
AD825 as part of the passive network.
Grounding and Bypassing
NEG
POS
C
F
VOUT
The AD825 is a low input bias current FET amplifier. Its high
frequency response makes it useful in applications such as photo
diode interfaces, filters and audio circuits. When designing high
frequency circuits, some special precautions are in order. Cir-
cuits must be built with short interconnects, and resistances
should have low inductive paths to ground. Power supply leads
should be bypassed to common as close as possible to the ampli-
fier pins. Ceramic capacitors of 0.1
µF
are recommended.
VNEG
Figure 27. Simplified Schematic
REV. C
–9–
AD [ ANALOG DEVICES ]
