AD825
DRIVING CAPACITIVE LOADS
VPOS
The internal compensation of the AD825, together with its high
output current drive, permits excellent large signal performance
while driving extremely high capacitive loads.
NEG
POS
1kΩ
+V
10µF
S
0.01µF
C
F
VOUT
7
V
IN
HP PULSE
GENERATOR
2
3
TEKTRONIX
P6204 FET
PROBE
TEKTRONIX
7A24
PREAMP
R
IN
1kΩ
6
AD825
4
V
OUT
0.01µF
10µF
C
L
50Ω
–V
S
VNEG
Figure 30. Inverting Amplifier Driving a Capacitive Load
Figure 32. Simplified Schematic
5V
500ns
The capacitor, CF, in the output stage, enables the AD825 to
drive heavy capacitive loads. For light loads, the gain of the
output buffer is close to unity, CF 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 CF adding to the dominant pole.
As the capacitive load is further increased, the amplifier’s
bandwidth continues to drop, maintaining the stability of the
AD825.
INPUT
OUTPUT
INPUT CONSIDERATION
5V
The AD825 with its unique input stage ensures no phase
reversal for signals as large as or even larger than the supply
voltages. Also, layout considerations of the input transistors
ensure functionality even with a large differential signal.
Figure 31. Inverting Amplifier Pulse Response
While Driving a 400 pF Capacitive Load
THEORY OF OPERATION
The AD825 is a low cost, wideband, high performance FET
input operational amplifier. With its unique input stage design,
the AD825 ensures no phase reversal, even for inputs that
exceed the power supply voltages, and its output stage is
designed to drive heavy capacitive or resistive loads with small
changes relative to no load conditions.
The need for a low noise input stage calls for a larger FET
transistor. One should consider the additional capacitance that
is added to ensure stability. When filters are designed with the
AD825, one needs to consider the input capacitance (5 pF to
6 pF) of the AD825 as part of the passive network.
GROUNDING AND BYPASSING
The AD825 (Figure 32) 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 AD825 is a low input bias current FET amplifier. Its high
frequency response makes it useful in applications, such as
photodiode interfaces, filters, and audio circuits. When
designing high frequency circuits, some special precautions are
in order. Circuits 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 amplifier pins. Ceramic capacitors of 0.1 µF are
recommended.
Rev. F | Page 10 of 12
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