AD825

Data Sheet

Rev. G | Page 10 of 12

DRIVING CAPACITIVE LOADS

The internal compensation of the AD825, together with its high

output current drive, permits excellent large signal performance

while driving extremely high capacitive loads.

Figure 30. Inverting Amplifier Driving a Capacitive Load

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 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.

Figure 32. Simplified Schematic

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 CONSIDERATION

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.

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 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.

TEKTRONIX

P6204 FET

PROBE

HP PULSE

GENERATOR

50

Ω

RIN

1k

Ω

CL

VOUT

VIN

TEKTRONIX

7A24

PREAMP

AD825

7

4

3

6

2

+VS

0.01

µF

10

µF

–VS

0.01

µF

10

µF

1k

Ω

INPUT

OUTPUT

5V

5V

500ns

POS

NEG

VNEG

VPOS

VOUT

CF