AD825

–9–

REV. C

TEKTRONIX

P6204 FET

PROBE

HP

PULSE

GENERATOR

50

0.01 F

10 F

TEKTRONIX

7A24

PREAMP

0.01 F

10 F

AD825

1k

1k

Figure 26a. Inverting Amplifier Driving a Capacitive Load

Figure 26b. Inverting Amplifier Pulse Response

While Driving a 400 pF Capacitive Loads

VNEG

VOUT

VPOS

POS

NEG

Figure 27. Simplified Schematic

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.

THEORY OF OPERATION

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.

drive heavy capacitive load. For light load, the gain of the out-

happens. As the capacitive load is increased, the gain of the

output buffer is decreased and the bandwidth of the amplifier is

As the capacitive load is further increased, the amplifier’s band-

width continues to drop, maintaining the stability of the AD825.

Input Consideration

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

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.

INPUT

OUTPUT