REV. C

OP270

–11–

CAPACITIVE LOAD DRIVING AND POWER SUPPLY

CONSIDERATIONS

The OP270 is unity-gain stable and capable of driving large

capacitive loads without oscillating. Nonetheless, good supply

bypassing is highly recommended. Proper supply bypassing

reduces problems caused by supply line noise and improves the

capacitive load driving capability of the OP270.

In the standard feedback amplifier, the op amp’s output resis-

tance combines with the load capacitance to form a low-pass

filter that adds phase shift in the feedback network and reduces

stability. A simple circuit to eliminate this effect is shown in

Figure 10. The added components, C1 and R3, decouple the

amplifier from the load capacitance and provide additional

stability. The values of C1 and R3 shown in Figure 10 are for a

load capacitance of up to 1,000 pF when used with the OP270.

OP270

–

+

R1

R3

50

C1

200pF

R2

C1

1000pF

PLACE SUPPLY DECOUPLING

CAPACITOR AT OP270

C5

0.1 F

C4

10 F

+

V–

C3

0.1 F

C2

10 F

+

V+

Figure 10. Driving Large Capacitive Loads

UNITY-GAIN BUFFER APPLICATIONS

£ 100 W and the input is driven with a fast, large

signal pulse (>1 V), the output waveform will look like the one

in Figure 11.

During the fast feedthrough-like portion of the output, the input

protection diodes effectively short the output to the input, and a

current, limited only by the output short-circuit protection, will be

≥ 500 W, the output is

£ 20 mA at 10 V);

the amplifier will stay in its active mode and a smooth transition

will occur.

capacitance (3 pF) creates additional phase shift and reduces

phase margin. A small capacitor (20 pF to 50 pF) in parallel

Figure 11. Pulsed Operation

APPLICATIONS

Low Phase Error Amplifier

The simple amplifier depicted in Figure 12 utilizes a monolithic

dual operational amplifier and a few resistors to substantially

reduce phase error compared to conventional amplifier designs.

At a given gain, the frequency range for a specified phase accuracy is

over a decade greater than for a standard single op amp amplifier.

The low phase error amplifier performs second-order frequency

compensation through the response of op amp A2 in the feed-

back loop of A1. Both op amps must be extremely well matched

in frequency response. At low frequencies, the A1 feedback

tor divider around A2. Note that like a conventional single op amp

amplifier, the dc gain is set by resistor ratios only. Minimum

gain for the low phase error amplifier is 10.

1/2

OP270E

A2

–

+

1/2

OP270E

A1

–

+

R2

K1

R2

R2 = R1

R2

R1

K1

ASSUME A1 AND A1 ARE MATCHED.

T

s

Figure 12. Low Phase Error Amplifier

Figure 13 compares the phase error performance of the low

phase error amplifier with a conventional single op amp ampli-

fier and a cascaded two-stage amplifier. The low phase error

amplifier shows a much lower phase error, particularly for fre-

quencies where

occurs at 0.002

0.11