OP270GS datasheet(11/16 Pages) AD

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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

–

VIN

200pF

VOUT

1000pF

PLACE SUPPLY DECOUPLING

CAPACITOR AT OP270

0.1 F

10 F

V–

0.1 F

10 F

V+

Figure 10. Driving Large Capacitive Loads

UNITY-GAIN BUFFER APPLICATIONS

When Rf

£ 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

drawn by the signal generator. With Rf

≥ 500 W, the output is

capable of handling the current requirements (IL

£ 20 mA at 10 V);

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

will occur.

When Rf > 3 k

W, a pole created by Rf and the amplifier’s input

capacitance (3 pF) creates additional phase shift and reduces

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

with Rf helps eliminate this problem.

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

loop forces V2 /(K1 + 1) = VIN. The A2 feedback loop forces

Vo/(K1 + 1) = V2 /(K1 + 1), yielding an overall transfer function

of VO/VIN = K1 + 1. The dc gain is determined by the resistor

divider at the output, VO, and is not directly affected by the resis-

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

–

1/2

OP270E

–

R2 = R1

VO = (K1 + 1) V IN

ASSUME A1 AND A1 ARE MATCHED.

AO(s) =

VIN

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

w/bwT < 0.1. For example, phase error of –0.1∞

occurs at 0.002

w/bwT for the single op amp amplifier, but at

0.11

w/bwT for the low phase error amplifier.

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