REV. A

OP271

–8–

1/2

OP271E

A2

ASSUME: A1 AND A2 ARE MATCHED.

R2

R1

1/2

OP271E

A1

R2

K1

R2 = R1

R1

K1

OP271

R1

8.5V/ s

PLACE SUPPLY DECOUPLING

CAPACITORS AT OP271

1000pF

V+

V–

C2

10 F

C3

0.1 F

R2

R3

50

R1

+

+

C1

200pF

C4

10 F

C5

0.1 F

OP271

Figure 2. Driving Large Capacitive Loads

Figure 3. Pulsed Operation

APPLICATION INFORMATION

Capacitive Load Driving and Power Supply Considerations

The OP217 is unity-gain stable and is 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 OP271.

In the standard feedback amplifier, the op amp’s output resistance

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 2. 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 8 are for a load capacitance of up to

1000 pF when used with the OP271.

Unity-Gain Buffer Applications

100

and the input is driven with a fast, large-signal

pulse (>1 V), the output waveform will look as shown in Figure

3.

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

500

, the output is

20 mA at

10 V); the amplifier will stay in its active mode and a smooth

transition will occur.

input capacitance (3 pF) creates additional phase shift and

helps eliminate this problem.

Computer Simulations

Many electronic design and analysis programs include models

for op amps which calculate AC performance from the location

of poles and zeros. As an aid to designers utilizing such a

program, major poles and zeros of the OP271 are listed below.

Their location will vary slightly between production lots.

Typically, they will be within

15% of the frequency listed.

Use of this data will enable the designer to evaluate gross

circuit performance quickly, but should not supplant rigorous

characterization of a breadboard circuit.

POLES

ZEROS

15Hz

2.5 MHz

1.2 MHz

4 X 23 MHz

2 X 32 MHz

-

8 X 40 MHz

-

APPLICATIONS

Low Phase Error Amplifier

The simple amplifier depicted in Figure 4, 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

feedback loop of A1. Both op amps must be extremely well

matched in frequency response. At low frequencies, the A1

resistor 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.

Figure 4. Low Phase Error Amplifier