REV. A

OP471

–13–

Low Phase Error Amplifier

The simple amplifier depicted in Figure 18 utilizes monolithic

matched operational amplifiers and a few resistors to substan-

tially 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

K1 + 1. The dc gain is determined by the resistor divider at

around A2. Note that similar to 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 19 compares the phase error performance of the low

phase error amplifier with a conventional single op amp amplifier

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

shows a much lower phase error, particularly for frequencies where

/

for the single op amp amplifier, but at 0.11 /

phase error amplifier.

For more detailed information on the low phase error amplifier,

see Application Note AN-107.

R2

1/4

OP471E

A2

R1

1/4

OP471E

A1

R1

K1

R2

K1

R2 = R1

ASSUME: A1 AND A2 ARE MATCHED.

T

s

V2

Figure 18. Low Phase Error Amplifier

FREQUENCY RATIO – 1/ ,

0

0.001

–1

–2

–3

–4

–5

0.01

0.1

1

0.005

0.05

0.5

–6

–7

SINGLE OP AMP

(CONVENTIONAL

DESIGN)

CASCADED

(TWO STAGES)

LOW-PHASE ERROR

AMPLIFIER

Figure 19. Phase Error Comparison