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
forces V
2
/(K1 + 1) = V
IN
. The A2 feedback loop forces Vo/(K1 +1)
= V
2
/(K1 + 1) yielding an overall transfer function of V
O
/V
IN
=
K1 + 1. The dc gain is determined by the resistor divider at
the output, V
O
, and is not directly affected by the resistor divider
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
/
T
< 0.1. For example, phase error of –0.1∞ occurs at 0.002 /
T
for the single op amp amplifier, but at 0.11 /
T
for the low
phase error amplifier.
For more detailed information on the low phase error amplifier,
see Application Note AN-107.
R2
1/4
OP471E
A2
R1
V
O
= (K
1
+ 1) V
IN
1/4
OP471E
A1
R1
K1
V
O
R2
K1
V
IN
R2 = R1
ASSUME: A1 AND A2 ARE MATCHED.
A
O
(s) =
T
s
V2
Figure 18. Low Phase Error Amplifier
FREQUENCY RATIO – 1/, /
T
0
0.001
PHASE SHIFT – Degrees
–1
–2
–3
–4
–5
0.01 0.1 10.005 0.05 0.5
–6
–7
SINGLE OP AMP
(CONVENTIONAL
DESIGN)
CASCADED
(TWO STAGES)
LOW-PHASE ERROR
AMPLIFIER
Figure 19. Phase Error Comparison
