OP270 Data Sheet
Rev. F | Page 16 of 20
LOW PHASE ERROR AMPLIFIER
The simple amplifier depicted in Figure 41 utilizes a monolithic
dual operational amplifier and a few resistors to substantially
reduce phase error compared with conventional amplifier
designs. At a given gain, the frequency range for a specified
phase accuracy is more than a decade greater than that of a
standard single op amp amplifier.
The low phase error amplifier performs second-order fre-
quency 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
feedback loop forces V
2
/(K1 + 1) = V
IN
. The A2 feedback loop
forces V
O
/(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, 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.
00325-042
1/2
OP270E
A2
1/2
OP270E
A1
R1
V
O
V
2
V
IN
R1
K1
R2 R2 = R1
ω
T
s
R2
K2
V
O
= (K
1
+ 1)V
IN
ASSUME A1 AND A2 ARE MATCHED.
A
O
(s) =
Figure 41. Low Phase Error Amplifier
Figure 42 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 ω/βω
T
< 0.1. For example, a phase error of
−0.1° occurs at 0.002 ω/βω
T
for the single op amp amplifier, but
at 0.11 ω/βω
T
for the low phase error amplifier.
00352-043
–7
–6
–5
–4
–3
–2
–1
0
PHASE SHIFT (Degrees)
FREQUENCY RATIO (1/βω)(ω/ω
T
)
0.001
0.005
0.01 0.1 1
LOW PHASE ERROR
AMPLIFIER
CASCADED
(TWO STAGES)
SINGLE OP AMP.
CONVENTIONAL DESIGN
0.05 0.5
Figure 42. Phase Error Comparison
FIVE-BAND, LOW NOISE, STEREO GRAPHIC
EQUALIZER
The graphic equalizer circuit shown in Figure 43 provides 15 dB
of boost or cut over a five-band range. Signal-to-noise ratio over
a 20 kHz bandwidth is better than 100 dB and referred to a 3 V
rms input. Larger inductors can be replaced by active inductors,
but consequently reduces the signal-to-noise ratio.
00325-044
1/2
OP270E
1/2
OP270E
R2
3.3kΩ
R1
47kΩ
R4
1kΩ
60Hz
TANTALUM
V
OUT
V
IN
R14
100Ω
R13
3.3kΩ
C2
6.8µF
L1
1H
C1
0.47µF
R3
680Ω
200Hz
R6
1kΩ
800Hz
R8
1kΩ
3kHz
R10
1kΩ
10kHz
R12
1kΩ
+
TANTALUM
C3
1µF
L2
600mH
R5
680Ω
+
C4
0.22µF
L3
180mH
R7
680Ω
+
C5
0.047µF
L4
60mH
R9
680Ω
+
C6
0.022µF
L5
10mH
R11
680Ω
+
Figure 43. Five-Band, Low Noise Graphic Equalizer
