OP27
Therefore, for low-frequency applications, the OP07 is better
than the OP27/OP37 when RS > 3 kΩ. The only exception is
when gain error is important. Figure 6 illustrates the 10 Hz
noise. As expected, the results are between the previous two
figures.
Figure 7 is an example of a phono pre-amplifier circuit using the
OP27 for A1; R1-R2-C1-C2 form a very accurate RIAA net-
work with standard component values. The popular method to
accomplish RIAA phono equalization is to employ frequency-
dependent feedback around a high-quality gain block. Properly
chosen, an RC network can provide the three necessary time
constants of 3180, 318, and 75 µs.1
For reference, typical source resistances of some signal sources
are listed in Table I.
For initial equalization accuracy and stability, precision metal
film resistors and film capacitors of polystyrene or polypropy-
lene are recommended since they have low voltage coefficients,
dissipation factors, and dielectric absorption.4 (High-K ceramic
capacitors should be avoided here, though low-K ceramics—
such as NPO types, which have excellent dissipation factors
and somewhat lower dielectric absorption—can be considered
for small values.)
Table I.
Source
Device
Impedance
Comments
Strain Gauge
<500 Ω
Typically used in low-
frequency applications.
Magnetic
Tapehead
<1500 Ω
<1500 Ω
Low is very important to
reduce self-magnetization
problems when direct coupling
is used. OP27 IB can be
neglected.
C4 (2)
R5
220ꢀF
100kꢅ
+
+
MOVING MAGNET
CARTRIDGE INPUT
LF ROLLOFF
OUT
C3
0.47ꢀF
IN
Magnetic
Phonograph
Cartridges
Similar need for low IB in
direct coupled applications.
OP27 will not introduce any
self-magnetization problem.
A1
OP27
Ca
150pF
Ra
R4
75kꢅ
C1
0.03ꢀF
47.5kꢅ
OUTPUT
R1
97.6kꢅ
Linear Variable <1500 Ω
Differential
Transformer
Used in rugged servo-feedback
applications. Bandwidth of
interest is 400 Hz to 5 kHz.
R2
C2
0.01ꢀF
7.87kꢅ
R3
100ꢅ
G = 1kHz GAIN
Open-Loop Gain
R1
R3
1 +
= 0.101 (
)
Frequency at
OP07
OP27
OP37
= 98.677 (39.9dB) AS SHOWN
3 Hz
10 Hz
30 Hz
100 dB
100 dB
90 dB
124 dB
120 dB
110 dB
125 dB
125 dB
124 dB
Figure 7.
The OP27 brings a 3.2 nV/√Hz voltage noise and 0.45 pA/√Hz
current noise to this circuit. To minimize noise from other
sources, R3 is set to a value of 100 Ω, which generates a voltage
noise of 1.3 nV/√Hz. The noise increases the 3.2 nV/√Hz of the
amplifier by only 0.7 dB. With a 1 kΩ source, the circuit noise
measures 63 dB below a 1 mV reference level, unweighted, in a
20 kHz noise bandwidth.
For further information regarding noise calculations, see “Minimization of Noise
in Op Amp Applications,” Application Note AN-15.
100
50
1
2
Gain (G) of the circuit at 1 kHz can be calculated by the
OP08/108
expression:
R1
OP07
10
G = 0.101 1+
R3
5534
1 RS UNMATCHED
e.g.RS = RS1 = 10kꢅ, RS2 = 0
2 RS MATCHED
For the values shown, the gain is just under 100 (or 40 dB).
Lower gains can be accommodated by increasing R3, but gains
higher than 40 dB will show more equalization errors because of
the 8 MHz gain-bandwidth of the OP27.
5
e.g.RS = 10kꢅ, RS1 = RS2 = 5kꢅ
OP27/37
RS1
RS2
REGISTER
NOISE ONLY
This circuit is capable of very low distortion over its entire range,
generally below 0.01% at levels up to 7 V rms. At 3 V output
levels, it will produce less than 0.03% total harmonic distortion
at frequencies up to 20 kHz.
1
50
100
500
1k
5k
10k
50k
R
– SOURCE RESISTANCE –ꢅ
S
Figure 6. 10 Hz Noise vs. Source Resistance (Includes
Resistor Noise)
Capacitor C3 and resistor R4 form a simple –6 dB-per-octave
rumble filter, with a corner at 22 Hz. As an option, the switch-
selected shunt capacitor C4, a nonpolarized electrolytic, bypasses
the low-frequency rolloff. Placing the rumble filter’s high-pass
action after the preamp has the desirable result of discriminating
AUDIO APPLICATIONS
The following applications information has been abstracted
from a PMI article in the 12/20/80 issue of Electronic De-
sign magazine and updated.
REV. A
–13–
ADI [ ADI ]
