OP27
Therefore, for low-frequency applications, the OP07 is better
than the OP27/OP37 when R
S
> 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.
For reference, typical source resistances of some signal sources
are listed in Table I.
Table I.
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 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.)
C4 (2)
220 F
+
+
MOVING MAGNET
CARTRIDGE INPUT
Ca
150pF
LF ROLLOFF
OUT
R5
100k
Device
Strain Gauge
Magnetic
Tapehead
Source
Impedance
<500
Ω
<1500
Ω
Comments
Typically used in low-
frequency applications.
Low is very important to
reduce self-magnetization
problems when direct coupling
is used. OP27 I
B
can be
neglected.
Similar need for low I
B
in
direct coupled applications.
OP27 will not introduce any
self-magnetization problem.
Used in rugged servo-feedback
applications. Bandwidth of
interest is 400 Hz to 5 kHz.
Magnetic
Phonograph
Cartridges
<1500
Ω
Ra
47.5k
A1
OP27
C3
0.47 F
IN
R1
97.6k
R2
7.87k
R3
100
R4
75k
C1
0.03 F
C2
0.01 F
OUTPUT
Linear Variable <1500
Ω
Differential
Transformer
Open-Loop Gain
Frequency at
3 Hz
10 Hz
30 Hz
OP07
100 dB
100 dB
90 dB
OP27
124 dB
120 dB
110 dB
OP37
125 dB
125 dB
124 dB
Figure 7.
G = 1kHz GAIN
R1
= 0.101 ( 1 +
)
R3
= 98.677 (39.9dB) AS SHOWN
For further information regarding noise calculations, see “Minimization of Noise
in Op Amp Applications,” Application Note AN-15.
100
50
TOTAL NOISE – nV/ Hz
1
2
OP08/108
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.
Gain (G) of the circuit at 1 kHz can be calculated by the
expression:
10
OP07
5534
R
1
G
=
0.101
1
+
R
3
1 R
S
e.g. R
S
2 R
S
e.g. R
S
UNMATCHED
= R
S1
= 10k , R
S2
= 0
MATCHED
= 10k , R
S1
= R
S2
= 5k
R
S1
5
OP27/37
REGISTER
NOISE ONLY
1
50
100
R
S2
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.
50k
10k
500
1k
5k
R
S
– SOURCE RESISTANCE –
Figure 6. 10 Hz Noise vs. Source Resistance (Includes
Resistor Noise)
AUDIO APPLICATIONS
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.
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
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–
AD [ ANALOG DEVICES ]
