REV. A

OP27

–13–

Therefore, for low-frequency applications, the OP07 is better

Ω. 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.

Source

Device

Impedance

Comments

Strain Gauge

<500

Ω

Typically used in low-

frequency applications.

Magnetic

<1500

Ω

Low is very important to

Tapehead

reduce self-magnetization

problems when direct coupling

neglected.

Magnetic

<1500

Ω

Phonograph

direct coupled applications.

Cartridges

OP27 will not introduce any

self-magnetization problem.

Linear Variable

<1500

Ω

Used in rugged servo-feedback

Differential

applications. Bandwidth of

Transformer

interest is 400 Hz to 5 kHz.

Open-Loop Gain

Frequency at

OP07

OP27

OP37

3 Hz

100 dB

124 dB

125 dB

10 Hz

100 dB

120 dB

125 dB

30 Hz

90 dB

110 dB

124 dB

For further information regarding noise calculations, see “Minimization of Noise

in Op Amp Applications,” Application Note AN-15.

10

50

10k

5

500

1k

5k

1

100

50

100

50k

OP07

5534

OP27/37

REGISTER

NOISE ONLY

OP08/108

1

2

Figure 6. 10 Hz Noise vs. Source Resistance (Includes

Resistor Noise)

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.

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

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,

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.)

Ca

150pF

A1

OP27

Ra

47.5k

R1

97.6k

MOVING MAGNET

CARTRIDGE INPUT

R2

7.87k

R3

100

C1

0.03 F

C2

0.01 F

C3

0.47 F

R4

75k

++

C4 (2)

220 F

LF ROLLOFF

OUT

IN

OUTPUT

R5

100k

G = 1kHz GAIN

= 0.101 (

)

R1

R3

1 +

= 98.677 (39.9dB) AS SHOWN

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 k

Ω 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:

G

R

R

=+









0 101 1

1

3

.

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.

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