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OP27

Rev. F | Page 17 of 20

33kΩ

5kΩ

TAPE

HEAD

0.47µF

0.01µF

10Ω

15kΩ

T1 = 3180µs

T2 = 50µs

OP27

–

Figure 42. Tape Head Preamplifier

While the tape equalization requirement has a flat high

frequency gain above 3 kHz (T2 = 50 μs), the amplifier need

not be stabilized for unity gain. The decompensated OP37

provides a greater bandwidth and slew rate. For many applica-

tions, the idealized time constants shown can require trimming

of R1 and R2 to optimize frequency response for nonideal tape

head performance and other factors (see the References

section).

The network values of the configuration yield a 50 dB gain at

1 kHz, and the dc gain is greater than 70 dB. Thus, the worst-

case output offset is just over 500 mV. A single 0.47 μF output

capacitor can block this level without affecting the dynamic

range.

The tape head can be coupled directly to the amplifier input,

because the worst-case bias current of 80 nA with a 400 mH,

100 μ inch head (such as the PRB2H7K) is not troublesome.

Amplifier bias-current transients that can magnetize a head

present one potential tape head problem. The OP27 and OP37

are free of bias current transients upon power-up or power-

down. It is always advantageous to control the speed of power

supply rise and fall to eliminate transients.

In addition, the dc resistance of the head should be carefully

controlled and preferably below 1 kΩ. For this configuration,

the bias current induced offset voltage can be greater than the

100 pV maximum offset if the head resistance is not sufficiently

controlled.

A simple, but effective, fixed gain transformerless microphone

preamp (Figure 43) amplifies differential signals from low

impedance microphones by 50 dB and has an input impedance

of 2 kΩ. Because of the high working gain of the circuit, an

OP37 helps to preserve bandwidth, which is 110 kHz. As the

OP37 is a decompensated device (minimum stable gain of 5), a

dummy resistor, Rp, may be necessary if the microphone is to be

unplugged. Otherwise, the 100% feedback from the open input

can cause the amplifier to oscillate.

Common-mode input noise rejection will depend upon the

match of the bridge-resistor ratios. Either close tolerance (0.1%)

types should be used, or R4 should be trimmed for best CMRR.

All resistors should be metal film types for best stability and low

noise.

Noise performance of this circuit is limited more by the Input

Resistors R1 and R2 than by the op amp, as R1 and R2 each

generate a 4 nV/√Hz noise, while the op amp generates a

3.2 nV/√Hz noise. The rms sum of these predominant noise

sources is about 6 nV/√Hz, equivalent to 0.9 μV in a 20 kHz

noise bandwidth, or nearly 61 dB below a 1 mV input signal.

Measurements confirm this predicted performance.

LOW IMPEDANCE

MICROPHONE INPUT

(Z = 50Ω TO 200Ω)

5mF

1kΩ

316kΩ

100Ω

316kΩ

1kΩ

30kΩ

OUTPUT

OP27/

OP37

–

10kΩ

Figure 43. Fixed Gain Transformerless Microphone Preamplifier

For applications demanding appreciably lower noise, a high

quality microphone transformer coupled preamplifier (Figure

44) incorporates the internally compensated OP27. T1 is a JE-

115K-E 150 Ω/15 kΩ transformer that provides an optimum

source resistance for the OP27 device. The circuit has an overall

gain of 40 dB, the product of the transformer’s voltage setup and

the op amp’s voltage gain.

JENSEN TRANSFORMERS

OP27

100Ω

121Ω

1100Ω

1800pF

OUTPUT

150Ω

SOURCE

T11

T1 – JENSEN JE – 115K – E

Figure 44. High Quality Microphone Transformer Coupled Preamplifier

Gain can be trimmed to other levels, if desired, by adjusting R2

or R1. Because of the low offset voltage of the OP27, the output

offset of this circuit is very low, 1.7 mV or less, for a 40 dB gain.

The typical output blocking capacitor can be eliminated in such

cases, but it is desirable for higher gains to eliminate switching

transients.

OP27

–18V

+18V

Figure 45. Burn-In Circuit

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