OP27
Noise performance of this circuit is limited more by the Input
+
0.47µF
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.
OP27
–
TAPE
HEAD
R
C
A
A
15kΩ
R1
33kΩ
R2
5kΩ
0.01µF
10Ω
T1 = 3180µs
T2 = 50µs
C1
5mF
R1
R3
R6
1kΩ
316kΩ
100Ω
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).
LOW IMPEDANCE
MICROPHONE INPUT
(Z = 50Ω TO 200Ω)
R7
10kΩ
R
OP27/
P
OUTPUT
30kΩ OP37
+
R2
1kΩ
R4
R3 R4
=
316kΩ
R1 R2
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.
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.
C2
1800pF
R1
R2
121Ω
1100Ω
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.
2
3
A1
OP27
6
1
OUTPUT
T1
150Ω
SOURCE
R3
100Ω
1
T1 – JENSEN JE – 115K – E
JENSEN TRANSFORMERS
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.
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.
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.
+18V
8
2
7
6
OP27
3
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.
4
–18V
Figure 45. Burn-In Circuit
Rev. F | Page 17 of 20
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