OP27GSZ Datasheet OP27GSZ, OP27GSZ-REEL, OP27GSZ-REEL7 | P16-P18

Data Sheet OP27

Rev. H | Page 15 of 21

COMMENTS ON NOISE

The OP27 is a very low noise, monolithic op amp. The out-

standing input voltage noise characteristics of the

OP27

are achieved mainly by operating the input stage at a high

quiescent current. The input bias and offset currents, which

would normally increase, are held to reasonable values by the

input bias current cancellation circuit. The OP27A/OP27E has

and I

of only ±40 nA and 35 nA at 25°C respectively. This

is particularly important when the input has a high source

resistance. In addition, many audio amplifier designers prefer

to use direct coupling. The high I

, V

, and TCV

of previous

designs have made direct coupling difficult, if not impossible,

to use.

Voltage noise is inversely proportional to the square root of bias

current, but current noise is proportional to the square root of

bias current. The noise advantage of the

OP27 disappears when

high source resistors are used. Figure 38, Figure 39, Figure 40

compare the observed total noise of the

OP27 with the noise

performance of other devices in different circuit applications.

2/1

)(

)

(

u



NoiseResistor

RNoiseCurrent

NoiseVoltage

NoiseTotal

Figure 38 shows noise vs. source resistance at 1000 Hz. The

same plot applies to wideband noise. To use this plot, multiply

the vertical scale by the square root of the bandwidth.

—SOURCE RESISTANCE (

)

50 10k

500 1k 5k

100

100 50k

OP07

5534

OP27/37

NOISE ONLY

OP08/108

1 R

UNMATCHED

e.g. R

= R

= 10k

, R

= 0

2 R

MATCHED

e.g. R

= 10k

, R

= R

= 5k

00317-038

TOTAL NOIS

E (nV/



Hz)

Figure 38. Noise vs. Source Resistance (Including Resistor Noise) at 1000 Hz

At R

< 1 kΩ, the low voltage noise of the OP27 is maintained.

With R

< 1 kΩ, total noise increases but is dominated by the

resistor noise rather than current or voltage noise. lt is only

beyond R

of 20 kΩ that current noise starts to dominate. The

argument can be made that current noise is not important for

applications with low-to-moderate source resistances. The

crossover between the

OP27 and OP07 noise occurs in the 15 kΩ

to 40 kΩ region.

Figure 39 shows the 0.1 Hz to 10 Hz p-p noise. Here the picture

is less favorable; resistor noise is negligible and current noise

becomes important because it is inversely proportional to the

square root of frequency. The crossover with the

OP07 occurs

in the 3 kΩ to 5 kΩ range depending on whether balanced or

unbalanced source resistors are used (at 3 kΩ the I

and I

error also can be 3× the V

spec).

—SOURCE RESISTANCE (

)

100

10k

p-p NOISE (nV)

500 1k

500

100 50k

1 R

UNMATCHED

e.g. R

= R

= 10k

, R

= 0

2 R

MATCHED

e.g. R

= 10k

, R

= R

= 5k

OP07

5534

OP27/37

NOISE ONLY

OP08/108

00317-039

Figure 39. Peak-to-Peak Noise (0.1 Hz to 10 Hz) as Source Resistance

(Includes Resistor Noise)

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 40 illustrates the 10 Hz noise. As expected, the results are

between the previous two figures.

10k

500 1k 5k

100

100 50k

OP07

5534

OP27/37

NOISE ONLY

OP08/108

1 R

UNMATCHED

e.g. R

= R

= 10k

, R

= 0

2 R

TCHED

e.g. R

= 10k

, R

= R

= 5k

00317-040

—SOURCE RESISTANCE (

)

TOTAL NOISE (nV/



Hz)

Figure 40. 10 Hz Noise vs. Source Resistance (Includes Resistor Noise)

Audio Applications

OP27 Data Sheet

Rev. H | Page 16 of 21

For reference, typical source resistances of some signal sources

are listed in Table 7.

Table 7.

Device

Source

Impedance Comments

Strain Gauge <500 Ω Typically used in low frequency

applications.

Magnetic

Tape Head

<1500 Ω Low is very important to reduce

self-magnetization problems

when direct coupling is used.

OP27 IB can be neglected.

Magnetic

Phonograph

Cartridges

<1500 Ω

Similar need for low I

in direct

coupled applications. OP27 does

not introduce any self-

magnetization problems.

Linear

Variable

Differential

Transformer

<1500 Ω Used in rugged servo-feedback

applications. Bandwidth of

interest is 400 Hz to 5 kHz.

Table 8. Open-Loop Gain

Frequency OP07 OP27 OP37

At 3 Hz 100 dB 124 dB 125 dB

At 10 Hz 100 dB 120 dB 125 dB

At 30 Hz 90 dB 110 dB 124 dB

AUDIO APPLICATIONS

Figure 41 is an example of a phono pre-amplifier circuit using the

OP27 for A1; R1-R2-C1-C2 form a very accurate RIAA network

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 µs, 318 µs, and 75 µs.

For initial equalization accuracy and stability, precision metal

film resistors and film capacitors of polystyrene or polypro-

pylene are recommended because they have low voltage

coefficients, dissipation factors, and dielectric absorption.

(High-k ceramic capacitors should be avoided here, though

low-k ceramics, such as NPO types that have excellent

dissipation factors and somewhat lower dielectric absorption,

can be considered for small values.)

150pF

OP27

N

MOVING MAGNET

CARTRIDGE INPUT

+ +

C4 (2)

220µF

0.03µF

0.01µF

0.47µF

LF ROLLOFF

OUT IN

OUTPUT

N

N

N

N



G = 1kHz GAIN

= 0.101 ( 1 + )

= 98.677 (39.9dB)

AS SHOWN

00317-041

Figure 41. Phono Preamplifier Circuit

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, generating 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:



G 1101.0

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 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 produces 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 roll-off. Placing the rumble filter’s

high-pass action after the preamplifier has the desirable result

of discriminating against the RIAA-amplified low frequency

noise components and pickup produced low frequency

disturbances.

A preamplifier for NAB tape playback is similar to an RIAA

phono preamplifier, though more gain is typically demanded,

along with equalization requiring a heavy low frequency boost.

The circuit in Figure 41 can be readily modified for tape use, as

shown by Figure 42.

Data Sheet OP27

Rev. H | Page 17 of 21

N

N

TAPE

HEAD

0.47µF

0.01µF



N

T1 = 3180µs

T2 = 50µs

OP27

–

00317-042

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, R

, 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

=  T2

5mF

N

N



N

N

N

OUTPUT

OP27/

OP37

–

N

00317-043

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





1

1800pF

OUTPUT



SOURCE

T1 – JENSEN JE – 115K – E

00317-044

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

00317-045

Figure 45. Burn-In Circuit

Capacitor C2 and Resistor R2 form a 2 µs time constant in this

circuit, as recommended for optimum transient response by the

transformer manufacturer. With C2 in use, A1 must have unity-

gain stability. For situations where the 2 µs time constant is not

necessary, C2 can be deleted, allowing the faster

OP37 to be

employed.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P18 P19-P21 P22-P22

OP27GSZ

Mfr. #:

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Precision Amplifiers LOW-NOISE PREC IC 2.8V/uS

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