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Design How-To

Op amps in small-signal audio design
- Part 2: Distortion in bipolar and JFET input op-amps

Douglas
Self

6/22/2011 05:08 PM EDT
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[Part
1 reviews a brief history of op amps and then
looks at various op amp properties from a perspective of audio
design.]

Bipolar input op-amps
Figure 4.4
shows the distortion from a 5532 working in shunt mode with
low-value resistors of 1 kΩ and 2k2 setting a gain of 2.2 times, at
an output level of 5 Vrms. This is the circuit of Figure 4.3(a) with
RS set to zero; there is no CM voltage. The
distortion is well below 0.0005% up to 20 kHz; this underlines what
a superlative bargain the 5532 is.

Figure 4.3: Op-amp test circuits with
added source resistance RS. (a) Shunt. (b)
Series. (c) Voltage-follower. (d) Voltage-follower with
cancellation resistor in feedback path

Figure 4.4: 5532 distortion in a
shunt-feedback circuit at 5 Vrms out. This shows the AP SYS- 2702
output (lower trace) and the op-amp output (upper trace). Supply
±18 V

Figure 4.5 shows the same situation but with the output increased
to 10 Vrms (the clipping level on ±18 V rails is about 12 Vrms) and
there is now significant distortion above 10 kHz, though it only
exceeds 0.001% at 18 kHz.

Figure 4.5: 5532 distortion in the
shunt-feedback circuit of Figure 4.3(b). Adding extra resistances
of 10 kΩ and 47 kΩ in series with the inverting input does not
degrade the distortion at all, but does bring up the noise floor a
bit. Test level 10 Vrms out, supply ±18 V

This remains the case when RS in Figure
4.3(a) is increased to 10 kΩ and 47 kΩ – the noise floor is higher
but there is no real change in the audio-band distortion behavior.
The significance of this will be seen in a moment.

We will now connect the 5532 in the series-feedback
configuration, as in Figure 4.3(b); note that the stage gain is
greater at 3.2 times but the op-amp is working at the same noise
gain. The CM voltage is 3.1 Vrms. With a 10 Vrms output we can see
in Figure 4.6 that even with no added source resistance the
distortion starts to rise from 2 kHz, though it does not exceed
0.001% until 12 kHz.

Figure 4.6: 5532 distortion in a
series-feedback stage with 2k2 and 1k feedback resistors, and
varying source resistances. Output 10 Vrms

But when we add some source resistance RS,
the picture is radically worse, with serious mid-band distortion
rising at 6 dB/octave, and roughly proportional to the amount of
resistance added. We will note it is 0.0085% at 10 kHz with
RS = 47 kΩ.

The worst case for CM distortion is the voltage-follower
configuration, as in Figure 4.3(c), where the CM voltage is equal to
the output voltage. Figure 4.7 shows that even with a CM voltage of
10 Vrms, the distortion is no greater than for the shunt mode.
However, when source resistance is inserted in series with the
input, the distortion mixture of second, third, and other low-order
harmonics increases markedly. It increases with output level,
approximately quadrupling as level doubles. The THD is now 0.018% at
10 kHz with RS = 47 kΩ, more than twice that of
the series-feedback amplifier above, due to the increased CM
voltage.

Figure 4.7: 5532 distortion in a
voltage-follower circuit with a selection of source resistances.
Test level 10 Vrms, supply ±18 V. The lowest trace is the analyzer
output measured directly, as a reference

It would be highly inconvenient to have to stick to the
shunt-feedback mode, because of the phase inversion and relatively
low input impedance that comes with it, so we need to find out how
much source resistance we can live with. Figure 4.8 zooms in on the
situation with resistance of 10 kΩ and below; when the source
resistance is below 2k2, the distortion is barely distinguishable
from the zero source resistance trace. This is why the low-pass
Sallen-and-Key filters in Chapter 5 have been given series resistors
that do not in total exceed this figure.

Figure 4.8: A closer look at 5532
distortion in a voltage-follower with relatively low source
resistances; note that a 1 kΩ source resistance actually gives
less distortion than none. Test level 10 Vrms, supply ±18
V

Close examination reveals the intriguing fact that a 1 kΩ source
actually gives less distortion than no source resistance at all,
reducing THD from 0.00065% to 0.00055% at 10 kHz. Minor resistance
variations around 1 kΩ make no difference. This must be due to the
cancellation of distortion from two different mechanisms. It is hard
to say whether it is repeatable enough to be exploited in
practice.

So, what's going on here? Is it simply due to non-linear currents
being drawn by the op-amp inputs? Audio power amplifiers have
discrete input stages that are very simple compared with those of
most op-amps, and draw relatively large input currents. These
currents show appreciable non-linearity even when the output voltage
of the amplifier is virtually distortion free, and, if they flow
through significant source resistances, will introduce added
distortion [4].

If this was the case with the 5532 then the extra distortion
would manifest itself whenever the op-amp was fed from a significant
source resistance, no matter what the circuit configuration. But we
have just seen that it only occurs in series-feedback situations;
increasing the source resistance in a shunt-feedback does not
perceptibly increase distortion. The effect may be present but if so
it is very small, no doubt because op-amp signal input currents are
also very small, and it is lost in the noise.

The only difference is that the series circuit has a CM voltage
of about 3 Vrms, while the shunt circuit does not, and the
conclusion is that with a bipolar input op-amp you must have both a
CM voltage and a significant source resistance to see extra
distortion. The input stage of a 5532 is a straightforward
long-tailed pair (see Figure 4.21 below) with a simple tail-current
source, and no fancy cascoding, and I suspect that the Early effect
operates on it when there is a large CM voltage, modulating the
quite high input bias currents, and this is what causes the
distortion. The signal input currents are much smaller, due to the
high open-loop gain of the op-amp, and as we have seen appear to
have a negligible effect.

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re: Op amps in small-signal audio design - Part
2: Distortion in bipolar and JFET input op-amps
Kinnar
6/24/2011 6:25:02 AM

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This
article give a very good insight for an audio designers. Generally
what the author has cover here is getting missed by most of the
designers. All the design parameters affecting the audio per
amplifier design is nice covered in this
article.

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