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

[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 R_S 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 R_S. (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 R_S 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 R_S, 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 R_S = 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 R_S = 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.