Op amps in small-signal audio design - Part 3: Selecting the right op amp

The 5532 is a low-noise, low-distortion bipolar dual op-amp, with internal compensation for unity-gain stability. The 5534 is a single version internally compensated for gains down to 3, and an external compensation capacitor can be added for unity-gain stability; 22 pF is the usual value. The common-mode range of the inputs is a healthy ±13 V, with no phase inversion problems if this is exceeded.

The 5532 has a distinctly higher power consumption than the TL072, drawing approximately 4 mA per op-amp section when quiescent. The DIL version runs perceptibly warm when quiescent on ±17 V rails.

Figure 4.20 shows that the 5532 deals well with loads up to its maximum 500 Ω. Its distortion performance is studied in detail in the section above on common-mode distortion.

Figure 4.20: Distortion is very low from the 5532, though loading makes a detectable difference. Here it is working in series feedback mode at the high level of 10 Vrms with 500 U, 1 kΩ loads and no load. The 'Gen-mon' trace is the output of the distortion analyzer measured directly. Gain of 3.23, supply ±18 V

The 5534/5532 has bipolar transistor input devices. This means it gives low noise with low source resistances, but draws a relatively high bias current through the input pins. The input devices are NPN, so the bias currents flow into the chip from the positive rail. If an input is fed through a significant resistance then the input pin will be more negative than ground due to the voltage drop caused by the bias current.

The inputs are connected together with back-to-back diodes for reverse-voltage protection, and should not be forcibly pulled to different voltages. The 5532 is intended for linear operation, and using it as a comparator is not recommended.

As can be seen from Figure 4.20, the 5532 is almost distortion free, even when driving the maximum 500 Ω load. The internal circuitry of the 5532 has never been publicly explained, but appears to consist of nested Miller loops that permit high levels of internal negative feedback. The 5532 is the dual of the 5534, and is more commonly used than the single as it is cheaper per op-amp and does not require an external compensation capacitor when used at unity gain.

The 5532 and 5534 type op-amps require adequate supply decoupling if they are to remain stable, otherwise they appear to be subject to some sort of internal oscillation that degrades linearity without being visible on a normal oscilloscope. The essential requirement is that the positive and negative rails should be decoupled with a 100 nF capacitor between them, at a distance of not more than a few millimeters from the op-amp; normally one such capacitor is fitted per package as close to it as possible.

It is not necessary, and often not desirable, to have two capacitors going to ground; every capacitor between a supply rail and ground carries the risk of injecting rail noise into the ground.

Deconstructing the 5532
To the best of my knowledge, virtually nothing has been published about the internal operation of the 5532. This is surprising given its unique usefulness as a high-quality audio op-amp. I believe the secret of the 5532's superb linearity is the use of nested negative feedback inside the circuit, in the form of traditional Miller compensation.

Figure 4.21 shows the only diagram of the internal circuitry that has been released; the component and node numbers are mine. This has been in the public domain for at least 20 years, so I hope no one is going to object to my impertinent comments on it.

The circuit initially looks like a confusing sea of transistors, and there is even a solitary JFET lurking in there, but it breaks down fairly easily. There are three voltage-gain stages, plus a unity-gain output stage to increase drive capability. This has current-sensing overload protection. There is also a fairly complex bias generator that establishes the operating currents in the various stages.

In all conventional op-amps there are two differential input signals that have to be subtracted to create a single output signal, and the node at which this occurs is called the 'phase-summing point'.

Q1, Q2 make up the input differential amplifier. They are protected against reverse biasing by the diode-connected transistors across the input pins. Note there are no emitter degeneration resistors, which would linearize the input pair at the expense of degrading noise. Presumably high open-loop gain (note there are three gain stages, whereas a power amplifier normally only has two) means that the input pair is handling very small signal levels, so its distortion is not a problem.

Q3, Q4 make up the second differential amplifier; emitter degeneration is now present. Phase summing occurs at the output of this stage at node 2. C1 is the Miller capacitor around this stage, from node 2 to node 1. Q5, Q6, Q7 are aWilson current mirror, which provides a driven current source as the collector load of Q4. The function of C4 is obscure but it appears to balance C1 in some way.

The third voltage-amplifier stage is basically Q9 with split-collector transistor Q15 as its current-source load. Q8 increases the basic transconductance of the stage, and C3 is the Miller capacitor around it, feeding from node 3 to node 2 – note that this Miller loop does not include the output stage. Things are a bit more complicated here as it appears that Q9 is also the sink half of the Class-B output stage.

Q14 looks very mysterious as it seems to be sending the output of the third stage back to the input; possibly it's some sort of clamp to ensure clean clipping, but to be honest I haven't a clue. Q10 plus associated diode generates the bias for the Class-B output stage, just as in a power amplifier.

The most interesting signal path is the semi-local Miller loop through C2, from node 3 to node 1, which encloses both the second and third voltage amplifiers; each of these has its own local Miller feedback, so there are two nested layers of internal feedback. This is probably the secret of the 5532's low distortion.

Q11 is the source side of the output stage and, as mentioned above, Q9 appears to be the sink. Q12, Q13 implement overcurrent protection. When the voltage drop across the 15 Ω resistor becomes too great, Q12 turns on and shunts base drive away from Q11. In the negative half-cycle, Q13 is turned on, which in turn activates Q17 to shunt drive away from Q8.

The biasing circuit shows an interesting point. Bipolar bias circuits tend not to be self-starting; no current flows anywhere until some flows somewhere, so to speak. Relying on leakage currents for starting is unwise, so here the depletion-mode JFET provides a circuit element that is fully on until you bias it off, and can be relied upon to conduct as the power rails come up from zero.