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Voltage Regulators for Line Level Audio. Part IV : The Error Amplifier

Posted 18th January 2014 at 07:05 AM by rjm
Updated 20th January 2014 at 11:21 PM by rjm

Part 4 for a series.

At the end of Part 3 I promised to introduce feedback, and I will, but what we are really talking about here is the addition of the error amplifier, the heart of all modern series pass regulators. The error amplifier is a non-inverting DC signal amplifier, and its function is simple: amplify and buffer the reference voltage. The twist is that the amplifier output is connected to the base of the pass transistor, while the feedback connection is taken from it’s emitter. The pass element is thus placed inside the feedback loop of the error amplifier, improving the ripple rejection and output impedance of the regulator dramatically.

So here we are, the three building blocks of a voltage regulator are in place: the reference voltage, the error amplifier, and the pass device. In the LTSpice circuit I’ve cheated, deliberately, in order to make the operation easier to follow. Instead of building a practical voltage reference I’ve used a perfect voltage source, Vref. Its a simulation: no rules to say it has to be a workable in the real world. Same thing for the voltage powering the error amplifier: a perfect voltage source V2 is used, sidestepping the tricky chicken-and-egg question of how to get clean power to the amplifier that makes the clean power…

You can see from the plots: under perfect conditions a high performance op amp like the LT1007 will generate fantastic numbers for ripple rejection and output impedance. So low that for a practical circuit the limitations are instead usually found elsewhere: noise and ripple on the voltage reference or op amp power supply, for example, or less obvious problems like oscillations and general instability of the op amp at high frequencies.

Note that the error amplifier doesn’t have to be an op amp … well-suited for the job though they are. A single transistor (or triode) can work as a reasonable substitute, and we’ll hopefully find time to look at an example later on. When an op amp is used, however, like in the example circuit below, please pay attention to what its power pins are connected to. Here it is a 14 V positive supply and ground. The inputs and output can only vary within a limited subset of that range, often no closer than 2.5 V from the supply voltages unless a rail-to-rail op amp is chosen. Very small reference voltages, or output voltages exceeding 11.5 V or so, will both leave you with a non-functioning regulator and quite possibly a puzzled look on your face. It confused the heck out of me the first time it happened.

The gain is just R4/R3+1, here G=10k/10k+1=2. The reference is 5 V, the regulator output is 10 V. Op amps operated in the non-inverting configuration tend to get unhappy at such low gains, especially driving highly capacitive loads. It is desirable then to use a higher gain and lower voltage reference, but remember a typical silicon bandgap reference (1.2 V) requires a rail-to-rail op amp if the op amp's negative power terminal is grounded.

Exercises for study:

1. Remove V2 and connect the positive power terminal of the op amp to V1 instead to observe the effect this has on the ripple rejection.

2. Remove the pass element from the voltage regulator. Its true: any DC non-inverting amplifier can double as a regulated power supply!
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