power amplifier as a voltage regulator - Page 1
Forum section: electronics / projects

author: cksa
author_rank: Regular Contributor
timestamp: December 06, 2011, 05:35:16 pm
content: Is it possible to use a power amplifier as an adjustable voltage regulator?i.e. microcontroller --> dac --> power amp --> output voltageI am sure it would be possible...but what would be the characteristics of such a circuit?

author: IanB
author_rank: Super Contributor
timestamp: December 06, 2011, 05:44:25 pm
content: What is meant by "power amplifier"?Power amplifiers are usually defined as delivering an AC signal over some defined frequency range (audio, RF, etc.).Power supplies are (usually) defined as delivering DC. So an amplifier would not work as a DC power supply, but it might work as an AC power supply in some specialized cases.

author: Simon
author_rank: Unknown
timestamp: December 06, 2011, 06:12:36 pm
content: have a look at daves recent videos on making a power supply, all of what you suggest is described (properly)

author: jahonen
author_rank: Super Contributor
timestamp: December 06, 2011, 06:46:36 pm
content: Actually, dc power supply is in many respects quite similar than an audio power amplifier (I'm assuming here that OP means one here). All feedback loops are similar, and design issues are pretty much the same. Although frequency range is not so high on a DC power supply. High bandwidth on a DC power supply might not be a good thing after all, since high bandwidth also invites stability problems with pathological loads, although it sounds attractive due to improved transient response. Also, power amplifiers are not usually DC accurate, so their internal gain is usually set to drop to unity near DC (below 5 Hz or so).Another main difference is that DC power supply usually works only in first quadrant (only source current with positive voltage), whereas power amplifier must tolerate all quadrant loads (that means sink and source current). Another way to say this is that DC power supply can only drive power out, not sink it.Although four quadrant operation sounds nice, that is not always desired. Problems arise if you connect the power supply output to something like battery. If the battery happens to be at higher voltage than the set voltage of the power supply, it starts to discharge it to its set voltage.There are some operational amplifiers suitable for this kind of operation, like TI's OPA541.Regards,Janne

author: ejeffrey
author_rank: Super Contributor
timestamp: December 06, 2011, 07:41:52 pm
content: A power amplifier is somewhat similar in principle to a linear DC power supply, but the practical considerations are quite different. The most obvious difference is bandwidth: audio amplifiers generally need high bandwidth and low distortion. DC power supplies don't need particularly great bandwidth, but it is often important to minimize overshoot. Audio amplifiers are usually designed to drive relatively linear resistive / inductive loads. Power supplies have to supply a capacitive load with potentially large load transients.General purpose power supplies usually only have one set of output transistors: from the positive rail to the load. Power supplies can only source current and rely on the load to sink current. True amplifiers always have push-pull drivers, although in single-ended class-A amplifiers the 'pull' is a passive resistor. This is the difference between a one-quadrant and four-quadrant supply that jahonen talkes about. That is why power supplies often have minimum load current specifications, while amplifiers do not. Amplifiers instead have to worry about biasing the output transistors properly to avoid crossover distortion when the load shifts from the N type output device to the P type.Power supplies to drive inductive loads like electromagnets are usually 4-quadrant supplies in order to safely ramp down the current without a huge inductive kick blowing everything apart. These devices are really built like amplifiers, just with lower bandwidth than audio amplifiers and much higher power than small-signal amplifiers.

author: Zero999
author_rank: Super Contributor
timestamp: December 06, 2011, 07:44:42 pm
content: A voltage regulator has to be stable under varying load conditions. A power amplifier it normally designed to drive a certain range of load impedances and that's it. An audio amplifier may be unstable if it's connected to a load with a constantly varying impedance such as a digital logic circuit.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 06, 2011, 08:20:00 pm
content: Technically, an audio amplifier is really just an inverter. And an inverter (most of them anyways) is really just 2 or 3 synchronous buck converters.

author: cksa
author_rank: Regular Contributor
timestamp: December 06, 2011, 08:26:39 pm
content: Quote from: Simon on December 06, 2011, 06:12:36 pmI have watched all of his videos.I have also listened to this week's amp hour...which is where this question is from. The CEE project linked by chris and dave involves the use of a OPA567 as it's output stage. (http://www.ti.com/lit/ds/symlink/opa567.pdf)Thank you jahonen, ejeffrey, and Hero 999 for your replies.How would I know whether an amplifier would be stable under varying load impedances?

author: Unknown
author_rank: Unknown
timestamp: December 06, 2011, 08:32:55 pm
content: Quote from: NiHaoMike on December 06, 2011, 08:20:00 pmThere's a huge difference in this regard between a class D (or proprietary versions like class T) amplifiers which use PWM similar to switching power supplies, and the traditional class A/B/AB amplifiers that use transistors in linear mode.

author: jahonen
author_rank: Super Contributor
timestamp: December 06, 2011, 08:40:32 pm
content: Quote from: cksa on December 06, 2011, 08:26:39 pmGenerally, those kinds of impedances which generate lag on the feedback are poison to the feedback phase/gain margin. That means a parallel capacitive load on a voltage output and an inductor in series of a current output.In practice, only way to be sure is to test it. Risk is that in order to find the stability limit, you must go past it. This may be destructive in some cases, so caution is required. Step response test might be good test to find out how far one is from the stability limit (longer the oscillation is, nearer the stability limit is).Regards,Janne