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Looking for advice on selecting voltage regulators for a project

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vxp036000:
What's the advantage of a power op amp over a simple single transistor audio amplifier?  Now you don't need to worry about +/- supplies.  It's not like you need such precise control over the gain. 

The reason I question the use of a regulator is two-fold.  First, most any ripple on the supply voltage can be taken out with a simple filtering cap.  Second, a regulator is going to try to maintain a constant voltage output.  That's not necessarily a good thing.  When your op amp gets warm, it draws more current.  With a simple resistive supply, the supply voltage drops, protecting the device.  Instead, with the regulator, the voltage remains the same.  Now the op amp dissipates more power, heats more, draws more current, and we have thermal runaway. 

Next problem.  Suppose your op amp fails with a short.  Your regulator is going to dump the maximum amount of current into the short, causing catastrophic heating and all sorts of other wonderful things.

As far as dropping 24 V to 5 V with a linear supply, it doesn't need to dissipate a lot of power.  What does the digital circuitry draw, a few mA?

Kremmen:
Sorry but that is a very confused post. This is why:


--- Quote from: vxp036000 on May 05, 2012, 05:30:27 pm ---What's the advantage of a power op amp over a simple single transistor audio amplifier?  Now you don't need to worry about +/- supplies.  It's not like you need such precise control over the gain. 
--- End quote ---
The answer to this question would be a long one if i went through every reason in detail. Basically the reasons are the same as with any op amp against a single transistor. The very reason op amps exist is to overcome the limitations and shortcomings of a earlier designs.

--- Quote ---The reason I question the use of a regulator is two-fold.  First, most any ripple on the supply voltage can be taken out with a simple filtering cap.  Second, a regulator is going to try to maintain a constant voltage output.  That's not necessarily a good thing.  When your op amp gets warm, it draws more current.  With a simple resistive supply, the supply voltage drops, protecting the device.  Instead, with the regulator, the voltage remains the same.  Now the op amp dissipates more power, heats more, draws more current, and we have thermal runaway.

--- End quote ---
True to some extent. The filter caps will never completely eliminate the ripple, but wit op amps it would not be critical due to their generally high PSRR. Another advantage compared to single transistors by the way.
Maintaining a constant voltage is always a good thing in a DC voltage supply. In fact that is one measure of their quality. The difference in current draw of a cold vs warm op amp is not significant and definitely not a factor in PSU design. That is not a way to provide overcurrent protection. High internal resistance just causes the PSU voltage to sag under load causing extra distortion. Definitely useless. Thermal runaway is a totally different concept, has nothing to do with the PSU as such.

--- Quote ---Next problem.  Suppose your op amp fails with a short.  Your regulator is going to dump the maximum amount of current into the short, causing catastrophic heating and all sorts of other wonderful things.

As far as dropping 24 V to 5 V with a linear supply, it doesn't need to dissipate a lot of power.  What does the digital circuitry draw, a few mA?

--- End quote ---
Well who cares if the amp already shorted. OK kidding but hey, fuses were invented a long time ago. Thats the way you do it, or in case you wish to be sophisticated, use a current foldback PSU.

There are reasons not to regulate a PSU but yours are not really any of them.

vxp036000:
I still question the justification for op amps/ regulation in an audio amplifier because I have designed and built high performance audio amps without regulators and without op amps.  The only justification for op amps is in circuitry requiring very precise voltage and current control, neither of which is necessary in an audio amplifier.  I would also point out that a well designed common collector amplifier can have a very high PSRR.  This is why I never understood the point of a power op amp in this sort of application.  I might also add that power op amps are generally a bit pricier than a discrete transistor.

My example of thermal runaway is not imaginary, I've seen it happen with op-amps.  Any transistor that gets hotter will have a higher base / gate voltage, further increasing the current draw.

As for the voltage sagging under load, find a decent DC supply and focus on designing the amplifier.  Bias circuitry doesn't need to have built-in regulation if you're using a decent supply.

Kremmen:

--- Quote from: vxp036000 on May 05, 2012, 07:24:21 pm ---I still question the justification for op amps/ regulation in an audio amplifier because I have designed and built high performance audio amps without regulators and without op amps.  The only justification for op amps is in circuitry requiring very precise voltage and current control, neither of which is necessary in an audio amplifier.  I would also point out that a well designed common collector amplifier can have a very high PSRR.  This is why I never understood the point of a power op amp in this sort of application.  I might also add that power op amps are generally a bit pricier than a discrete transistor.

My example of thermal runaway is not imaginary, I've seen it happen with op-amps.  Any transistor that gets hotter will have a higher base / gate voltage, further increasing the current draw.

As for the voltage sagging under load, find a decent DC supply and focus on designing the amplifier.  Bias circuitry doesn't need to have built-in regulation if you're using a decent supply.

--- End quote ---
I have no argument with unregulated supplies because careful regulation is not usually needed. I too have built numerous amps over the years and always with unregulated supplies. My reply concerned the specific points you made in your previous which i felt were somewhat inaccurate.
I have built amps from discretes and op amps and haven't really seen big differences in principle. Of course there are crap op amps but then crap discrete circuits dive up as well every now and then. Properly implemented both will work nicely.
Regarding themal runaway - sure, but if we wish to be painfully precise it is the carrier density i.e. conductivity of the silicon that goes up, not necessarily base/gate voltage (why would that happen?). My point was that runaway is an overload or load sharing problem, not so much related to the PSU. I don't know if you can effectively mitigate runaway by using an unregulated supply (from your first post) at least i never seen that or tried it.

vxp036000:
Hmm, you bring up an interesting point with thermal runaway.  I always assumed that it was caused by resistance in the emitter (or source) increasing, resulting in the base (source) voltage going up, but I'm probably mistaken. 

For a bipolar device, Ic = Is e^(Vbe / Vt) or Ic = u k T ni^2 e^(Vbe / Vt).  Since u is roughly u0 T^(-3/2), we have
Ic = u0 T^(-3/2) k T e^(Vbe / Vt), which simplifies to Ic = u0 T^(-1/2) k e^(Vbe / Vt).  Now, ni^2 is roughly T^3 e^(-Eg / kT), so Ic = u0 T^(5/2) k e^(Vbe / Vt - Eg / kT).  So now it becomes obvious that the collector current goes up with with temperature because the carrier concentration increases.  Yep, I was clearly mistaken in my original understanding.  Thanks for pointing that out :-)


--- Quote from: Kremmen on May 05, 2012, 08:13:14 pm ---I have no argument with unregulated supplies because careful regulation is not usually needed. I too have built numerous amps over the years and always with unregulated supplies. My reply concerned the specific points you made in your previous which i felt were somewhat inaccurate.
I have built amps from discretes and op amps and haven't really seen big differences in principle. Of course there are crap op amps but then crap discrete circuits dive up as well every now and then. Properly implemented both will work nicely.
Regarding themal runaway - sure, but if we wish to be painfully precise it is the carrier density i.e. conductivity of the silicon that goes up, not necessarily base/gate voltage (why would that happen?). My point was that runaway is an overload or load sharing problem, not so much related to the PSU. I don't know if you can effectively mitigate runaway by using an unregulated supply (from your first post) at least i never seen that or tried it.

--- End quote ---

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