Making an amplifier without knowing the load

Thread Starter

Gibson486

Joined Jul 20, 2012
336
I am trying to design an amplifier. I decided to just try a push pull Class AB mosfet amp. The issue is that I am not sure the load will stay constant. Essentially, the voltage out is going to be dependent on what the load is because we are relying in the current through the drain for amplification. Am I thinking about this correctly? Sorry if this is a dumb question....
 

crutschow

Joined Mar 14, 2008
27,755
If you are referring to an audio amp, they normally have negative feedback that makes the dynamic output impedance very low (typically <1Ω). so the output signal voltage is largely independent of the (speaker) load.
 
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WBahn

Joined Mar 31, 2012
26,398
Could you provide a schematic that you are working from?

I don't have a lot of experience with these kinds of amplifiers (and what I do have is now a couple decades in the past), but most of these amplifiers that I've worked with (IIRC) were essentially source-followers, which would mean that they controlled the voltage applied to the load and the load then determined the current, not the other way around.

The proper approach depends on what it is you are trying to achieve. For the same input signal, what do you want (and really you should focus on what you NEED) to have happen at the load if it is replaced by an impedance twice/half as large? Do you want the voltage across the load to remain the same? The current through the load to remain the same? The power delivered to the load to remain the same? Does it really matter (for instance, if someone replaces the speakers with ones that result in increased loudness than the previous speakers with the same input signal, the person using it just turns the input signal down)? Then you need to consider and decide over what range of loads you need this and how precise you need it.
 

Thread Starter

Gibson486

Joined Jul 20, 2012
336
It is literally just a standard push pull, but with split rails instead of VCC and GND. All I have done so far is glance over the idea, so no specific parts yet. Surprisingly, looking up class AB amps yield 98% BJT instead of mosfets.... I put this in LTspice and I never could get bigger better than unity gain....but then again, all i did was pick the standard NMOS and PMOS part, so I may have just biased them wrong. I just need the AC voltage to remain stable, but as crutschow states, that may require feedback.


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Alec_t

Joined Sep 17, 2013
12,212
I put this in LTspice and I never could get bigger better than unity gain....but then again, all i did was pick the standard NMOS and PMOS parts
Those generic devices hardly ever do what you want. Select models of real-world FETs instead.
 
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Ohmlandia

Joined Mar 2, 2020
32
A modern transistorised audio power amplifier is essentially a voltage source, as already mentioned in other replies. The current taken by the load, and therefore the power delivered, will be determined by the load resistance. Most amplifiers will run quite happily with no load at all, or a high impedance load. Just be aware that there must be a specified lowest load resistance that the output devices can safely drive without exceeding their current handling ability.
 

dl324

Joined Mar 30, 2015
13,142
Surprisingly, looking up class AB amps yield 98% BJT instead of mosfets....
Likely because they're easier to bias. The Class B amplifier you posted is going to have a lot of crossover distortion. MOSFETS don't have tightly controlled threshold voltages, and not having them slightly on at all times (Class AB) does nothing to mitigate distortion.
 

crutschow

Joined Mar 14, 2008
27,755
Look at Class AB audio amplifier schematics to see how they bias the BJT emitter follower outputs.

As dl324 noted, it's much more difficult to bias MOSFET source follower outputs to operate Class AB.
What you have in your simple circuit is a Class B stage, which gives a large amount of crossover distortion (essentially the output doesn't conduct around zero volts for the sum of the two MOSFET threshold voltages).
 

Thread Starter

Gibson486

Joined Jul 20, 2012
336
Look at Class AB audio amplifier schematics to see how they bias the BJT emitter follower outputs.

As dl324 noted, it's much more difficult to bias MOSFET source follower outputs to operate Class AB.
What you have in your simple circuit is a Class B stage, which gives a large amount of crossover distortion (essentially the output doesn't conduct around zero volts for the sum of the two MOSFET threshold voltages).
I think the way you fix that is with feedback....but with that said, my outputs are going to swing to beyond 100V. So feedback will have to go through resistor dividers and I am pretty sure that will lead to errors. I will try BJTs as well.
 

Thread Starter

Gibson486

Joined Jul 20, 2012
336
And you never will since it is composed of two source followers.

Class AB amps of this type need a gain stage before the output stage if voltage gain is needed.

Bob
Here is an extremely simple audio amplifier project using Mosfets in the output stage:
http://www.redcircuits.com/Page123.htm
Thanks....

That helped a lot. That boot strapping is pretty cool. I have done it before to fix a problem with a darlington, but I never knew the terminology.

This is what I am trying to do, but not to the tune of 2kV pk-pk. I only need 900V pk-pk.

https://www.edn.com/high-voltage-amplifier-uses-simplified-circuit/

I was expecting that you would somehow isolate the the low side from the high side, but I guess that is another project in itself. Also, I question how correct that schematic is. First thing that caught my eye was that Q12 is biased the same exact way Q5 is even though they are complementary types.
 

Ohmlandia

Joined Mar 2, 2020
32
So your cards are on the table at last. An amplifier with 900v pk-pk output. Most of the advice given would have been different I think if responders knew that detail. You need now to specify how much distortion is acceptable in your application, which is obviously not an audio speaker amplifier after all.
 

Audioguru again

Joined Oct 21, 2019
3,544
The article has a list of errors in the schematic but they do not say that I think R12 connects to -12V, not the +12V shown.
Will you use it to electrocute somebody?
 

Thread Starter

Gibson486

Joined Jul 20, 2012
336
So your cards are on the table at last. An amplifier with 900v pk-pk output. Most of the advice given would have been different I think if responders knew that detail. You need now to specify how much distortion is acceptable in your application, which is obviously not an audio speaker amplifier after all.
Is it really that different? It is basically an audio amplifier that needs to put one frequency. It is still just a push pull amp, but just at pretty high rails.
 

Ohmlandia

Joined Mar 2, 2020
32
A single frequency amplifier can be a different kettle of fish to a broadband amplifier. You can have transformers, resonant circuits, etc, etc. Might not need push pull circuitry at all. You haven't said what the frequency is, or the waveform, or the power.
 
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