Electronics > Beginners

Adding filters to class AB audio amp

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fourfathom:

--- Quote from: Zero999 on August 19, 2019, 09:41:09 am ---Sorry, I should have reread your reply more carefully. It was obvious that's what you meant.

Yes, it will soften the crossover distortion, but it'll still be there. I doubt it'll do much in this application because the op-amp would have to supply a significant output current. It's more effective in applications where the output current is under an order of magnitude greater than what the op-amp can supply.
--- End quote ---

Yes, this would work better with a more powerful opamp, but I'll bet that it would noticeably improve this design as well.  With a 100-Ohm resistor, the opamp only has to drive an additional 6mA (approx) before the transistor takes over.  I don't have sim models for the TL082, but the spec sheet shows that it can sink/source this level of current within 3V of the supply/ground.  Given the 10V supply here, that gives about 3.5VP-P output at the load, or 190mW power out (8 Ohm load).

Tell me I'm wrong -- I can take it!  It's just a fun problem.

Mark Hennessy:

--- Quote from: Zero999 on August 17, 2019, 08:20:12 pm ---
--- Quote from: d4n13l on August 17, 2019, 08:14:22 pm ---doesn't the op amp make this work as a class AB? as I understand it a class B amplifies only after the 0.6V that takes to activate the transistors thus causing distortions, however due to the negative feedback the op amp put enough voltage to overcome that

--- End quote ---
No the op-amp doesn't cause the output stage to work in class AB. Each transistor conducts for less than half of the waveform, so it's definitely class B.

The op-amp will reduce the crossover distortion somewhat, but it will still be there, especially at higher frequencies/slew rates. The only way to reduce it further is to bias the output transistors so both conduct slightly all the time, making it class AB.

--- End quote ---

Actually, it's class C because the output devices conduct for less than 180 degrees.

Class B is exactly 180 degrees. Some of the lowest distortion amplifiers are class B.

Class AB is more than 180 degrees. When everything else is optimally implemented, moving from class B to AB actually increases distortion because of gm-doubling.

See page 55 of this PDF (page 35 of the book): https://www.desmith.net/NMdS/Data/Books%20and%20Manuals/Self%20-%20Audio%20Power%20Amp%20Design%20Handbook%204th%20Edn.pdf

Chapter 5 goes into much more detail - page 131 (109).

For anyone with even a passing interest in audio, Douglas Self's works are well worth seeking out. As well as the power amp book, "Small Signal Audio Design" is also excellent  :-+

Zero999:

--- Quote from: fourfathom on August 19, 2019, 04:05:32 pm ---
--- Quote from: Zero999 on August 19, 2019, 09:41:09 am ---Sorry, I should have reread your reply more carefully. It was obvious that's what you meant.

Yes, it will soften the crossover distortion, but it'll still be there. I doubt it'll do much in this application because the op-amp would have to supply a significant output current. It's more effective in applications where the output current is under an order of magnitude greater than what the op-amp can supply.
--- End quote ---

Yes, this would work better with a more powerful opamp, but I'll bet that it would noticeably improve this design as well.  With a 100-Ohm resistor, the opamp only has to drive an additional 6mA (approx) before the transistor takes over.  I don't have sim models for the TL082, but the spec sheet shows that it can sink/source this level of current within 3V of the supply/ground.  Given the 10V supply here, that gives about 3.5VP-P output at the load, or 190mW power out (8 Ohm load).

Tell me I'm wrong -- I can take it!  It's just a fun problem.

--- End quote ---
I've simulated it in LTSpice. The model for the TL072 is embedded in the file. the red trace is when R5 is 1G which is near enough open circuit and the blue trace is when R5 is 100R. It does soften the edges somewhat, but most of the crossover distortion is still there.

 Class B.asc (3.85 kB - downloaded 85 times.)



--- Quote from: Mark Hennessy on August 19, 2019, 05:53:51 pm ---
--- Quote from: Zero999 on August 17, 2019, 08:20:12 pm ---
--- Quote from: d4n13l on August 17, 2019, 08:14:22 pm ---doesn't the op amp make this work as a class AB? as I understand it a class B amplifies only after the 0.6V that takes to activate the transistors thus causing distortions, however due to the negative feedback the op amp put enough voltage to overcome that

--- End quote ---
No the op-amp doesn't cause the output stage to work in class AB. Each transistor conducts for less than half of the waveform, so it's definitely class B.

The op-amp will reduce the crossover distortion somewhat, but it will still be there, especially at higher frequencies/slew rates. The only way to reduce it further is to bias the output transistors so both conduct slightly all the time, making it class AB.

--- End quote ---

Actually, it's class C because the output devices conduct for less than 180 degrees.

Class B is exactly 180 degrees. Some of the lowest distortion amplifiers are class B.

Class AB is more than 180 degrees. When everything else is optimally implemented, moving from class B to AB actually increases distortion because of gm-doubling.

See page 55 of this PDF (page 35 of the book): https://www.desmith.net/NMdS/Data/Books%20and%20Manuals/Self%20-%20Audio%20Power%20Amp%20Design%20Handbook%204th%20Edn.pdf

Chapter 5 goes into much more detail - page 131 (109).

For anyone with even a passing interest in audio, Douglas Self's works are well worth seeking out. As well as the power amp book, "Small Signal Audio Design" is also excellent  :-+

--- End quote ---
I'd say you're being a bit pedantic. Quoting the book..


--- Quote ---Class-C implies device conduction for significantly less than 50% of the time, and is normally only usable in radio work, where an LC circuit can smooth out the current pulses and filters harmonics.
--- End quote ---
What is significant, is open to interpretation.

One way of looking at it is, for large signals, where crossover distortion is less significant, it will be operating closer to class B (each device conducting for nearly 50% of the cycle) and for smaller signals, it will be closer to class C (each device conducting for much less than 50% of the cycle). I suppose if one really wants to be pedantic, they could say it's a class BC output stage.

It is true that, the ideal mode of operation is class B, but is it actually possible to perfectly obtain class B? I doubt it. A compromise must be met, hence class AB.

Mark Hennessy:
In the context of this circuit:


--- Quote ---Class B is subject to much misunderstanding. It is often said that a pair of output transistors operated without any bias are working in Class-B, and therefore generate severe crossover distortion. In fact, with no bias each output device is operating for slightly less than half the time, and the question arises as to whether it would not be more accurate to call this Class-C and reserve Class-B for that condition of quiescent current which eliminates, or rather minimises, the crossover artefacts.

--- End quote ---

In other words, given that the crossover distortion is gross enough to be seen on a 'scope, that absolutely makes it class C - no ifs, no buts. If we were debating something that needed a distortion or spectrum analyser to distinguish, then yes, perhaps we might be in the pedantic zone...  :)

As you suggest, class AB is about avoiding the lesser of two evils (gm-doubling (AB) vs crossover distortion (C)). Pragmatic enough, especially if the rest of the design isn't all that clean...

In a better amplifier, maintaining that exact bias point is worth doing, but is one of the harder things to get right. So much so that an entire chapter is devoted to it (chapter 13). For an EF stage, the quiescent voltage needs to be held to around 100mV, and for a CPF, it's 10mV (ballpark figures) - obviously very easy if these were simple fixed voltages, but of course these voltages must track the changes in output stage Vbe caused by temperature. It can be done, and Douglas Self has put many such designs into production - and I'm sure he's not the only one.

Zero999:
For completeness, here's the amplifier biased in class AB.

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