distortion with Class B (with NFB) vs Class AB

I was shocked to see such low distortion figures on this Class B audio amplifier:

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Is negative feedback sufficient to eliminate crossover distortion in Class B?

Reply to
mrdarrett
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If you are talking about the first circuit on that page, then I'm not sure I'd call it class B, there is a path from the opamp output to the speaker. (I also thought it should have a bit of resistance in there, opamp to speaker, but I don't know... JL has posted circuits like that.)

George H.

Reply to
George Herold

Yep, the first circuit. Sure there's a path from opamp pin 6 (output) to the speaker, but the heavy lifting is done by Q1 and Q2, right?

Michael

Reply to
mrdarrett

On second thought, you raise a good point... shouldn't the output from the opamp drive the bases of the transistors somehow?

Reply to
mrdarrett

Yep. It's really not quite class-B.

This scheme was first done in the mid '60's in the Motorola MC1524.

I used it in the power supply regulator for the Hughes TOW missile somewhere around 1970-73.

The THD specs are not particularly surprising. What you have to watch out for is TIM (transient intermodulation) distortion... particularly to those of us who played clarinet in orchestra, sitting in front of the oboes and French horns, TIM glares at you when playing a recording of a Mozart woodwind ensemble. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Nope. What happens is the OpAmp _tries_ to drive the speaker, with the result that the current in the OpAmp power lead increases driving the appropriate bipolar device. You want to choose the B-E resistors such that the bipolar devices conduct before the OpAmp current limits, but not so large that you have rail-to-rail conduction thru the bipolar's. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Correction, it wasn't the MC1524 as I thought. Finally found a datasheet for the MC1524... it was a classic architecture, implemented as a hybrid in a TO-5 package, with three chips, two P-type substrate, one as N-type.

I haven't found what number the rail-driven amplifier was yet. I loaned out my ancient data book and it never returned :-( ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Huh. Not quite Class B... does this scheme have a name? B+ would be neat, haha...

Reply to
mrdarrett

Here's a different one. Is this more accurately Class B with NFB?

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Thanks,

Michael

Reply to
mrdarrett

It's sorta class-AB, _except_ that the loop gain jumps up when the bipolar's come active... which means you have to compensate the loop based on that extra gain and lose effective feedback when only the OpAmp is active. It's probably something I should analyze in detail... for amusement-sake ;-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
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Reply to
Jim Thompson

Yep. More closely class-B, though I don't know if the 0.22uF capacitor helps or hinders. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

It's a boosted opamp, not really "class B". Voltage regulators, like LM7805s and such, are sometimes "helped out" by a parallel PNP, similar idea.

The rightmost 0.22 uF cap allows the opamp to drive the load some directly at higher frequencies. A resistor there makes more sense.

The gain control is insane. So is the transistor biasing. It's impressive how many really, really bad audio circuits there are around. And there is a positive correlation between circuit badness and web page pontification.

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John Larkin         Highland Technology, Inc 

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Reply to
John Larkin

...

Dang. What's a beginner to do, then? I'd like to make a 12-volt wall-wart-powered class B, or AB, or boosted-opamp thingie, and would like to know *why* it works, too (so that kind of defeats the purpose of chipamps).

Any suggestions? :)

Thanks!

Michael

Reply to
mrdarrett

** Only zero bias class B designs have incurable crossover notches and that schem is something different - class AB with a twist if you like. The 5534 drives the speaker direct and the current in its supply rails bring the two boosters on as needed.

Bias current in the boosters is not defined - it depends on the 5543's supply current and that varies from sample to sample and brand to brand. It also very much depends on the Hfe and temperature of the boosters - they will need a decent size heatsink to prevent thermal runaway.

Seeing as the collectors are linked, the boosters can be bolted to the same heatsink with no insulators ( just a smear of silicone grease) - but the heatsink cannot be grounded.

The THD figures are indeed very low and that is mainly down to the exceptional performance of the 5534.

.... Phil

Reply to
Phil Allison

Ohm's and Kirchhoff's Laws never fail you.

(Kirchhoff's Law is particularly applicable to circuit analysis... I call it, "What gozinta must gozouta" ;-) ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

Norton and Thevenin too, I bet. The stuff that made me hate electronics after taking the electronics class at Davis. Took me a long time to like electronics again after that.

Michael

Reply to
mrdarrett

Look for some of the older literature, like the GE Transistor manuals and the National Linear Applications books. They were into discrete transistor design.

Start with a simple 1-transistor class A amp and understand its bias and gain.

Horowitz and Hill, The Art of Electronics, is a good book to have around. Chapters 1 and 2 start with electrical basics and make it up to bipolar transistor amps.

Build this and listen to it. It's a real class B amp.

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Then try things to make it less horrible.

An oscilloscope is worth having here. Like a Rigol or something.

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    
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Reply to
John Larkin

Ok, I think I'll go ahead and grab Horowitz and Hill then.

Ooh, thanks! TIP31A and its PNP spouse are OK for output transistors?

Much appreciated!

Michael

Reply to
mrdarrett

There should be lots of used ones around, 2nd edition.

Sure. You'll understand what crossover distortion is really about.

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John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    
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Reply to
John Larkin

UC Davis? Good professor there knowledgeable in electronically commutated DC motors. Can't remember his name now, but I visited him there in the late '90's when I was designing the Bosch motor control chip...

...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

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