Class B distortion

Does this sentence make sense? The output stage is the output of the opamp.

In any event, you can use any number of ways to make the circuit AB:

1) VBE multiplier 2) diode string 3) Complementary class A buffers to eliminate the drop

I'm trying to think of any integrated class B amps. Maybe motor drivers where you wouldn't care about the notch.

LM386?

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yes, or if you reduced the frequency, or input amplitude. anything that makes the curve flatter helps against slew-rate limitations.

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umop apisdn

Jan's circuit works. R is actually there for stability not to drive the speakers (check it out 20ma output current into 8 ohms - a whopping 3.2mW, into a standard speaker is below the noise floor in most indoor locations). An R about 4.7k +/- 50% should be fine.

If you already tried this and crossover was really bad there must have been something very wrong in your circuit. Even with a 741 the cross over distortion is above 20k.

Looks like AB:

NT

I think I follow. I can see possible issues, will spice it later and see

NT

If you add a C so it limits mean current, that's still going to be much too high for idling.

NT

I did sim it. Putting that R in made little difference, still heavy crossover distortion. Maybe I need to go much lower with R. How well your average opamp can cope with very low R I'm not so sure, lets hope Spice models it ok.

NT

opamp's output goes to op tr bases. Op tr emitters are the speaker output terminal. If you connnected those 2 points there would be no op tr drive.

So far I've run into issues with those 3.

1: hot tr Vbe=0.35v, vbe multiplier isnt self adjusting and theres no feedback to deal with excessive idle current 2: ditto 3: will sim this, though I suspect issues there too

NT

Post your LTspice file so we can check it.

I guess it's technically class AB as I guess the output couldn't bias up to Vcc/2 at quiescense without some current through the output devices. It seems there is some difference in terminology when two diodes are used to bias the transistors right at the edge of of conduction so each is conducting exactly 180 degrees; I've seen it called either class AB or class B. Maybe this distinction doesn't matter.

If it is class AB it is a very "lean" AB as the LM386 also has nasty crossover distortion.

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maybe you should build it

I never used the compound PNP, but in theory the diode drop scheme should work the same. They probably don't bias it very far in Class A.

The LM321 looks kind of class B, though they claim it is AB.

On a sunny day (Sat, 20 Dec 2014 19:59:46 -0000) it happened "Kevin Aylward" wrote in :

• | |/ c -------| NPN | |\ e

| >------===----| -- - |/ | |-------- out | | |/ e | | --------| PNP | | |\ c | | | | | - | --------------------------

The classic way to do this ahmmm.. err...correctly is the "Quad Current Dumper" by Peter Walker

Kevin Aylward

- SuperSpice

On a sunny day (Mon, 22 Dec 2014 20:00:27 -0000) it happened "Kevin Aylward" wrote in :

Yes I am familiar with that, a lot actually, used it in a low frequency transmitter in the UK. That was around 1982?

I will - not been able to spice anything in last 24 hours tho, its probably not the best time of year. Will post it as soon as its simmed and have sorted the hardware issue. Computers!

NT

nasty crossover distortion. Now, the nfb is taken from the circuit's output terminal, not from the opamp's output, so surely an ideal opamp should cor rect for the 0.6v Vbe drops. Question is why doesn't it?

s the output directly from -0.6 to +0.6v

Thanks to everyone for all the help/explanations/ideas so far. I don't know what I'll get time to do over the hols, but when I can I'll sim the sugges tions and see what I can get to work.

One of the issues with some circuits is that output tr Vbe drops to about 0 .35v at 175C (Tj). Diodes glued onto output trs would only rise to Tcase, r esulting, in a basic implemetation, in runaway bias. Everything can be fixe d with more bits of course, but I'm trying to do this one as rock bottom as possible.

Enjoy your xmas!

NT

Um 175 dec C? You have ceramic packages? You have characterized a diode on the chip to determine Tj? Diodes on the output? Glue?

This would go far better if you just stated what you are trying to achieve rather than discuss op amp design.

ut

se,

n

Standard plastic/metal trannies. Real life Tj will vary a good bit between samples, since the specs of the parts used are either unknown or ignored. W hether its 175 or 125, Vbe still shifts a long way.

e

Like my other recent projects, getting a job done at the lowest possible pa rts cost with labour as unskilled and ill equipped as possible, and using o nly scrap parts with no data. I've got a simple class A amp, now want a B f or battery apps. The point of the exercise is to provide designs that some people in poverty might learn to build to sell.

NT