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Old 27th April 2014, 08:04 AM   #1
arifs is offline arifs  Pakistan
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Default Simple class b amplifier problem

Hi,

I have made this simplest class B amplifier I can, in hobby.

https://www.dropbox.com/s/8j8agsagmo...ont%20move.JPG

The problem is it seems to work fine in simulation but when i made it in practical the quiescent current on negative side jumps upto 2Ampere (like a short circuit) .

Pleas put me out of my misery, What am i doing wrong here

Supply is +/- 30V


After testing around the problem i think might be is that the voltage on biasing diodes is -29V instead on few volts (simulation says 1.5 volt approx)



Import code for circuit simulator (just incase)

$ 1 5.0E-6 10.20027730826997 53 10.0 42
t 704 128 768 128 0 1 -29.42362773884907 0.5709866700909675 100.0
w 768 144 768 176 1
w 768 384 768 432 1
R 128 304 80 304 0 1 40.0 5.0 0.0 0.0 0.5
w 704 432 768 432 0
w 704 64 768 64 0
w 768 64 768 112 0
R 416 48 416 0 0 0 40.0 30.0 0.0 0.0 0.5
R 416 448 416 496 0 0 40.0 -30.0 0.0 0.0 0.5
w 768 272 816 272 2
r 864 272 864 384 0 6.0
g 864 384 864 416 0
t 704 368 768 368 0 1 -29.568259322530185 0.43673437431786866 100.0
r 768 176 768 224 0 0.47
r 768 272 768 320 0 0.47
t 672 112 704 112 0 1 -28.968019081682147 0.4556086571669238 100.0
t 672 352 704 352 0 -1 29.246654236843813 -0.3216050856863711 100.0
w 768 320 704 320 0
w 704 336 704 320 0
w 768 320 768 352 1
w 576 144 608 144 0
w 608 144 672 112 2
w 704 64 576 64 0
w 576 304 576 352 0
w 704 432 576 432 0
w 672 352 576 352 2
w 576 64 464 64 0
w 464 64 416 64 0
w 416 48 416 64 0
w 464 128 464 288 0
w 464 320 464 448 0
w 416 320 416 368 0
c 176 304 304 304 0 1.0E-5 0.0010000000002076126
r 304 304 304 368 0 22000.0
w 464 448 416 448 0
r 496 304 528 304 0 22000.0
w 416 128 416 288 0
w 528 304 640 304 0
w 640 304 640 272 0
w 640 272 768 272 0
g 304 368 304 400 0
w 464 448 576 448 0
w 576 432 576 448 0
s 128 304 176 304 0 1 false
w 816 272 864 272 1
w 768 224 768 272 0
w 704 64 704 96 0
d 576 144 576 256 1 0.805904783
d 576 256 576 304 1 0.805904783
t 304 304 416 304 0 -1 29.53013549085043 -0.5658348399109794 100.0
t 496 304 464 304 0 -1 30.14785051924438 -0.5651000975112301 100.0
r 576 64 576 144 0 5600.0
w 464 128 416 128 0
r 416 64 416 128 0 22000.0
r 416 368 416 448 0 1000.0
t 544 368 576 368 0 1 -29.066408325167483 0.6169802859941989 100.0
w 576 384 576 432 0
w 544 368 416 368 0
o 3 64 0 34 6.0 9.765625E-5 0 -1
o 10 64 0 35 32.73390607896142 5.237424972633828 1 -1
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Old 27th April 2014, 08:09 AM   #2
Mihkus is offline Mihkus  Estonia
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Isnt it a class AB because of those diodes and both outputs are turned on a bit...?
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Old 27th April 2014, 08:20 AM   #3
Mooly is online now Mooly  United Kingdom
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A few possible problems.

1) You are running with 100% feedback (unity gain) and so the amp may well be unstable.

2) There is no frequency compensation shown. Again stability problem.

3) It probably needs a zobel network on the output.

4) The bias could be a problem. Diodes are a poor solution for a real build. You need a proper vbe multiplier instead, however for testing you can just use a single diode to be sure there are no issues. It will all work but with a little distortion.
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Old 27th April 2014, 08:22 AM   #4
Mooly is online now Mooly  United Kingdom
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One more thing. If the current jumps on one side only then that is because you have a load attached and there is significant DC offset or hugely assymetric stability issue.
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Old 27th April 2014, 08:32 AM   #5
arifs is offline arifs  Pakistan
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Thanks for responses.
Mihkus , Pardon my bad. it was biased so low i thought it might fell in b category.

Mooly , Yes the current jumps on -ve supply side on load attachment.

@feedback. I have seen several schemes using 22k direct from output to feedback capacitor.

How come it works fine on simulator but not on breadboard, should i not rely on simulator ?

Can you kindly refers to some easy cicruit for hobby purpose. I have seen several with current source that make the build far more complex.
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Old 27th April 2014, 08:42 AM   #6
Mooly is online now Mooly  United Kingdom
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You should be able to make this one work and there are many reasons why simulation and practice vary, such as wiring interactions and so on with the ral version.

First thing to do with this one is short all the bias diodes out apart from one. Then add a feedback return network (any power amp will need this). To do this add a 1k resistor in series with a 100uf cap from the base of the second input transistor to ground.

Also, are they proper power darlingtons or are they made up from separate transistors ?

Like this...
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Old 27th April 2014, 08:43 AM   #7
arifs is offline arifs  Pakistan
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any idea on how to bias it correctly or look for some more complex schematic.
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Old 27th April 2014, 08:45 AM   #8
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by arifs View Post
any idea on how to bias it correctly or look for some more complex schematic.
Yes, but lets get your amp working first. If you short out the bias diodes leaving just one in place then the bias will be OK to get it all working.
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Old 27th April 2014, 08:45 AM   #9
arifs is offline arifs  Pakistan
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Thanks Mooly

mead it on breadboard, right on it.

using old 2SD313 as power stage and C945 and A733 just before power transistors, saw them in local some amp kits.
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Old 27th April 2014, 08:52 AM   #10
Mooly is online now Mooly  United Kingdom
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OK Breadboard layouts can be tricky. Make sure you don't have a load attached until all the DC conditions are proved good. That under about 200mv DC at the output. With the diodes shorted the output transistors should not be drawing current (but the amp will still all work).

It might be worth adding some resistors to the base of the output transistors. Anything from 220 ohm to 680 ohm.
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