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Old 21st July 2003, 08:14 PM   #1
subwo1 is offline subwo1  United States
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Default Class AB

Hello all. I just finished a simulated design of an amp. I am incapacitated now and cannot build it and test it. I plan to check back later to see comments. Thanks.
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Old 22nd July 2003, 05:55 AM   #2
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Default Changes to ClassAB-N-Ch

Hello all,
I have made some changes to the circuit. I want to keep the part count down to the minimum which will still do a good job. Any suggestions or comments?
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Old 22nd July 2003, 06:26 AM   #3
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Default Re: Changes to ClassAB-N-Ch

Hi!

Quote:
Originally posted by subwo1
Any suggestions or comments?
Yeah! What semiconductors to use? Any special types in mind, or just rough specifications?

Bye

Arndt
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Old 22nd July 2003, 06:45 AM   #4
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Hi.

The BJTs are general "generic" types. The junction FET is given as a 2N5545, but you could use whatever you have and adjust the value of R3 to compensate for the different pinch-off voltge. The mosfets are IRF510, but the breakdown voltage is set higher than the rated 100V in the simulations I did.
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Old 22nd July 2003, 07:27 AM   #5
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Interesting. Any simulation results?

Jan Didden
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Old 22nd July 2003, 08:26 AM   #6
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Default Basic Simulation

I am not the greatest with simulations, but I hope this one helps. I overdrove it a little to observe how the clipping is handled. I think the transitions into and out of ckipping is a useful indicator.
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Old 22nd July 2003, 08:51 AM   #7
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The first version probably has a dead zone (class C operation causing cross-over distortion). It looks like the quiescent current of the output stage in the second version is very dependent on the current gain of Q6 and Q8, as well as on the device parameters of M1, M2 and the JFET. At least you should add a bias adjustment potmeter somewhere, for example in the JFET tail current source. Try it with some form of current limited supply when you first switch it on, and see what happens if you use a hair dryer or so to change the temperature, because it is not clear to me how stable the quiescent current is going to be.
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Old 22nd July 2003, 08:59 AM   #8
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Default Q4,5,6,8 operating point.

Hi ,
I noticed that the Base currents of Q6 and Q8 are the collector currents of Q5 and Q4 respectively.
Is this your design point ? I would have expected the base currents to be much lower than the operating collector current in Q5 or Q4. Unless Ic of Q6 and Q8 is quite high and they have a low hfe.
Or maybe I have not understood the circuit well enough.
Cheers.
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Old 22nd July 2003, 09:10 AM   #9
subwo1 is offline subwo1  United States
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Quote:
Originally posted by MarcelvdG
The first version probably has a dead zone (class C operation causing cross-over distortion). It looks like the quiescent current of the output stage in the second version is very dependent on the current gain of Q6 and Q8, as well as on the device parameters of M1, M2 and the JFET. At least you should add a bias adjustment potmeter somewhere, for example in the JFET tail current source. Try it with some form of current limited supply when you first switch it on, and see what happens if you use a hair dryer or so to change the temperature, because it is not clear to me how stable the quiescent current is going to be.
Yes, the first version can be forgotten about. Your idea about the potential drift of the quiescent current is good; R3 could be lowered a little and a potentiometer placed in series with it. That would make the adjustment easier.
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Old 22nd July 2003, 12:53 PM   #10
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Default Re: Q4,5,6,8 operating point.

Quote:
Originally posted by ashok
Hi ,
I noticed that the Base currents of Q6 and Q8 are the collector currents of Q5 and Q4 respectively.
Is this your design point ? I would have expected the base currents to be much lower than the operating collector current in Q5 or Q4. Unless Ic of Q6 and Q8 is quite high and they have a low hfe.
Or maybe I have not understood the circuit well enough.
Cheers.
Hi.

The base currents of Q6 and Q8 can be allowed to decay without pull down resistors because they drive the MOSFETs, which have much higher input capacitance than the output capacitance of Q6 and Q8. The MOSFETs are allowed to operate fairly slowly, so I wasn't concerned about speed for Q6, and Q8. I hope I explained it well enough.
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