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Old 8th June 2004, 08:11 AM   #1
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Lightbulb Need Help On Push Pull Amplifier

Hi Everybody

I need your help on the following issue.
I have to construct a Class AB - push pull amplifier with MOSFET
Amplifier should be able to provide 21Watt output power (10.5
Watt RMS) at 8 ohm load (i.e speaker) .
The circuit suppose to be able to amplify input signal from 1 Vrms at
a range of 10 Hz to 40 KHz .
Requirements
1) Vdc should not exceed 50 Volts
2) Dc Current not to exceed 2 Amperes.
3) Construction of Amplifier should be with MOSFET , even if we can find
easier ways!

As a matter of fact is is a school assignment , and there is a plan given by teacher to work on it. Ofcourse we can propose our new schematic , if there is any , that will absolutely comply with the above rules.


Your opinion would be reall help for me .


Regards

George Vasiliou
GREECE
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File Type: zip push_pull_amplifier.zip (67.5 KB, 181 views)
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Old 8th June 2004, 11:55 AM   #2
djk is offline djk
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In no order:

Q4 will blow up, needs to be rated at peak to peak voltage swing at a minimum. 2N3904 is a 40V part, try a MPSA06 (80V).

Get rid of the 20dB pad on the input. Make R1 1K5, R2 15K

D1N4003 needs to be a fast diode, 1N4934.

R18 needs to be more like 4R7~10R

C6 and C7 will need to be more like 8000F for a 50hz line, and should be paralleled with about 10F.

R15 should be more like 2K0, and R14 100R.

C3 should have an RC time constant one octave above C6 or C7 and the load Z, or about 5hz, or 330F.

C5 will need to be increased, probably to between 22pF~100pF, use a 'scope.

Mount Q3 to track the heat of the outputs.

R3 looks like about 1K.
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Old 8th June 2004, 04:40 PM   #3
azira is offline azira  United States
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* Change R1 and R2 like prev post suggests but also change R15 to match R2 and scale R14 appropriately. This will help keep the base resistances of Q1/Q2 matched.

* C1 should have some kind of R with it to filter just frequencies above 100Khz or so (RF stuff). Othewise you'll shunt your entire input signal.

* C3 should probably be a non-polar cap or 2 bipolars back to back.

* What is C4 for? I don't understand bootstrapping so maybe it's a good thing to have. Seems like you can achieve your goals without needing anything like that.

* One idea you might consider, purely up to you, is swapping R9, R10, C4 for a current source. This will boost your open loop gain pretty significantly which is good especially since you seem to have a near unity or very small gain front end.

* There's a lot of oppinions about thermal runaway with MOSFETS. I don't understand a lot of it but if you're planning on your Vgs multiplier following your output devices then you might consider swapping the BJT for a small FET instead. It'll should work the same but will have more similar thermal tracking. On the otherhand you might not need to track your output devices at all.

Good luck, looks like a pretty straight forward uncluttered topology. Let us know how it sounds!!
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Danny
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Old 8th June 2004, 06:18 PM   #4
ekaerin is offline ekaerin  Sweden
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Default Azira, regard.Vbe multiplier

Azira,
I believe FETs usually have a positive temco and BJTs suffer from
a negative one.
Maybe it's OK in this app. it just struck my mind.....

Regards / Mattias

btw seems like a lot of fun in school nowadays.
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Old 8th June 2004, 07:04 PM   #5
azira is offline azira  United States
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Default Re: Azira, regard.Vbe multiplier

Quote:
Originally posted by ekaerin
I believe FETs usually have a positive temco and BJTs suffer from a negative one.
Maybe it's OK in this app. it just struck my mind.....
I'm aware of that perception a lot of people have. In general many MOSFET amps have been developed with no temp comp involved but I suspect it's not that simple. There have been quite a few active threads on this subject in the past few months. My take on it is that MOSFETS seem to have a pos temp coeff after a certain current but that current rating can change between different types of FETS. In some cases it's 100mA and in other cases it's 2A. That's a huge disparity that could made a difference in this design.

Add to this, you can also deal with thermal runaway in other ways than just a temp tracker such as larger emitter resistors or sufficient heatsinking.
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Danny
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Old 8th June 2004, 08:36 PM   #6
ekaerin is offline ekaerin  Sweden
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Danny,

I was referring to, what I thought, was your idea of using a FET
for thermal tracking.
I have never seen that before. (did I get it all wrong (usually))

Regarding output stages I agree with you, some type of MOS FET devices change sign of temco at certain levels of current.
I don't think everybody is aware of that.
Personally I have a feeling that the DC operating point of the devices is critical weather to have positive temco or not.

/ Mattias
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Old 8th June 2004, 11:04 PM   #7
azira is offline azira  United States
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I misread your post Mattias. Turns out you were just talking about using the FET for a Vgs multiplier. I've never actually tried doing it in a real circuit but here are my thoughts on it.
In terms of creating a "rubber diode" or a device which holds a constant voltage accross it's terminals, a FET or BJT can be hooked up in a similar fashion to do so. The difference being that inherently the FET creates a Vgs drop and the BJT creates a Vbe drop. This I've done many times in simulations.
I assumed that since the small FET was more like the output devices it would track tempurature related effects more similarily to the output devices than a BJT would. If the temperature effects of the output FETs make it self regulating then the FET Vgs multiplier should self regulate and eventually settle at a value that won't drift too much.
On the other hand, if the output FETs turn out to suffer from thermal runaway... then a FET Vgs multiplier might perform better drift tracking to a FET output device.
I suppose that is more of a theory of mine than an actual fact, maybe I shouldn't have suggested it to him
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Old 9th June 2004, 09:55 AM   #8
djk is offline djk
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"* Change R1 and R2 like prev post suggests but also change R15 to match R2 and scale R14 appropriately. This will help keep the base resistances of Q1/Q2 matched. "

That's all fine and good, but don't forget to change C3 if you change R14. With his DC trim pot the impedances don't have to match.

"* C1 should have some kind of R with it to filter just frequencies above 100Khz or so (RF stuff). Othewise you'll shunt your entire input signal."

What do you think R1 is for?

"* What is C4 for? I don't understand bootstrapping so maybe it's a good thing to have. Seems like you can achieve your goals without needing anything like that.

* One idea you might consider, purely up to you, is swapping R9, R10, C4 for a current source. This will boost your open loop gain pretty significantly which is good especially since you seem to have a near unity or very small gain front end."

A bootstrap is a constant current source, that's one of the main reasons its used. The other reason is to get higher drive voltage swing, very important in this kind of circuit with low power supply voltage and an output that needs very high bias voltage.

"* There's a lot of oppinions about thermal runaway with MOSFETS. I don't understand a lot of it but if you're planning on your Vgs multiplier following your output devices then you might consider swapping the BJT for a small FET instead. It'll should work the same but will have more similar thermal tracking. On the otherhand you might not need to track your output devices at all."

I always wonder why people make remarks like this. Pull the data sheet and look. Then there is less chance you will make an inappropriate remark. By accident you almost made a good remark, but it shouldn't be a small FET, it should be of the same type the outputs are.

I'll give you a hint, in addition to the tempco of the RDS, you also must consider the tempco of the Vgs.

Lateral MOS types are the only easy ones to use in a class AB amplifier, they are easy to spot because the Source is connected to the case. Vertical MOS are easiest to use in class A designs, the Drain is connected to the case.

Required Reading:

http://www.passdiy.com/pdf/citation.pdf
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Old 9th June 2004, 10:01 AM   #9
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R6 anyone?
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Old 9th June 2004, 10:48 AM   #10
djk is offline djk
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"R6 anyone?"

Probably doesn't need to be there, but if you yank it be sure to increase C5.

Here is where the original schematic was ripped from:

http://www.irf.com/technical-info/appnotes/an-948.pdf
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