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Old 3rd June 2005, 07:29 AM   #21
AndrewT is offline AndrewT  Scotland
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Hi,
I will have to look again as you suggest. There is something I cannot understand here.
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Old 3rd June 2005, 01:07 PM   #22
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Quote:
Originally posted by mod_evil
Hi friends, he necessite a simple amplifier because in rio de janeiro, we arent found a diverse components.

Portuguese version;
Olá amigos, ele necessita de um simples amplificador, porque aqui no rio de janeiro, nós não achamos diversos componentes"
Thanks for the post. I have the book: "Designing Power Amplifier and Hi Quality Amplifier"

It's grate, but i want a more amplifier base.
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Old 3rd June 2005, 02:55 PM   #23
Mr Evil is offline Mr Evil  United Kingdom
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If you want simple, then they don't come any simpler than this one. One op-amp, two MOSFETs and two resistors; just five components for a complete amp (best to use at least two 100nF supply bypass capacitors too, although it is often possible to get away without them depending on exact component choices and layout). I often throw this one together on a breadboard for testing purposes.

Use lateral MOSFETs for lower Vgs. Yes, crossover distortion will stil be high with no biasing at all, but it's surprising just how low it can be with a good fast op-amp. Certainly good enough for non-critical applications and it can be all but unnoticable at high output powers. For high output power use an op-amp like OPA445, for up to +/-45V supply, or thereabouts.

Minor increases in complexity can yield improvements. For instance, a single resistor from the op-amp output to the MOSFET sources will partially compensate for crossover distortion by allowing the op-amp to supply the first few mA of current to the load. A high output current op-amp helps with that. Alternatively, for slightly more complexity, a few diodes and a couple of resistors can bias the output stage into class AB.
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Old 3rd June 2005, 03:22 PM   #24
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Quote:
Originally posted by Mr Evil
If you want simple, then they don't come any simpler than this one. One op-amp, two MOSFETs and two resistors; just five components for a complete amp (best to use at least two 100nF supply bypass capacitors too, although it is often possible to get away without them depending on exact component choices and layout). I often throw this one together on a breadboard for testing purposes.

Use lateral MOSFETs for lower Vgs. Yes, crossover distortion will stil be high with no biasing at all, but it's surprising just how low it can be with a good fast op-amp. Certainly good enough for non-critical applications and it can be all but unnoticable at high output powers. For high output power use an op-amp like OPA445, for up to +/-45V supply, or thereabouts.

Minor increases in complexity can yield improvements. For instance, a single resistor from the op-amp output to the MOSFET sources will partially compensate for crossover distortion by allowing the op-amp to supply the first few mA of current to the load. A high output current op-amp helps with that. Alternatively, for slightly more complexity, a few diodes and a couple of resistors can bias the output stage into class AB.
So good and simple this amplifier. To decrease the crossover distortion, can i put a 1.2V bias in gate of MOSFETs ?

For the MOSFET can i use the IRF640 ?

I am using the Circuit Maker program to test circuits. is it a good program ?
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Old 3rd June 2005, 04:34 PM   #25
Mr Evil is offline Mr Evil  United Kingdom
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Quote:
Originally posted by PicancoNet
So good and simple this amplifier. To decrease the crossover distortion, can i put a 1.2V bias in gate of MOSFETs ?
1.2V is about right for lateral MOSFETs. You'll want to set it so that quiescent current anything up to about 100mA. Less gives more efficiency, more gives lower distortion.


Quote:
Originally posted by PicancoNet
For the MOSFET can i use the IRF640 ?
You could (plus its complement for the P-channel). Being a vertical MOSFET you will need a lot higher bias voltage, plus you need to ensure thermal stability via source resistors and thermally coupled Vbe multiplier.
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Old 3rd June 2005, 05:33 PM   #26
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Quote:
Originally posted by Mr Evil

1.2V is about right for lateral MOSFETs. You'll want to set it so that quiescent current anything up to about 100mA. Less gives more efficiency, more gives lower distortion.



You could (plus its complement for the P-channel). Being a vertical MOSFET you will need a lot higher bias voltage, plus you need to ensure thermal stability via source resistors and thermally coupled Vbe multiplier.

I made this circuit. Is it realy can give 50W at 2V@4ohms in input ?
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Old 3rd June 2005, 05:41 PM   #27
AndrewT is offline AndrewT  Scotland
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Hi,
you just blew up your 741 putting 80v across it.
you have to limit the Vrail to the same voltage as your opamp requirements.
OR use a voltage amplifier between the opamp and the output stage.
That one by Amplifier guru does that, even though I still can't understand it.
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Old 3rd June 2005, 05:45 PM   #28
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Quote:
Originally posted by AndrewT
Hi,
you just blew up your 741 putting 80v across it.
you have to limit the Vrail to the same voltage as your opamp requirements.
OR use a voltage amplifier between the opamp and the output stage.
That one by Amplifier guru does that, even though I still can't understand it.

I put the 741 only to reference. It is a hypotetic Op Amp
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Old 3rd June 2005, 05:49 PM   #29
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Why are you all still fascinated by some simply ***** ? Wake up guys, now is the time, when useable transistors cost only 5 cents ! Simply Zen, simply GC, simply ... Beauty is in complicated connections, only with them you can make right amp.
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Old 3rd June 2005, 06:15 PM   #30
edl is offline edl  Hungary
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What about this?

With a 2*12V, cheap and small psu trafo.
With cheap darlingtons (BDW93/94C for example)
In class AB or class B, or even in class A
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