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Old 22nd December 2009, 01:18 AM   #11
Bonsai is offline Bonsai  Taiwan
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Glen and richie00boy have given good advice. You have to drop 25mV across your emmitter degen resistors for best linearity and thermal stability. For this reason, a good emitter resistor value is 0.22 Ohms with 100 to 110mA of quiescent current per device. This will also moderate somewhat the transistor to transistor differences you are seeing, since at 0.1Ohm, you need bigger variations in quiscent current (I x R) to develop any emitter degeneration voltage, unless you set the quiescent current at a high intial value. OTOH, making Re too high is also problematic

Get hold of Self's Amplifier Design Handbook - he covers this subject in quite some detail.
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Old 22nd December 2009, 01:19 AM   #12
Bonsai is offline Bonsai  Taiwan
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I jsut saw AndrewT's post - also spot on!
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Old 22nd December 2009, 03:45 AM   #13
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your biasing method suits the deaf only.
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Old 22nd December 2009, 04:28 AM   #14
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Itīs easy to be a believer when you see a prove like this:
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Old 23rd December 2009, 12:29 AM   #15
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Quote:
Originally Posted by nigelwright7557 View Post
I usually find 5mA through each MOSFET is enough to get rid of crossover distortion.
Any more is just wasted as heat.
Mosfets are notoriously very non-linear in the low conductance region. When you measure output with the scope and determine it is no longer showing crossover, did you ever look at the gate drive signal? I bet the fb loop is at task to 'correct' lots of error around crossover. (of course fb doesn't actually correct errors ) Mosfets have lower transconductance than BJTs so they need more bias, at least a couple hundred mA. I prefer a bit more.

I have observed in my mosfet EC amp the gate drive error signal or 'distortions' at different bias levels. Even with class B bias, the output showed no crossover but the gate drive had much 'error'. With a few mA, the error is still very significant. It took about 300mA to almost completely get rid of the crossover error in the gate drive. Unless you are class A, there will always be some bit of crossover distortion in there. Your fb just shifts it to an inaudible frequency band. BJT's have very high transconductance compared to mosfet and do not need lots of bias to have optimum crossover. If youíre fretting over a few Watts of power, BJTís are your friend, I suppose. As for me, the substantial increase in SOA for the mosfet more than makes up for a few extra Watts of heat.


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Old 23rd December 2009, 12:34 AM   #16
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Quote:
Originally Posted by CBS240 View Post
Mosfets are notoriously very non-linear in the low conductance region. When you measure output with the scope and determine it is no longer showing crossover, did you ever look at the gate drive signal?
So long as the sine wave looks good into a speaker and it sounds good to the ear then I am happy with that.
The crossover distortion clearly goes around 5mA on my complimentary MOSFET amps.
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