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Old 11th February 2002, 02:26 AM   #1
hugeli60 is offline hugeli60
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Default MOSFET Help

Question for any of you out there (Mr. Pass please help) about MOSFETS in a Class AB configuration.

What is the most perfered method of providing bias for the devices?

1. A couple of resistors
2. Some Diodes (Zener or others)
3. Vbe Multiplier
4. Any other wierd config.

Is it neccessary to provide lots of thermal protection like BJT's? Is it nessary to mount the Bias device(s) near the output devices?
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Old 11th February 2002, 03:32 AM   #2
GRollins is offline GRollins  United States
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The best method for bias will depend on the rest of the circuit.
MOSFETs are not prone to thermal runaway in the manner that bipolars are. With reasonable care they'll stay well within bounds.
As long as your bias scheme remains stable, you're free to put it wherever you like.

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Old 11th February 2002, 03:48 AM   #3
jam is offline jam  United States
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I have found that a couple of resistors or a trimpot work fine with lateral mosfets (eg. Toshiba and Hitachi).

This is not so true with vertical mosfets (eg. International Rectifier) because their temperature co-efficient is positive till quite high bias currents. In this case I would advise the use of a Vbe multiplier.

Mr. Pass should be able to shed more light on the subject.

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Old 11th February 2002, 12:29 PM   #4
traderbam is offline traderbam  Europe
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There are two main things to consider: how to control the bias voltage and how to make the bias circuit look invisible to the signal path. In most configurations the drive current for one of the FETs has to flow through the bias circuit. If the circuit has any impedance then the voltage across it will be modulated by the gate current (plus any other currents, eg: if you are using a resistor load on the Vgain stage or a bootstrap). Normally this is undesirable, so you want to make the bias circuit have low impedance.

I would say if you need a fixed bias voltage then use a diode chain with a good HF capcaitor in parallel (10uF or more). If it needs to be adjusted then use a BJT Vbe multipler. A single BJT with resistor, pot and parallel cap will do just fine. Both these circuits will have better voltage stability and lower impedance than a simple resistor chain or zeners. If your FETs are thermally unruly then you can mount the semiconductors on the heatsink or use a thermistor with the BJT circuit. You'll have to study the FET datasheet to work out whether you need this or not.

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Old 11th February 2002, 07:47 PM   #5
Nelson Pass is offline Nelson Pass  United States
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I like a Vgs multiplier; it looks just like a Vbe
multiplier except it uses a MOSFET and typically
higher resistor values. It gives voltages more
in the range you will want, and the thermal character
of the MOSFET is more similar to the MOSFETs on the
output stage.

The other approaches work also, so just pick
something and see.
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Old 7th June 2002, 11:36 AM   #6
Circlotron is offline Circlotron  Australia
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I like the Vgs multiplier too, but with fixed equal value resistors, and fed from an adjustable current source. It and the output devices together form a current mirror. See my post here. If it has holes in it please tell me!

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