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?
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.

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.


Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
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.