Re: [Amps] MOSFET biasing

[Steve Thompson <g8gsq@eltac.co.uk>]

Danny Pease wrote:
> Anyone have any experience with biasing MOSFET's for linear service,
> specifically the MRF174? 
>
> I have a 144 MHz 600 watt amp that uses 3 boards of pairs in push-pull.
> The existing bias arrangement switches between a negative cut off
> voltage and a positive voltage high enough to cause the transistors to
> draw some idle current.  All transistors see the same bias voltage.
> Since the transistors area not matched to each other in any way, the
> bias is fixed at a level that causes the lowest transistor idle current
> to be about 300 mA .  Some other transistors may run as high as 1 amp of
> idle current. The power source is 28 VDC.
>
> Am I correct in assuming that I would be better of with a circuit that
> would allow me to set the bias on each individual transistor or should
> the bias voltage be the same for each push-pull pair?

It's best to have individual control for each device. If you have that, 
then in all normal situations devices with the same batch code will be 
well enough matched for push-pull use.

Negative bias for cutoff shouldn't be needed unless you need to hold the 
output off while rf drive is present. If you use a simple circuit with a 
pot and the wiper to the gate, simply removing the +ve supply from the 
top of the pot will be sufficient if drive is removed at the same time. 
Always add a fixed resistor from gate to ground for each transistor - I 
usually use 100k. Without it, if your pot wiper goes open for an 
instant, the device can self destruct.
> What amount of idle current is needed to provide linear service with the
> MRF174?

The data sheet recommends 100mA, and this would be a good starting 
point. FETs can be a bit strange with bias - more current doesn't always 
mean better linearity.

Steve
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