[AMPS] Biasing MOSFET Power Transistors

[Jon Ogden]

>
>
>I acquired a commercial power amplifier at Dayton, and I'd like to use it on
>6.  It's obviously biased for Class C right now, however.  Looking in my 
>data 
>books, nowhere is there a procedure given for properly biasing an 
>amplifier to 
>Class AB.

It will probably be a bit of work.  Class C bias circuits are typically 
much simpler.  So be prepared to add parts.  Probably the best thing to 
do is to just give you a typical class AB bias schematic and re-work the 
whole thing from there.

>
>The amp has 8 MOSFETs, each with their own bias pot.  But I don't have 
>individual current metering for each device, obviously.  My thought is to
>bias each device to cutoff, measure the remaining current draw, then one at a
>time set the bias pots to draw some amount of current.  Is there another 
>way to
>properly bias a multi-stage amplifier like this?

Hmm....I would be nervous about this.  What sort of current limiting does 
the amp have on the drain?  Many class AB circuits (at least for 
bipolars) have a diode in the base or gate I believe.  It's more than 
just changing the value of the bias current.
>
>Second question.  To what value should I set the quiescent drain current 
>to put
>the device (MRF150) in the linear region?  If the maximum rating is 16A, 
>do I 
>set it for 1/10 of that?  Is there a rule of thumb for this?  The common 
>source 
>unity gain frequency peaks at Id of about 7.5 A, but that doesn't seem 
>reasonable to use.

Well, when I was looking at this sort of thing, I would put my devices on 
a curve tracer and figure things out from there.  1.6 amps seems kind of 
high for quiescent current in the amp, so I don't know if you need to go 
that high.


One last thing:  The large signal impedance parameters of the devices are 
quite possibly going to change depending on the bias conditions.  So 
while the matching circutis are all designed to match 50 Ohms to a class 
C amp, they may not be correct for matching 50 Ohms to a class AB 
amplifier.  You may have to retune the input and output matches.

I'll see if I can dig around in a text book of mine on solid state amps 
and see if I can find anything else to help you.

73,

Jon
KE9NA



--------------------------------------------------------------------------
Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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