Dave Phillips wrote:
>
>
>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.
>
>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?
>
>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.
>
>Third, does anyone have the schematic? It contains 2 channels of 2 MRF148s
>driving 2 MRF150s, and was used in a medical application to drive 3cx800s.
>
Take a look at 'Radio Frequency Transistors' by Dye & Granberg of
Motorola (published by Butterworth-Heinemann, ISBN 0-7506-9059-3) and
also at the individual Motorola data sheets and Application Reports. The
D&G book doesn't contain detailed circuits, but it does explain *why*
the circuits in the App Reports are designed the way they are.
The big book of 'Motorola RF Application Reports' (HB215/D) is
obtainable free from the Motorola web site (sorry, I don't have the
address for the USA).
Why is this amp "obviously" Class C, Dave? If it has bias pots, it's
already capable of being forward-biased, especally if the bottom leg of
each bias network includes a thermistor.
One way to set up all the standing currents would be to use an accurate
digital ammeter, start with all transistors cut off, and then keep
adding precisely the same amount of current to the grand total.
As D&G say, "the idle current required to place the device in the linear
mode of operation is usually given in the data sheet."
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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