Transistor biasing


From: W. Daniel (pandora!daniel@Think.COM)
Date: Wed Mar 30 1994 - 00:17:45 EST

Hi Gang,

        Ok, more technical questions to keep the grey matter busy. I have
built a push-pull MOSFET RF Tx Amp which is supposed to take 2 watts and
output 20 watts. The original design is a linear one but since I will be using
it for CW work only, can I omit the biasing circuitry?

        Secondly, I am driving the amp with 8 watts instead of 2 watts and
getting some 30 watts at the output. Does overdriving a MOSFET type amplifier
at CW present any problems?

        Thirdly, can I get rid of the negative feedback network since I will
be using this on only one band (20m)?

        Fourthly, the transistors get REALLY HOT! And I mean really really
hot. I am surprised it didin't melt. This is during actual operation for some
considerable time. Can I replace the MOSFETs with another type in order to
have lower power dissipation? What else can I do to keep the amp cool other
than use a bigger heatsink?

        Fifthly, the toroids get hot too! Does this mean I need to use a
bigger toroid (binocular types)?

        I have checked and there are no parasitic oscillations so that I know
the heating is due to the driving and amplifying action, not due to stray

        Finally, can anyone send me any articles for constructing HF (20m) RF
Amps with output power in the region of 40 to 80 watts? Preferably MOSFET
designs but bipolar design is also acceptable. Or refer me to any GOOD kits
for such a purpose.

        FINAL Finally, in high power amplifiers, are QSK done with PIN usually
or with relays?

        Tks in advance for any advice :)


p.s. I still have an OPTO-ELECTRONICS Model 2300 Handheld counter which I
        am willing to trade or sell for anything reasonable. 1 week old.

| Daniel Wee  |             | ** Man needs more
| UUCP1.12b   | |  than a new start, he
| SNEWS 1.91  |!pandora!daniel             |  needs a new heart! **

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