Re: µBITX & QRP labs 50W PA

Ashhar Farhan
 

Hans,
Even 25 ma idle current should be enough, given that it is a push pull amplifier and a large part of the power cycle keeps one or the other transistor switched on.
- f

On Fri, Dec 13, 2019 at 10:36 AM Hans Summers <hans.summers@...> wrote:

Hi all

Changing amplifier class is firstly a matter of altering the idle current. It requires an increase in bias to increase the idle current (NOT a decrease in bias). That is the first thing to try. 

But it would help to have capability to measure the results to see how good the linearity is and what the limitations are. Two tone audio test signal and RF spectrum analyzer. 

The circuit is essentially similar to my 10W Linear kit http://qrp-labs.com/linear 

Yes biasing the transistors for 125mA each (for example) would dissipate power straight away even with no input. That would be 2.5W per transistor at 20V supply. Whether it needs MORE cooling or not is not so straightforward to answer though. CW is a full power, intermittent carrier. SSB operates at a lower average level but without so many breaks (except when you take a breath etc). Any speech processing you have will increase the average power. Which is more, CW or SSB... I think it needs some experiment. Probably this amp can be biased to AB but I haven't had time to test all this as yet. 

73 Hans G0UPL 

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