Wednesday, February 1, 2012

Update on Power Amplifier

Um...  Ok...  I lied...  It is just plain ugly construction...

But, hey...  We are trying to prove a concept, right?  :)  Well, time to see if I can let the magic smoke out of it...  I have not built the output 5 pole filter yet, there will be time for that once things check out a bit.

I hooked up the 12V current limited supply and with the current limit just above the minimum current shutoff, slowly turned up the voltage to 12 VDC and there was no current draw.  So far so good.

Next I hooked up a dummy load, scope to the output and my beacon on the input.  Here is the lashup:

I had set the bias pot all the way down.  With the beacon supplying about 1.2 V of signal to the input, I slowly adjusted the bias until the finals kicked in.  There is a noticeable threshold effect from the unbiased output stage, but it kicked in at about 2.1 VDC of bias.  I ran it up to around 2.8 VDC and the current limiter kicked in (remember it is set to turn off at a very low current value.  For the moment I have bias set a 2.75 VDC and am seeing this on the output.  Remember there are no low pass filters yet in place and the bias is just rough set.  Pretty ratty looking, rather terrible really but about 5.5 V P-P into 50 ohms.  There is no skipping a low pass filter on this device.

This is would represent about 75 mW, given the unfiltered output.  There is quite a bit of energy going to  harmonics with this waveform and we really have not explored the bias setting or allowed the entire amp to consume more than about 6 mA, so the finals are not even getting slightly warm yet.  More testing to come.

So, it's time to build a low pass filter and do some more adjustments and performance tuning.

Note to self:  Gorilla glue sucks...  Don't use it for breadboarding circuits...

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