2n7000 lab - aka:letting the smoke out

I have been reading a lot of material around class E amps. The reality is that until you start to play with a transistor at the workbench some of the research is simply academic. It is important, but you need to apply some power and see how it works for yourself.  --Maybe even let the smoke out.


I decided to start simple. Forget tuned circuits and the magic math. I plugged in a 2n7000 on a breadboard and wired up a simple amp. Since a FET is a voltage component my plan was to adjust the voltage on the bench power supply to change the gain of the circuit. In this case I am trying to drive something between class C and class E but I am nearly sure what I am going to get until try it.  In this case I am using about +10dBm to turn on the gate during the positive side of the signwave. (A lowpass or bandpass filter would be needed for real world use to bring the harmonics within legal limits.)


I was amazed to see about 13dB of gain with 12vdc and .04a. The FET never even got warm which is a good thing. (I suspect that probably had a very poorly match circuit and might have seen more power if I had a transformer at the output to convert the low impedence output of the FET to something closer to the 50 ohms of the test gear and dummy load.  I also could probably use a smaller inductor value but in this case I choose to use a component from the parts bin vs. calculating and winding an inductor on a toriod.)

After the basic single FET version of this I added a second FET in parallel to the first one with poor results.  I suspect that the output impedance was too low compared to the load. I then made a transformer (6:4 ratio) to see if that would improve the performance. It did help to improve the performance almost back to where I was at with the single FET up until the point where I let the smoke out of the FET and inductor. I suspect that inductor was too large and acting as a resistor and the output impedence of the parallel FETs was too low for the load at the output.

When time permits I need to revist this experiement and try to achieve a better match for the output of the parallel FETs. I also need to play with a output network to see if I can create a flywheel effect for the output of the FETs.

I have a new oscope on order which will help evaluate what class the amp is really running in. It is probably closer to C than E right now.  :-)   This a fun way to burn some time tonight after dinner.


73 de NG0R