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2n7000 lab - night two, no smoke

I decided to revist the experiement from last night. Since I let the smoke out of the parts last night I wanted to see if I could reproduce similar results (power levels) and maybe try tweaking a few things.

 

 

The initial schematic is similar to last night except that 100uH molded inductor was replaced with a 7.5uH inductor made with a toriod. (FT50-43 with 4 turns) That has a Z=476 @ 10MHz which seemed more realistic.

 


I then measured the power output for 1 FET and then reran the test with 2 FETs. I experimented with several different variations of broadband transformers to try to find a better impedence match... it had very little impact on the power level so I left it out.

Two realizations:
1. I am consuming less current than last night. (Why?)
2. I am generating less power/gain than last night.  (Why?)

 

 

Using my old oscope I tried to look at the waveform to see if I could tell what class the amp is running in. Frankly I can't tell. It looks sort of like class E or just a really ugly class C. --Hmmm... something to research.

 

Two variables from last night:
1. New FETs (I let the smoke out of the 2N7000 from last night)
2. The inductor is different (I let the smoke of the 100uH inductor last night)

Reflection:
I don't know if I had some awesome parts last night, a flaw in the test, something to do with the inductor, or some other wild card.    Last night consumed 4x the current and generated another 8.31dB.

I don't think that I am turning on the FET enough with just 10dBm of RF drive. I might need to look at biasing the gate a bit. That is an experiment for another night.

PS... I did some testing a few days later. I think that I am running into a couple of issues.

1. Some variablility differences between parts. (They are not RF/HF rated parts.)
2. The low output impedence without a matching circuit is causing the parts to fail in some cases immediately.

--I tried about 10-12 different parts and most of them were failing and had low gain values. Once I added an transformer to out the output I was getting HUGE amounts of gain (16-20db) but the efficancy was low because I was running in the class A-->C range not class E. At one point I had 400mW with 0dBm of drive at 10MHz.  I will retest this new scenario and likely post it on the blog once I have some better info.


73 de NG0R

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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