I received some new N-JFETs today from Mouser.com. This time the component tester shows the $4 J310 part as an N channel JFETs, like it is supposed to.
I replaced the burned out resistors in my RF signal amplifier, and installed one of the new J310 FETs. This time before I powered up the circuit, I made one additional check to make sure I did not get smoke again. This is a check that can be used with any circuit, to make sure there’s not some short circuit or other catastrophic problem, and it uses Ohms Law. It only requires a multi-meter.
I measured the resistance across the two power connectors (+ and -) on the circuit board and got 161 ohms. Then I calculated the expected current draw based on using a 12.6v battery using Ohm’s Law:
Because this is a small-signal amplifier (for an HF receiver) I would expect the current draw to be very small–like a fraction of an amp. Anything else would suggest a short-circuit, or some other kind of problem (like a fake part, for example)!
By my calculations, the circuit should draw roughly 78 milliamps, which is a small amount of current–certainly not enough to blow a hole in a transistor like last time! After powering up the circuit, and seeing no smoke… and touching the transistor carefully to make sure it was not getting hot, I turned on the signal generator, and connected it to the input terminals of the amplifier board.
I set the signal generator to send a sine wave signal of 500mv (1/2 of one volt) at a frequency of 2MHz, and connected the oscilloscope to the output terminals of the amplifier board.
The oscilloscope showed the output of the amplifier to be a sine wave of 4 volts, an 8 times increase in voltage. Using the formula of dB=20 x log10(vout/vin) we get 18dB of gain, which is actually more than I was looking for. So I turned the adjustment potentiometer on the circuit board which changes the voltage on the gate of the N-JFET transistor, and reduced the voltage of the output to 2 volts, giving us a 12dB boost, which is what I was looking for.
I did one final test of this circuit. I had also purchased a bag of 100 cheaper N-JFETs (J113), costing about 25 cents each. (I bought these from mouser.com). I wanted to see if the circuit performed any differently with the cheap part, versus the much more expensive part.
Let’s see: $4 versus $0.25 would be a 16 times decrease in price, or -24dBd (dB in dollars. That’s a joke)
The end result is that the circuit performed identically with the cheaper part. Even the frequency cutoff was the same. Go figure… 73