I'm making a contact microphone circuit and they say to use a MPF 102 transistor. I was wondering is it ok to use a BJT (that's what I have on hand) instead of the FET. If so would I hook the circuit up differently with the BJT or would it be the same as the FET. Thanks.
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2 Answers
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It's not a great idea and that 3.3Megohm resistor gives you a hint why...
The FET has a very high input impedance, so that 3.3Megohm resistor defines the input impedance of the amplifier.
Designing a bipolar transistor stage with such a high input impedance is possible but not easy, and I believe you would need at least two transistors, possibly connected as a darlington.
The FET shown is a simpler solution; if you can't find the MPF102, there should be alternatives (2N5459 is probably suitable, I have used it in microphone applications)
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\$\begingroup\$ So what your saying is that there should be two transistors instead of 1 connected as a darlington like this... Using the 2N5459 transistors [link] (ischematics.com/webspicy/share/…) @BrianDrummond or that it's ok to change the transistor? \$\endgroup\$ Jul 12, 2014 at 13:36
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\$\begingroup\$ The 2N5459 is another JFET so if you can get it you can simply change the transistor. If you MUST use BJTs then a Darlington may be the easiest option. (There is no point attempting a Darlington connection with JFETs as in that schematic) \$\endgroup\$– user16324Jul 12, 2014 at 13:57
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You would need a different bias circuit to use a BJT. You'd also need at least 2 stages of gain to get from the high input input impedance to the 1.5k output impedance your circuit has.
