JFET PREamp

  
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The input transistor Q2 is an N-channel JFET. Each particular JFET sets a particular output bias voltage, so the JFET must be selected from among multiple devices for the desired bias value. Fortunately, due to the low current involved, desirable devices can come from a wide range of device types. JFETs also vary with respect to gain and noise lev
JFET PREamp - schematic

el, so best operation requires some selection there as well. However, other than exceptionally bad noise in particular devices, a wide range of JFETs function surprisingly similarly. The input resistor R1 just drags the JFET gate to ground potential and so reverse-biases the FET. The value should be at least 10x the signal source impedance, but probably could be as high as 100 megohms, if that was useful for some reason. Larger resistors do produce somewhat more Johnson noise, but that is normally "shunted" or "shorted out" by a low-impedance signal source. The second transistor Q3 is a bipolar PNP. One might think to make it a low-noise low-current-linear device, but in practice that does not seem to matter much. Many different PNP devices can be used with no circuit change and remarkably little difference. The resistor across the base-emitter junction of the PNP R2 sets the "standing" or "quiescent" current for the input transistor. The PNP operates on the "knee" of turn-on, at about 0. 6V. The recommended value of 5K allows about 0. 6/5k = 120uA to flow through the JFET before the PNP starts to turn on, which then increases the output voltage and starts to turn off the input transistor. Since feedback keeps the current through the input transistor almost constant, resistor R2 can be seen as a particularly simple form of constant-current source. The low operating current allows the JFET gate to operate at a bias voltage...



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