high impedance input JFET/BJT buffer circuit

Posted on Feb 6, 2014

this is a rather common cookbook circuit that you can find in books and on the net, yet personally i have yet to see it in stompbox circles, so i thought i would post it. it uses an npn JFET DC coupled with a pnp BJT . the FET allows a a much higher input impedance than a BJT normally would. R1 could realistically be anything from 1M to 10M, this will only really affect the low end coupled with C1(which i have left open for you to decide the value).

high impedance input JFET/BJT buffer circuit
Click here to download the full size of the above Circuit.

the BJT is "inverted" so that a pnp transistor can be used with a negative ground circuit, for what i presume is for it's lower noise specs. i haven't measured the output current of this configuration versus a single FET buffer, but that could be something i'll do in the future and report back. the real reason i am posting this buffer is because it can easily be turned into an amplifier stage, which can subsequently be used for a sweet booster/overdrive. all that and more in the next installment of CIRCUIT SPLINTERS! bonus question: anyone want to tell me where to place the resistor in order to turn this buffer into an amplifier?

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