FET Preamplifier Circuit

Field Effect Transistor is an amplifying device in which the output current depends on the input voltage. The FET Pre Amplifier described here is a sensitive and stable Preamp circuit using an N-Channel FET and a PNP Bipolar transistor. This combination gives high input impedance and low output impedance with stabilized gain.

The Junction FET

FET Pre Amp Circuit diagram

The circuit is a sensitive preamplifier using an N-Channel FET and a PNP transistor. This combination gives high input impedance and low output impedance with stabilized gain. Any general purpose PNP transistors like BC 557 or BC558 can be used according to the required gain. The gain of the Preamp depends on the Feedback network and with the given values of R4 and R5, it is around 20. Input of the preamp is sensitive and readily matches with microphones and ceramic pick up.



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  • Arturo Esparza

    I am looking ideas and trying to keep myself learning,so while in an attempt to fix and improve an old stereo equipment(Fisher)that is almost gone,I Was trying to read and understand the different types of preamps and circuits,so I found this ones.I still don’t understand why we prefer to use discrete transistors when the IC’s supposedly have improved,evolved and are getting many advantages(according to the manufacturers,which are well known,have brand names and prestige,like National,ST,etc)
    In your personal opinion,are these still good options to build today? I am old fashioned guy. Honestly I like the simplicity of this circuits.But would it be better to look at for example what they call “Lo-Noise”,or “Jfet input Op amps”?Thanks in advance for your comments.

  • eko

    hello mr mohan
    please mr mohan, I find it difficult to get the transistor sk100, sk100 transistor is there a substitute?

    if there is let me know via email

  • aneel

    i am a student of electronic engineering in 2nd year.
    i am willing to join your group so, send me free differnt and latest electronic circuit diagrams.
    very thankful to you

  • D.Mohankumar

    Thanks for the suggestions. There was an error during drawing.R1 should go to ground to allow any ac signal applied through capacitor C2 to develop across R1. As you suggested, any general purpose PNP transistors can be used.Diagram corrected

  • Chinmoy Mitra

    The FET preamp circuit by Mr.D.Mohan Kumar is interesting and a bit unconventional. Though I have not yet built or tried the circuit, I am sure it will work. Yet I have a couple of points to mention about the circuit. The connection of the 1 Meg resistor in the gate of the FET is unusual; normally this biasing resistor should be connected between the gate and ground (negative) of the circuit. In this circuit, this resistor is connected in series with the input signal source, which is not recommended. Secondly, the output transistor used is the type SK100, which is overkill. This transistor is a medium power transistor, capable of handling upto 4 watts of power at a case temperature of 25 degrees centigrade, and it has a max current handling capacity of 500mA. A much better and cheaper alternative would be the popular low noise transistor BC549 or BC548 or any other general purpose transistor.

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