FET Pre Amp

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 has a conducting channel .It’s one end is the Source and the other end is Drain. The junction FET has direct ohmic contact at the gate. The enclosing material forms the Gate. If the channel is formed of N- material and the gate is formed of P -material, the FET is called as N-channel FET. If the arrangement is opposite type, then it is called as P-channel FET. The conductance of the channel can be controlled by the Gate-Source potential and then current flows from drain to source.

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.