G4IZH Preamp

   Copied on 2014-01-12 from G4IZH’s website with permission

Simple Four Metre Pre-Amplifier

This is a simple dual gate mosfet pre amp for use in the four metre (70Mc/s) band.
There is nothing original about the arrangement but it is based on TOKO MC111 coils which are available on the internet. These coils are small and have dust cores for tuning, the primary winding is 6.25 turns and the secondary 1.25 turns which is Ideal for this purpose as fixed capacitors can be used to tune them avoiding the rather expensive trimmers.
The device used is a BF981 mosfet which provides about 24dB gain which is too much for the band due to the high noise floor experienced so a 6dB attenuator is used after the device in the form of R5/R6/R7.
The value of R3 in the device source was selected to give a drain current of about 10mA with a 13.8v supply, NB that the maximum current specified for this device is 20mA.
The layout of the pre amp is not critical and can be made on copper board or a pcb can be made.
It is important that a shield be fitted so that L1 cannot “see” L2 otherwise instability will surely result.
Many other mosfets may be used but the value of R3 will need to be adjusted to suit the parameters of the device used.


Leave a Reply