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, if less gain is required these values are adjusted accordingly.
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.
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, if less gain is required these values are adjusted accordingly.
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.
With thanks to G3EJS for the artwork.
Parts List
R1 8k2
R2 8k2
R3 180r
R4 150r
R5 39r
R6 150r
R7 150r
C1 33pf
C2 1n
C3 1n
C4 10n
C5 10n
C6 33pF
TR1 BF981 (or equiv, see text)
L1/L2 TOKO MC111 (6.25t PRI, 1.25t SEC)
NB, By using the 33pF capacitors (C1 and C6) to tune L1 and L2 it should be found that the dust cores are about level with the top of the formers which also means that maximum coupling exists between the primary and secondary windings.
