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                        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.
Picture
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.











































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