The following single stage amplifier may be used as a fixed or variable frequency RF or IF amplifier. The gain will depend on the device used and final circuit values but should be a minimum of 15dB at 150MHz and typically 20-30dB at lower frequencies. AGC or manual gain control may be implemented by making the Gate 2 voltage variable from 0 to + 4v.

Notes:

• The tuned frequency will be controlled by L2 / C1 and L3 / C2 and for most purposes these can be made the same value, I.e. C1 = C2 and L2 = L3.
• Additional RF selectivity may be implemented by using a multi-section bandpass filter instead of the single tuned circuit that is shown.
• The low impedance points will be determined by the turns ratio of L2 : L1 and L3 : L4 which would typically be between 7:1 and 10:1 for a 50 ohm environment.
• If this amplifier is required to be manually tuned from a front panel control then C1 and C2 should be variable and ganged together with C2 connected between drain and ground instead of as shown - a DC isolating capacitor may be used if required.
• If this amplifier is to be used ahead of the crystal set shown elsewhere on this site then the tuned circuit comprised of C2 / L3 / L4 should be replaced with an RF choke and the High-Z output connected to the tuned circuit in the crystal set. C1 should be ganged to the tuning capacitor in the crystal set.
• Keep the input and out circuits apart and if necessary, put a screen between the two to ensure stability. The inductors may be mounted in screened assemblies.
• As the 40673 is now very difficult to obtain it may be substituted with another type of dual gate MOSFET - in this case check the operating characteristics to ensure that the voltage and current ratings are not exceeded. UHF types should be avoided if the amplifier is to be used at much lower frequencies in order to minimise the risks of instability.