Circuit 70MHz RF Stage (4m amateur band use) | Electronic Circuits

Circuit 70MHz RF Stage (4m amateur band use)

This is primarily intended for 4m amateur band use, and has a grounded gate FET, Figure 6. A grounded gate stage of this type is particularly stable, and no isolation other than that provided by a layout similar to Figure 4B is required, to avoid oscillation. The gain obtained is less than with a grounded source stage. The tuning of L2 is particularly flat.

R1, with the by-pass capacitor C1, is for source bias, and must be tapped down L2 as the input impedance of TR1 is low with this circuit. It is possible to obtain a slight improvement in results by tapping the drain down L3. The supply can be 9v to 12v, and can generally be from the equipment with which the stage is used.

L2 and L3 could be tuned by trimmers, and be air cored. However, solid cores suitable for 70MHz are easily obtained, so C2 and C3 may be fixed, resonance being obtained by adjustment to the cores then employed with L2 and L3.

L2 and L3 each have ten turns, of 26swg enamelled wire, side by side on 3/16th in diameter (or 4mm to 5mm) cored formers. L1 is overwound on the grounded end of L2, or tightly coupled to it, and has three turns. L4 has two turns, similarly coupled.

TR1 is a VHF type transistor with an upper frequency limit of at least 200MHz. The BF244, MPF102, and similar types can be used. For best possible results with individual samples, R1 and the tap on L2 can be modified, but are not critical.

This circuit is readily adapted for 144MHz use. Self supporting air cored coils, with parallel 10pF trimmers, can then be fitted. L1/L2 can be five turns in all, of 20swg wire, wound to have an outside diameter of 8mm, and with turns spaced so that the coil is 10mm long. A tap for the aerial connection is one-and-a-half turns from the grounded and of L2, and the source tap (C1, R1) it two turns from the grounded end. L3 is similarly wound. The drain lead can now be tapped to L3, three turns from the C4 end of this winding. L4 is a single turn of insulated wire, closely over L3.

As mentioned, the grounded gate stage will not be found to increase signal strength to the extent obtainable with circuits as Figure 4, but it has the merit of simplicity, or for isolation from the aerial.

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