MOSFET preselector circuits

Drain

Drain

MOSFET transistor

MOSFET transistor

Source

Preselector Circuit
7-6 MOSFET active preselector circuit.

The inductors are of the same sort as described. It is permissible to put and Ls in close proximity to each other, but these should be separated from L4 in order to prevent unwanted oscillation because of feedback arising from coil coupling.

The circuit in Fig. 7-7 is a little different. In addition to using only input tuning (which lessens the potential for oscillation), it also uses voltage tuning. The hard-to-find variable capacitors are replaced with varactor diodes, also called voltage variable-capacitance diodes. These PN junction diodes exhibit a capacitance that is a function of the applied reverse bias potential, VV Although the original circuit was built and tested for the AM broadcast band (540 to 1610 kHz), it can be changed to any band by correctly selecting the inductor values. The designated varactor (NTE-<518) offers a capacitance range of 440 pF down to 15 pF over the voltage range of 0 to +18 Vdc.

The inductors might be either "store-bought" types or wottnd over toroidal cores. I used a toroid for Li/L> (forming a fixed inductance for and "store-bought" adjustable inductors for L? and Lv There is no reason, however, why these same inductors cannot be used for all three uses. Unfortunately, not all values are available in the form that has a low-impedance primary winding to permit antenna coupiijig.

Figure 7-7 shows the connection points for the dc power (+9 to +12 Vdc). These points are 0.001-pP ceramic feedthrough capacitors. These are a little liard to find locally, but can be bought from Newark Electronics or other distributors.

in both of the MOSFET circuits, the fixed bias network used to place gate G<> at a positive dc potential can be replaced with a variable voltage circuit, such as Fig. 7-8. The potentiometer in Fig. 7-8 can be used as an RF gain control to reduce

+ii Vic

+ii Vic

Varactor Tuned Vfo Circuit
7-7 Varactor-tuned MOSFET active preselector circuit.

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