Solid State Shortwave Receivers For Beginners - The Listeners Guide

CHAPTER 3
General Purpose Receivers
In this chapter a number of general purpose S.W. receiver designs
will be described. These will all give good results on both the amateur
and broadcast bands. Some of the circuits are extremely simple and
inexpensive to construct, and others are a little more sophisticated. In
general, and as one would expect, the more complex designs offer the
best performance and greater flexibility in use, but even the more simple
designs will provide excellent results.
Regenerative FET Receiver (Fig.17)
Field effect transistors (FETs) have several advantages over
ordinary bipolar transistors in the stages of a S.W. that handle R.F.
signals. There are two basic types of FET, Junction Gate FETs, and
Metal Oxide Silicon FETs. These names are usually abbreviated to
JUGFET and MOSFET respectively. Most MOSFETs currently in use
have two gates, and are called Dual Gate MOSFETs. Any of these
devices will work well in simple S.W. receiver designs, provided of
course, that they are intended for use at high frequencies. In the
present design a JUGFET is used, and this is the widely available
and inexpensive 2N3819 device.
The complete circuit diagram of the ‘Regenerative FET Receiver’
is shown in Fig.17, and as will be seen from this, apart from the FET,
only one other active device is used. This is a 748C operational amplifier
which is used here as a high gain audio amplifier.
If we consider this circuit in greater detail, the aerial signal couples
to the primary winding of T1, and the secondary winding of T1 together
with VC2 forms the tuned circuit. VC2 is the normal tuning capacitor.
The signals induced into the tuned circuit are coupled direct into the gate
of Tr1. An FET has three terminals, and these are termed the gate, drain
and source. These are the equivalents of the base, collector and emitter,
respectively, of a bipolar transistor.
The direct coupling to the gate of Tr1 will seem a little unusual
to those who are unfamiliar with FETs. Unlike a bipolar device, a FET
has quite a low resistance between its drain and source terminals with no
bias applied to its gate. Whereas an ordinary transistor must be forward
biased before it can be used as a practical amplifier, a FET must be
reverse biased. This is achieved by using source resistor R2 and holding
the gate at chassis potential. Usually a resistor is used to tie the gate to
chassis, but in this case the tuned winding of T1 performs this task.
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