Solid State Shortwave Receivers For Beginners - The Listeners Guide

The output of Tr1 is developed across R1, and some of this
output is fed to the third winding on T1. This provides the regeneration
for the circuit, and VC2 is the regeneration control. The signal is inverted
between the gate and drain of Tr1, and so it is necessary to connect T1
with the phasing shown, so that it also inverts the signal, and provides
positive rather than negative feedback. For this reason it is essential that
the connections to the windings of T1 are as shown in the circuit diagram,
and that, for instance, the connections to pins 5 and 2 of T1 are not
reversed.
There must be a low level of loading on the output of Tr1 at radio
frequencies by theaudio circuit, or it may be found that sufficient regeneration
cannot be obtained. R.F. choke L1 has therefore been interposed
between Tr1 drain and the R.F. filter capacitor. L1 and C3 provide very
effective R.F. filtering, but allow an easy passage for audio signals. The
high impedance of L1 at R.F. ensures a very low level of R.F. loading on
Tr1’s output.
Operational amplifiers such as the 748 I.C. used here have two
inputs, a non-inverting one (+) and an inverting one (-). Normally these
devices are used with a centre tapped power supply when used in the
configuration employed here. The non-inverting input wouldconnect
to the centre tap. In this circuit a single supply is used, and the junction
of R4 and R5, in effect, forms the necessary centre tap.
The voltage gain of the I.C. is extremely high, being typically
some 100 dB (100,000 times!), and in a practical circuit it is necessary
to use a lange amount of negative feedback between the output and the
inverting input in order to reduce the gain to the required level.
The most simple way of doing this is to connect the input signal
to the inverting input of the I.C. via a resistor which has a value which
is equal to the desired input impedance of the amplifier. The inverting
input forms what is termed a ‘virtual earth’, and so the input impedance
of the circuit will be approximately equal to the value given to this
resistor. A resistor having a value equal to that of the required voltage
gain multiplied by the value of the input resistor is connected between
the output and inverting input of the I.C.
Unfortunately, this simple arrangement is often unsatisfactory
when a fairly high voltage gain is required, as it is here. This is because
the feedback resistor has to be extremely high in value, and this can
seriously upset the D.C. biasing of the amplifier. To overcome this
problem the feedback resistor has been made up from two resistors
in series (R6 and R7), and these have a low enough combined value
to provide stable D.C. biasing. Some of the A.C. feedback is decoupled
by R8 and C8, and this boosts the gain at audio frequencies to the
required level.
44