27-08-2013, 04:44 PM
PRACTICAL RADIO CIRCUITS
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BASICS
For a regenerative receiver to perform
well, three basic requirements have to be
met. (1) Its regeneration control must be
smooth, completely free from backlash,
and have a minimal effect on tuning.
(2) The tuned circuit to which the Q
enhancing positive feedback is applied
must be isolated from the aerial: failure to
do this will result in reaction dead spots on
frequencies (or harmonics) at which the
aerial is resonant.
(3) Provision must be made, at the
receiver input, for the attenuation of pow-
erful signals, otherwise the regenerative
circuit will lock onto strong carriers and it
will not be possible to receive weak trans-
missions on adjacent channels.
ARMSTRONG
UPDATED
A modern, transistor version of
Armstrong’s circuit, patented in 1913, is
shown in Fig.3.1. In the original, the feed-
back coil (tickler coil in the USA), L3, was
connected in place of the radio frequency
choke, L4, and regeneration controlled by
adjusting its proximity to tuning coil L2.
This was eventually superceded by the
variable capacitor system shown here,
where increasing the capacitance of VC1
increases feedback through L3.
Choke L4, in the drain (d) circuit of
TR1, acts as a load for the radio frequency
component of the amplified signal, con-
trolled amounts of which are fed back
through L3 to overcome losses in the tuned
circuit (L2/VC2) and increase its efficien-
cy or Q factor.
SEPARATION
During the valve era, the functions of
signal detection and Q multiplication or
regeneration were invariably carried out
by a single device. This combining of
functions can make it more difficult to
obtain the smooth, backlash-free control
of regeneration which is crucial to the
efficient operation of a receiver of this
kind. Best modern practice uses separate
transistors.
The dual-gate MOSFET circuit illus-
trated in Fig.3.2 can be used just as a Q-
multiplier by increasing the value of
capacitor C4 to 100nF. Filter components,
R4, C6 and coupling capacitor C7, can be
omitted when the stage is configured in
this way.
The “hot” end of the tuned circuit must,
of course, be connected to gate g1 of the
transistor, and resistor R1 is best retained
to hold gate g1 at 0V during coil changing.
SEMICONDUCTORS
The pnp, bipolar transistor used as the
r.f. amplifier, TR1, is not particularly criti-
cal. Any small signal device with an fT of
at least 100MHz and an Hfe of 200 or more
should perform well. The audio preamplifi-
er, TR4, can be almost any small signal npn
silicon transistor, but low-noise, high gain
(Hfe at least 400) devices are to be
preferred.
Most n-channel field effect transistors
should function in the detector (TR2) and
Q multiplier (TR3) stages. In addition to
the specified 2N3819’s, the BF244A,
BF245B, J304, J310, TIS14, K168D and
MPF102 have all been “in circuit” tested
and found to be satisfactory.
Note that base connections for all of
these devices vary and should be checked.