Coll 35/31 'Bahrain: application for W/T amateur transmitting licence for employee of Bahrain Petroleum Company' [‎86v] (172/250)

90
A GUIDE TO AMATEUR RADIO
around the circuit. This current depends on the
applied voltage and the R.F. resistance of the coil
and condenser. Obviously the lower this resistance
the greater the current at the resonance frequency,
and at the same time the smaller, compared with
the resonant current, will be the current at any
frequency other than the resonant one ; the ratio
of the current at the resonance frequency to the
current at some other frequency is known as the
“ selectivity " of the circuit. In practice the
resistance of the coil and condenser determines the
selectivity, and thence the current in the coil and
the voltage across it; obviously the most selective
circuit will provide the most voltage to apply to the
grid of a valve and at the same time reduce the
input of any undesirable frequency, i.e., interference,
in proportion. We can reduce this resistance to a
fairly low value by good coil and condenser design,
but a practical limit is reached and we have to
resort to another method.
Fig. 5.
Anode Bend Detector.
If the grid of a valve is connected to a tuned
•circuit to amplify the voltage across this circuit,
and some of the amplified voltage in the anode
circuit is returned to the grid in such a direction
as to increase the voltage already on the grid, the
grid total voltage is increased in effect, which is
the same thing as lowering the resistance of the
tuned circuit. If the amount of amplified voltage
feedback is increased until the amount of feedback is
such that all the losses in the tuned circuit are can
celled out, then the voltage and the selectivity will
be infinitely great ; actually in practice oscillation
of the valve takes place just before this ideal
condition is reached. Nevertheless, a considerable
reduction of the R.F. resistance and an increase
in the voltage is effected before oscillation occurs.
The actual increase of voltage is usually of the
order of 20 times. The effect of reaction, as this
process is called, is to introduce negative resistance
into the tuned circuit. In practice the reaction is
applied as a rule to detector valves and to the
I.b. amplifier of superheterodyne receivers. In
Fig. 1 the reaction on the detector valve V, is
applied through a variable condenser C 4 and the
separate coil (reaction coil) L 3 coupled to L,. In
some types of detector circuit the condenser" C. is
fixed, and the amount of feedback is controlled
by altering the amplification of the valve V 2 by
varying the anode voltage, or, when a pentode valve
is used, by varying the screen voltage. For the
reception of C.W. signals the detector valve is
allowed just to oscillate by increasing reaction, and
the tuning altered to give an audio beat or note
with the incoming carrier. Under these conditions
the detector is operating in its most sensitive and
selective state, which accounts for the generally
better reception of C.W. signals on T.R.F. receivers.
Audio Frequency Amplification
The output of any of the above types of detector
is usually amplified by an audio frequency amplifier
in order to increase the strength of signals, particu
larly where a loud-speaker is employed. In such an
amplifier the coupling between the valves may be
a resistance capacity network or, as in Fig. 1, an
A.F. transformer. Where a volume control is
required it is usual to use a potientiometer across the
secondary of the transformer or as a grid leak for
the amplifier valve V 2 in the case of resistance
capacity coupling, arranged in either case so that
the grid of the valve is connected to the slider
of the potentiometer. The valves for the A.F.
amplifier are given a grid bias such that they
operate over the straight portion of their charac
teristic curve. This grid bias may be obtained from
Fig. 6.
Leaky Grid Rectification.