Figure 3-1.
IF amplifier.
a. Circuitry.
Most IF amplifiers have rather simple circuit configurations.
They normally use inductive coupling through input and output transformers.
The
remainder of the circuit is associated with the amplifying element, in this case,
the transistor.
(1) The input transformer, T1, is usually double-tuned and coupled so as to
achieve the bandwidth required of the signal being passed. The secondary
is tuned to improve selectivity.
Tuning capacitors C1 and C2 are
individually adjusted. Many IF transformers use fixed capacitors and the
inductance is varied by adjustable slugs.
The primary winding of
transformer T1 is the tuned output circuit of the mixer. The secondary is
tapped at a low-impedance point to match the impedance of the transistor
input.
(2) The transistor is an NPN type connected in a common-emitter circuit.
(3) The output transformer, T2, is similar in most respects to the input
transformer. In communication-type receivers, one of the two windings in
the IF transformers is sometimes made movable so that the degree of
coupling can be varied.
The bandwidth can be adjusted to accommodate
narrow-band signals in this manner.
A second method of controlling the
bandwidth is to insert a band-limiting filter in the string of IF
amplifiers. Still another way is to insert into the IF channel a crystal
that is cut to pass a narrow portion of the IF band.
b. Biasing.
The secondary of T1 is connected to ac ground through C3.
A dc
ground is not used at this point because it would prevent the needed +0.7 volt from
appearing at the base-emitter junction.
This voltage is developed by resistance
network R1-R2 connected in series with the +6.5-volt dc source.
The emitter
voltage of +0.5 volt is developed by a collector current of about 0.5 milliampere
(ma) flowing through R3.
Also applied to the emitter through R4 is the AGC
voltage. This voltage reduces the forward bias on strong
322 L3
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