TC 9-62
Figure 6-18. Typical RF Amplifier for AM Radio Receiver
6-75. You may notice that no neutralization is shown in this circuit. This circuit is
designed for the AM broadcast band (535 KHz to 1605 KHz). At these relatively low RF
frequencies the degenerative feedback caused by base-to-collector interelectrode
capacitance is minor. Therefore, the amplifier does not need neutralization.
6-76. Figure 6-19 is a typical RF amplifier used in a VHF television receiver. The input
signal developing circuit for this amplifier is made up of L1, C1, and C2. The inductor
tunes the input signal developing circuit for the proper TV channel (L1 can be switched out
of the circuit and another inductor switched into the circuit by the channel selector). R1
provides proper bias to Q1 from the base supply voltage (VBB). Q1 is the transistor. Notice
that the case of Q1 (the dotted circle around the transistor symbol) is shown grounded. The
case must be grounded because of the high frequencies (54 MHz to 217 MHz) used by the
circuit. R2 provides proper bias from the emitter of Q1 and C3 is used to bypass R2. C5
and L2 are a parallel LC circuit that acts as the load for Q1. The LC circuit is tuned by L2
that is switched into and out of the LC circuit by the channel selector. L3 and C6 are a
parallel LC circuit that develops the signal for the next stage. The parallel LC circuit is
tuned by L3 that is switched into and out of the LC circuit by the channel selector along
with L1 and L2 (L1, L2, and L3 are actually part of a bank of inductors). L1, L2, and L3
are in the circuit when the channel selector is on channe12. For other channels, another
group of three inductors would be used in the circuit. R3 develops a signal that is fed
through C4 to provide neutralization. This counteracts the effects of the interelectrode
capacitance from the base to the collector of Q1. C7 is used to isolate the RF signal from
the collector power supply (VCC).
6-20
TC 9-62
23 June 2005
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