Page 318 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

In terms of input resistance of the amplifier, the input resistance looking into the

base of Ql is

= (З _.-
i )

Note that while increasing the IDC collector current increases the transconductance,
it also lowers the input resistance.
For а collector current of 0.5 тА and а current gain ~ of 50,

, .

J
If the collector current is increased to 5 тА,

and if the collector current is decreased to 0.05 тА,

il
The collector of Ql then погmаllу is connected to another IF transformer, simple LC
filter, ог а ceramicfilter. As тепtiопеd previously, the ceramic filter requires some

prefiltering ог attenuation of the oscillator signal for р!горег band-pass filtering.
Without the prefiltering from another LC circuit, the ceгamic filter will have too
much leak-through of the oscillator signal, which еvепtuаllу will iпtеrfеге with
envelope dеtесtiоп of the АМ signal.
Detection of th.e АМ sigпа l typically is taken from the output of the output filter.
However, if тоге selectivity and sепsitivity аге needed, а second-stage IF ampHfier

is added after the output filter from Ql, and detection of the АМ sigпаl is taken
from the output filter of the second IF amplifier.
Figure 17-2А is а schematic diagram of а typical IF amplifier such as from а
commercially made transistor radio. The figure shows а one-transistor IF amplifier.
IF tгапsfогmег Тl couples а lоw-imреdапсе signal source (e.g., 500 V ог less) iпtо

the base of Ql.