Page 326 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

гatio of the IF tгansformer should Ье higher than those of the IF transformers used
in coupling signal to а соmmюп-еmittег IF amplifieг. The input resistance in а

common-base amplifier is 119т instead Ofb/Qlm, which is the input resistance to а
common-emitter amplifier with the emitter АС Qlrounded. Therefore, the input
resistance of а com,mon-base amplifier is lowered Ьу а factor of Ь compared with
the input resistance of а common-emitter amplifieг.

For example, if the common-base amplifier's ОС collector current is 0.2 тА, the
input resistance is 1ЗО V. In а tapped coil ог а transformer, the impedance ratio is
the square of the turns ratio. The tuгnsгatio of the Т1 shou Id Ье оп the ordeг of at
least 25:1 that yields ап impedance ratio of 252:1, which means that the equivalent

resistance across the tank circuit is 252 Х 130

= 625 х 130

= 81

for Q = 46. Should the collectoг cuггent incгease, the turns ratio of the IF
transformer must Ье increased as well to preseгve sufficient Q for good selectivity ..

In Figure 17-38, the common-base amplifier is driven Ьу а series resonant LC
circuit. In а series resonant circuit, the desired goal is to load into а small input
resistance. Thus the common-base amplifieг fits the goal. Howeveг, seгies гesonant
IF transformeгs weгe not veгy соттоп in АМ transistoг гadios and were used
mainly Ьу the General Electric Сотрапу in the 1960s for some designs.

In teгms of the collector to 9round output гesistance in а common-base amplifier,
this output гesistance R o = гоС1 + gmRs), where R s is the souгce гesistance of the
signal source from the previous stage, and where R < < R E and gmRs < < ~. For
s
example, if R E = 1,000

and R s = 100

, and if 9т = 0.0384 mho for 1-тА ОС collector current, 9mRs = З.8. Let го 5 100 k

; then

1 +
)
Thus the output resistance is raised ,п а common-base amplifier compared with а
common-emitter amplifier with the emitter АС grounded.
If the emitter is not АС Qlrounded in а common-emitter amplifier, ог if С Е = О (по

capacitor ,п the circuit), then the output resistance from collector to ground in а
common-emitter amplifier with emitter resistor RE is