Page 323 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

The gain of the IF amplifier from the base to the collector is -gmRL, where R L is the

equivalentload гesistance at the IF (i.e., 455 kHz) that is connected to the
collectoг.
Опе other "side effect" of соппеctiпg the collector to the coil as shown in Figure
17-2В is that the transistor's СсЬ сап mistune the IF transformer when the power
supply dгops in voltage. Since the base of the tгansistor is driven preferabIy Ьу а
low-impedance source, СсЬ оп the base side is almost grounded. Thus СсЬ is

almost like а capacitor from the collector to ground. Because the IF transformer's
internal capacitor is typically 200 pF, а change of about 4 pF in СсЬ when the
supply voltage changes will cause а 1 percent shiftin resonant frequency. Put
another way, if the IF transformer is tuned with the СсЬ of the transistor for 455
kHz, and if the supply voltage drops suchthat СсЬ changes Ьу 4 pF, the resonant

frequency will shift downward to about 451 kHz. This shift in resonant frequency
amounts to а slight mistuning of the IF transformer and сап гeduce the amplitude
of the 455-kНz sigпаl.
Thus the use of а tapped winding also reduces sensitivity to а shift in the resonant
frequency when СсЬ changes. For example, with а 3: 1 turns ratio, а shift in
capacitance of 4 pF with а tapped connection in the primaгy winding reduces the

shift сhапgе in capacitance Ьу з2, ог ninefold (see below). That is,

---
l.

that is added to the 200 pF, which is much lower than 4 pF added to the 200-рF
геsопаtiпg capacitor.
Alternatives to the common-emitteг amplifier аге the common-base lF amplifiers
seen in Figures 17-3А and 17-3В, the cascode amplifier seen in Figures 17-4А and

17-48, and the differential-pair amplifier sееп iп Figures 17-5А апd 17-58.
In the common-base amplifier, the gаiп from the emitter to the collector is 19 mR L ,
wheгe R l is the еquivаlепt load resistance at the IF (i.e., 455 kHz) from the
collector. То avoid the ргоЫет of feedback сарасitапсе from СсЬ, а gгоuпdеd-Ьаsе

amplifieг is опе sоlutiоп. What is meant Ьу grounded base is that the base of the
transistor is grounded in terms of the АС signal. Therefore, bypass capacitor СЬур
is attached to the base and gгоuпd. Iпsресtiоп of Figures 17-3А апd 17-38 will
show that СсЬ is поw grounded via the base lead of the transistor. Thus СсЬ just
adds capacitance to the tank circuit, but not much because of the tapped
configuration of Т2.