Page 317 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

amplifier transistor сап handle and amplify both signals in а mostly linear таппег.

А genera'lized IF system using а one-transistor IF amplifier is shоwл in Figure 17-1.

IF Filter 2

Converter (") а1
IF Filter 1 Rin
(e .g.,2N3904)
Mixer

RE

FIGURE 17-1 А bIock schematic diagram of а typicallF system for а radio.
This figure shows ап IF amplifier system in which the first IF filteris а single tuned
circuit that couples into the base of Ql. А соттоп emitter amplifier generally is
used to allow for moderately high input re5istance (e.g.{ 500

to 2,000

) 50 that the first IF filter i5 not exces5ively loaded down. А heavy load оп the IF

filter will cause low signal and reduced Q. Becau5e the emitter i5 alternating-current
(АС) gгоuпdеd via capacitoг СЕ, the small-signa'l tгапsсопduсtапсе of the amplifier
i5 gm == I CQ1/(O.026 volt), where I CQ1 i5 the quiescent direct-current (ОС) collector
current.

The gain of the соттоп emitter amplifieг then is -gm R L , where R L is the
equivalent load resistance at the IF О.е., 455 kHz). Since the ОС current is set up
Ьу the voltage VE across RE,

<> 1

Note that changing the DC curгent also changes the transconductance of Ql. Ог
put another way, changing the base voltage at Ql re5ults in а change in
tгапsсопduсtалсе. For example, increasing the ОС voltage at the Ьа5е increa5es the
transconductance, wherea5 lowering the ОС voltage at the Ьа5е reduces the
tra ncond ucta псе.