to the emitter through ground and R3 (the stabilizing resistor).
causes a voltage drop of 2.1 volts across R3, which reduces the forward bias
between base and emitter to the difference between +2.5 and +2.1 volts (base minus
emitter voltages), or +0.4 volt.
This small voltage is increased by the AGC
voltage connected to the emitter when a strong signal is received. Thus, a strong
signal causes the AGC voltage to become more positive; this increases the emitter
positive voltage, thereby reducing stage gain.
c. Output Signal Path.
The +6.5 volts applied to the collector circuit is
reduced to +5 volts at the collector because of the voltage drop in the collector
series resistors R4 and R5. The collector-current signal variations appear across
L2 and the collector resistors.
Note that capacitors C1 and C3 are ganged to
achieve tracking. Tank circuit L2-C5, tuned by C3, forms a high-impedance output
circuit. Resistor R4 and capacitor C6 combine to form a low-impedance circuit to
match the impedance of the mixer circuit.
A mixer stage using an NPN transistor is shown in figure 2-2.
Like the RF
amplifier just described, this mixer uses a grounded emitter common to both input
and output (base and collector) circuits.
a. Input Circuit. The input circuit receives the signal from the RF amplifier
through coupling capacitor C1. The signal voltage developed across R2 is impressed
across the base-emitter junction. The local oscillator signal that is to be mixed
with the input RF signal is coupled into the emitter-base junction by capacitor C2.