Service manual

STP 11-25R13-SM-TG
D - 5
c. R2 Increase and Decrease. Since R2 is used for stabilization, increasing or decreasing R2 will
have a smaller effect on the common emitter's Q point. Nevertheless, it will change. When R2 increases
in value, the Q point will move towards saturation, because the bias point will increase slightly. If R2
decreases in value, the Q point will move toward cutoff, since the base bias point will decrease slightly.
Once again, this means there will be a change in the class of operation.
d. R3 Increase and Decrease. Since R3, the collector load resistor, is where the output is being
taken, a change in the value of R3 will change the slope of the load line. If R3 increases in value, the
current amplification factor will decrease and the voltage amplification factor will increase. The output
voltage swing (peak-to-peak) will be higher than normal. Likewise, if R3 decreases in value, the output
voltage swing will be lower than normal, thus decreasing the voltage gain of the circuit. As you can see,
the value of R3 is directly proportional to the voltage gain of the circuit.
e. R4 Increase and Decrease. R4 is used for thermal stabilization; increasing or decreasing its
value will have some effect on the circuit's operation. Increasing R4 decreases the gain and decreasing
R4 increases the gain. This change in the gain is not as large as changing R3, but it still affects the
amplification factor.
f. Cl, C2, and C3.
(1) If C1 opens, there will be no input signal applied to the base of Q1; therefore, no output signal
will appear at the collector. If Cl shorts, the DC potential of the previous stage will cause severe distortion
at the output of Ql.
(2) If C2 opens, although the circuit is still operating, the output signal cannot be coupled to the
next stage. If C2 shorts, the DC potential at the collector will be passed on to the next stage, causing
distortion.
(3) If C3 opens, the gain of the circuit will decrease. If C3 shorts, the thermal stabilization resistor
will be effectively shorted. This short will cause Ql to move toward saturation and a burnout of Ql may
occur.
(4) Generally speaking, if the components in the CB and CC circuits change, they will have very
similar effects on each of their respective circuits.