Service manual

STP 11-25R13-SM-TG
D - 2
(2) If the bypass capacitor opens, there will be an inverse current feedback in the emitter and this
will decrease the gain. When the bypass capacitor is open, all the DC potentials remain the same.
Besides human error (measuring the wrong point), the only way you can observe the output of a common
emitter in phase with the input is to have a short between the base and the collector and thus, you would
have no gain.
(3) The components which may cause amplitude changes are R3 (load resistor), the transistor, and
C3 (bypass capacitor if open). The component that can cause a phase change is the transistor itself, if
shorted base to collector.
D-4. FET Waveform Troubleshooting.
Figure D-1. Common Source Amplifier
a. Review of Component Functions. Before we discuss waveform troubleshooting of an FET
amplifier, we will review the basic component functions of the common source amplifier. Refer to Figure
D-1 for the following explanation. Rl develops the input signal, R2 provides self bias for Ql, R3 develops
the output as the load resistor for Q1, Cl couples the input signal to the gate of Q1, C2 is the bypass
capacitor to prevent inverse current feedback, C3 couples the output from Ql, and Ql is the active
component responsible for the amplification and the phase shift.
b. Distorted Outputs. Once again, the components which affect the bias of the amplifier will distort
the output (change the class of operation). Since there is only one component (R2) that can change the
bias of the common source amplifier, then increasing, decreasing, or shorting R2 can cause a distorted
output. If R2 were to open, there would be no output because there would be no current. When talking
about transistors, we mentioned that the transistor could cause distortion. In the case of the FET, when it
breaks down, there is no output.
c. Amplitude and Phase Changes. As in the CE amplifier, the load resistor develops the output.
So, changes in the load resistor's value will cause changes in the output amplitude. If the load resistor
opens or shorts, there will be no output, the same as in the CE circuit. The bypass capacitor has the
same function as the common source amplifier. If it opens there will be a reduced output since there will
be an inverse current feedback.
d. Last of all, if there is a short between the gate and the drain of Ql, there will be both a drastically
reduced output and no phase shift.