Page 189 - Troubleshooting Analog Circuits

I 76 14. Real Circuits and Real Problems

Figure 14.2. Basic Single-Transistor Amplifier.

\ I+VW
R2 vc
c, 47K

J.
12v
Table 14.2 Troubleshooting Single-Transistor Amplifiers
Indication of Trouble Possible Cause Solution
Output at wrong DC level
Collector at +12 V? R2 broken or missing? Check ohms and volts.
Touch in 47 k across R2.
R3 shorted? Look for shorted foil.
R1 shorted? Look for shorted foil.
Collector at +10 V? Ql's base shorted? Measure base voltage.
Q1 's collector open? Check c-b diode.
Collector at +0.7 V? R3 broken or missing Check resistor.
or open? Touch in 120 k across R3.
RI or R2 has wrong Check resistors.
value?
Collector-base short? Look for shorted foil
or transistor.
Collector at 0 V? Short from collector Look for shorted foil or
to ground? shorted transistor.
Base at wrong voltage
Base at +3 V? Solder connection Make sure base and emitter
missing? are actually connected.
Base-emitter junction Replace Q 1.
probably blown?
PNP transistor? Double-check.
Base at -3 V? Wrong resistor values? Check resistor values.
Input signal much Check with scope.
too big?
No gain or bad gain Capacitor missing or Add a good capacitor
too small? across C 1 or C2.
Bad DC bias? Check for DC levels, as
above.
Q1 installed backwards? Check.
Oscillation. General: Study frequency of osc'n.
Power supply oscil- Check power supply; add
lating? more bypasses.
Load is oscillating? Short Q1 base to ground,
look at collector?
Load causes Q1 to Remove load, study load.
oscillate?
Low gain (non-inverting) Transistor broken? Check as above.
PNP transistor? Check.
etc., etc., etc.