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Trouble-Shooting Transistor Circuits
As you work your way through a circuit looking for problems, you can make a few simple checks with a voltmeter, to determine if each transistor is working correctly. Once you have identified a suspect transistor, a few additional checks with an ohmmeter will tell you if you should try a replacement device. (See the page called Testing Transistors ).
Base-Emitter Diode Test:
With the exception of saturated logic circuits, the base-emitter diode is always forward biased in a correctly operating bipolar transistor circuit. (In saturated switching circuits, the transistor is sometimes turned on hard, sometimes turned off)
Make this measurement with instrument power turned on. With your voltmeter set on a low voltage range (say 2V full scale), measure between the base and the emitter of the suspect transistor. For a germanium transistor, you should see 0.2 to 0.3 volts. For a silicon transistor, you should see 0.6 to 0.7 volts. If you don't see the forward bias voltage of the base-emitter diode, then the device is damaged or the bias circuit is faulty. The diagram to the right shows the polarities of these voltages for NPN and PNP devices, respectively.
Current Flow Test:
In a properly operating linear amplifier, collector current will be nearly the same as emitter current. To be precise: emitter current = collector current + base current. (Base current should be a small fraction of the total.)
Make this measurement with instrument power turned on. With your voltmeter set on a medium range, (perhaps 20V full scale) measure the voltage across the emitter resistor. In the circuit to the left, you would measure across R3. As an example, lets assume that you measure 10 Volts. If so, according to Ohm's Law, there must be 1mA flowing through R3. ( I = E/R )
According to our emitter vs. collector current approximation, there should be about 1mA of collector current too. Using Ohms Law again, we calculate that there should be 15 Volts across R4. ( E = I*R )
If the voltage across R4 is too high or too low, there is more or less collector current than we expected. There is probably something wrong with this device (very low beta, for example) or there is some other path where significant current is flowing (into or out of the next stage, for example).
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Last modified on  2/29/00 10:06:08 PM