R

base voltage = R1 , R9 x's collector

111 T *" voltage

In the normal low-level amplifier stage with a quiescent collector current of approximately 1 mA, the base-emitter voltage is of the order of 200 millivolts in a germanium transistor. or 700 in V in a silicon transistor. The emitter voltage can be estimated by subtracting this voltage drop from the base voltage.

The emitter current is equal to the emitter voltage divided In the value of the emitter resistor, and the collector current will he about the same. The same analysis applies to the circuits of Fig, 2B-C.

Current Measurements

Direct current measurements sometimes provide useful clues to the operation of a circuit than voltage measurements. To make current tests, however, the circuit must be broken to insert the milliammeter, For this reason, current measurements are often used only as a last resort.

We can measure the emitter current by connecting dc milliammeter in series between the emitter resistor and the emitter electrode, To do this, remove the transistor from its socket disconnect the emitter resistor from the emitter electrode and insert the milliammeter leads. If the transistor is soldered in to the circuit, simply disconnect the emitter resistor from its ground return and connect the milliammeter from the pround return to the open end of the resistor. You can also calculate the emitter current by measuring the emitter voltage and the resistor value.

Finding Open Circuits

When the emitter circuit of a transistor is open the collector current circuit is broken. The floating emitter lead will assume the same voltage as the base terminal if the BC diode is not shorted. The-emitter-base bias voltage will be zero volts, and the base volt-ape to ground will be fairly normal.

If the base circuit of a germanium transistor opens, the transistor may go into thermal runaway. A silicon transistor will simply stop conducting under normal conditions, but may also go into the thermal runaway if operating temperatures are high.

When the collector circuit opens, the collector and emitter electrodes will assume the same potential, provided with normal base bias, the transistor has gone into saturation. Another condition which will cause the emitter and collector electrodes to have the same voltages is a shorted transistor. This is more likely to happen with power transistors.

Vari ous Tests and Measurements s test a transistor or diode before inserting it in a circuit. It may be defective.

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