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Unformatted text preview: L AB 5: B IPOLAR J UNCTION T RANSISTOR T RANSISTOR S WITCH I Purpose and Background This laboratory has three objectives: 1. To measure the collector characteristic (V CE- I C curve) for a bipolar junction transistor; 2. To build and test three transistor switch circuits from the simplest one-transistor switch to a more complex two-transistor switch exhibiting hysteresis. 3. To diagnose simulated transistor circuits to identify possible faults in the circuits. The npn BJT transistor is operated with the base-emitter pn junction forward-biased and the base-collector junction reverse-biased. In normal operation, the base-emitter voltage is approximately 0.7V. The BJT is a current amplifier device: a small base current is amplified into a larger collector current. The DC of the transistor is defined as I C /I B ; it is ~ 100. In a transistor switch, the transistor is operated either in cutoff or in saturation. In cutoff, the collector current is 0 and the transistor acts like an open switch (V CE is maximal). In saturation, the collector current is maximal and the transistor acts like a closed switch (V CE is small: V CE(sat) , about 0.2V). The data sheet for the 2N3904 BJT transistor are attached to the handout. II Collector Characteristic Curve Prelab: Consider the circuit shown on the following graph: VB B 5 V RB VC C 100 2N3 904 1. Compute the values of R B that give a base current of 50 A, 100 A, 150 A. Enter the values in a table. For each R B , determine the single standard 10% precision resistor or combination of two standard 10% precision resistors that when added yield the value nearest to R B . (A table of resistor values is available at the end of your textbook in appendix.) 2. The data sheet of the transistor lists two maximum values for V CE(sat) ; which value would you use for this circuit given the expected base currents....
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- Fall '08