Interview Question

Electrical Engineer Intern Interview

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Tektronix

A schematic shows a BJT in a common-collector configuration with the collector tied to ground, the base attached to a signal source, and the emitter connected through a resistor to -5V. In between the emitter and resistor is one end of a 75 Ohm transmission line. The other end is attached to a 75 Ohm resistor that terminates it to -5V. The signal source is a step with an initial voltage of -5V and a step voltage up to 0V. Determine the voltage at the emitter before and after the step. Then, draw the path of the loop current when the voltage transient has traveled halfway down the transmission line. Then, determine the voltage at the terminated end of the transmission line just before and just after the transient arrives. Determine how much power is reflected by the termination.

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The emitter voltages are -5V and about -0.8V, respectively, creating a 4.2V transient wave. As the time when the transient is only halfway down the transmission line, the current loop is from ground, through the collector and emitter of the BJT, into the transmission line, and back to ground at the halfway point of the transmission line where the voltage transient is. The voltage at the terminated end of the line is -5V and -0.8V, respectively. There is no power reflected from the load.

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