Bipolar transistors II, Page 1
Bipolar Transistors II
Transistor circuits can be used to obtain stable sources of constant voltage. The
following sections trace the development of a simple voltage source using a single
transistor.
The pass transistor
Build the emitter follower circuit below, called the “pass-transistor” circuit.
(a) Calculate the output impedance. (Hint: It is approximately equal to the output
impedance of the circuit that supplies the base current divided by β.)
(b) Measure the output impedance by finding the change in voltage when the circuit
is loaded by 150 ohms.
(c) Reduce the input voltage from 15 volts to 10 volts, a 33% change. What is the
percentage change in the output voltage?
1. The Zener-regulated pass transistor
Replace the 6.8K resistor by a reversed-biased 5.6 volt Zener diode.
1K
6.8K
10K
2N2219
15 V In ~5 V Out
Figure 1: Emitter Follower Circuit.
