Semiconductor Fundamentals Unit 6 – Transistor Load Lines and Gain
Exercise 3 – Transistor Circuit DC Voltages
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to demonstrate dc operating conditions
of a transistor circuit by using an NPN transistor. You will verify your results with a multimeter.
DISCUSSION
- Three possible dc voltage conditions for a transistor circuit are (1) at the saturation point, (2)
in the active (linear) region, or (3) at the cutoff point. - An NPN transistor circuit is used in this exercise. The base voltage supply is 15 Vdc, and the
collector circuit supply is 10 Vdc. - All voltage measurements are referenced to the emitter voltage. Since there is no emitter
resistor, the following is true: VB = VBE and VC = VCE - Base current is set by adjusting potentiometer R2.
- Saturation conditions are present when the base-collector junction is forward biased. VCE =
0 Vdc. In this state the transistor behaves like a closed switch. Collector current is at
maximum and is equal to the collector supply voltage divided by the total collector circuit
resistance. At saturation, collector current does not equal the base current times beta. - A transistor circuit operating in the active region can have an operating point anywhere
between saturation and cutoff. - In the active region, the base-emitter junction is forward biased and the base-collector
junction is reverse biased. Collector current is linearly proportional to the base current.
Collector current is equal to the base current times beta. - Cutoff occurs when the base voltage and current become zero because the base-emitter
junction is no longer forward biased. Collector current is zero and VCE is equal to the
collector supply voltage. The transistor circuit behaves as an open switch.