Semiconductor Fundamentals Unit 5 – Transistor Junctions & PNP DC Bias
Exercise 2 – PNP Transistor Current Control Circuit
EXERCISE OBJECTIVE
When you have completed this exercise, you will be able to demonstrate transistor current
control by using a PNP transistor circuit. You will verify your results with a multimeter.
DISCUSSION
- A bipolar transistor can operate as a switch by changing the base current from zero to the
maximum value. - To forward bias a base-emitter junction, the base, of a silicon PNP transistor must be about
0.5 Vdc to 0.8 Vdc more negative than the emitter. - Collector-emitter junction resistance depends upon the base current.
- A forward biased base-emitter junction causes the collector-emitter resistance to be very low
allowing current to flow in the transistor collector circuit. Analogous to a closed switch. - When the forward bias is removed (base-emitter junction is reverse biased) the base current
becomes zero. Zero base current causes the collector-emitter resistance to be very high,
which blocks the flow of current in the transistor collector circuit. Analogous to an open
switch. - When maximum collector current flows, the transistor is said to be in saturation and VCE is
nearly zero volts. - When the base current is zero and there is no collector current flow, the transistor is said to
be in cut-off and VCE is equal to the supply voltage. - The emitter current is equal to the sum of the base current and the collector current. Since the
base current is negligible, the collector current and emitter current are virtually equal.