TRANSISTORS

• TRANSISTORS

• Bipolar Junction Transistor Model

• The Ideal Bipolar Junction Transistor

The Ideal Bipolar Junction Transistor

Because the current gain is typically unknown or varies greatly with temperature, time, collector–emitter

potential, and other factors, good designs should not depend on it. In this laboratory, we assume that is

sufficiently large (i.e.,amplification ≫ 1, where amplification ≈ 100 in our laboratory) so that

iB ≈ 0 and iC ≈ iE.

These simple rules are similar to the rules we use with operational amplifiers. The analysis approach usually

follows these steps:

1. Calculate the transistor base potential vB by assuming that no current enters the base (i.e., iB ≈ 0).

2. Calculate the potential vE at the emitter of the transistor using vB. For an npn transistor,

vE = vB − 0.65V,

and for a pnp transistor,

vE = vB + 0.65V.

3. Calculate the emitter current iE using the emitter voltage vE and the rest of the circuit.

4. Assume that iC ≈ iE and analyze the rest of the circuit.

• Because we know vE, we usually know iE as well. So our iE dictates what iC should be.

However, keep these notes in mind.

• For an npn transistor, active mode requires vC − vE > 0.2V. For a pnp transistor, active mode

requires vE − vC > 0.2V. If this condition is violated, the transistor is saturated, and the analysis

cannot continue using these simple rules. In design problems, change parameters (e.g., resistors, supply

rails, etc.) to prevent saturation.

• Sometimes it’s easier to find vE first and use it to calculate vB.

• How “small” iB must be to neglect its effect depends on the circuit. In particular, iB × RB must be

very small, where RB is the the Th´evenin equivalent resistance looking out of the transistor base.

Bipolar Junction Transistor Model

A bipolar junction transistor (BJT) can be in three modes:

Transistor acts like an open switch between collector and emitter (i.e.,
collector–emitter “resistance” is infinite).


Transistor acts like a dynamic resistor between collector and emitter that
adjusts its resistance in order to keep collector current at a set level (i.e.,

collector–emitter resistance is finite and positive).

Transistor acts like a closed switch between collector and emitter (i.e.,
collector–emitter “resistance” is very low).


In the active mode, the transistor adjusts the collector current to be a version of the base current amplified
by some constant > 0. If the base current falls to 0, the transistor enters cutoff mode and shuts off. When
the base current rises too far, the transistor loses its ability to decrease the collector–emitter resistance
to linearly increase the collector current. In this case, the transistor enters saturation mode. To keep the
transistor out of saturation mode, the collector and emitter should be separated by at least 0.2V.
A simple model for the operation of a transistor in active mode is shown in Figure. It requires knowing
the current gain in order to design the circuit. In both of these models,
iC = iB and iE = ( + 1)iB,
and the emitter is separated from the base by a diode. In order for this diode to conduct current, it must
be forward biased with 0.65V1.

TRANSISTORS

Transistors can be regarded as a type of switch, as can many electronic components. They are used in a variety of circuits and you will find that it is rare that a circuit built in a school Technology Department does not contain at least one transistor. They are central to electronics and there are two main types; NPN and PNP. Most circuits tend to use NPN. There are hundreds of transistors which work at different voltages but all of them fall into these two categories.