Transistor Operation

A transistor in a circuit will be in one of three conditions

Cut off (no collector current), useful for switch operation.

In the active region (some collector current, more than a few tenths of a volt above the emitter), useful for amplifier applications

In saturation (collector a few tenths of a volt above emitter), large current useful for "switch on" applications.

Collector Current Determination

The base-emitter voltage can be considered to be the controlling variable in determining transistor action. The collector current is related to this voltage by the Ebers-Moll relationship (sometimes labeled the Shockley equation):

The saturation current is characteristic of the particular transistor (a parameter which itself has a temperature dependence). This relationship is stable over a wide range of voltages and currents. A further useful relationship is

where ß can be called the current gain. The value of ß is not highly dependable since it depends on Ic , Vce and the temperature.

Base-Emitter Junction Details

Some useful "rules of thumb" which help in understanding transistor action are (from Horowitz & Hill):

1.- A base emitter voltage Vbe of about 0.6 v will "turn on" the base-emitter diode and that voltage changes very little, < +/- 0.1v throughout the active range of the transistor which may change base current by a factor of 10 or more.

2.- An increase in base-emitter voltage Vbe by about 60 mV will increase the collector current Ic by about a factor of 10.

3.- The effective AC series resistance of the emitter is about 25/ ohms.

4.- The base-emitter voltage is temperature dependent, decreasing about 2.1 mV/C

5.- The base-emitter voltage varies slightly with the collector-emitter voltage at constant collector current : Ic=∆Vbe=-0.01∆Vce

Transistor Action

More about transistor regions

Collector Current

Normal transistor action results in a collector-to-emitter current which is about 99% of the total current. The usual symbols used to express the transistor current relationships are shown.

The proportionality β can take values in the range 20 to 200 and is not a constant even for a given transistor. It increases for larger emitter currents because the larger number of electrons injected into the base exceeds the available holes for recombination so the fraction which recombine to produce base current delines even further.

Use of the Current Gain

Any circuit that depends on a specific value of the current gain is a bad circuit because that value varies for a given transistor as well as between different transistors of the same type.