A bipolar junction transistor (BJT) can be used in many circuit configurations; as an amplifier, oscillator, filter rectifier or just used as a switch. If the transistor is biased into the linear region, it will be used as an amplifier or other linear circuit, if biased into saturation or cut-off, then it will be used as a switch, controlling other circuits. This article describes the BJT when used as a switch.
BJT Characteristic Curves
For a transistor there are input, output and transfer characteristic curves. The output characteristic curve is especially useful as it shows the variations in collector current, Ic for a given base current, Ib over a range of collector-emitter voltage, Vce. A sample plot for a 2N3904 is shown below. The centre, cyan curve shows that for a base current of 42uA the collector current is about 8.5mA. This gives a static dc current gain, (also called beta or HFE ) of 8.5/0.042 = 202.
When used as an amplifier, the biasing is arranged so that the transistor operates in the linear region ( shown above as almost horizontal sections). The linear region is for Vce > 0.5V and extends to the full supply voltage. An amplifier will usually be biased to about half the supply voltage to allow for maximum output swing.
For use as a switch, the BJT operates in the regions of the output curves called saturation and cut-off. See the diagram above.
The yellow shaded area represents the "cut-off" region. In cut-off the BJT is fully off and its operating conditions are zero base current, zero collector current, and collector-emitter voltage Vce will be high.
In "saturation" as depicted by the red shaded area, the transistor is fully on, and conditions are maximum base current, maximum collector current, and minimum collector-emitter voltage Vce. In both cut-off and saturation, minimum power is dissipated in the transistor.
Current Gain in Saturation
In the linear region, the dc current gain, HFE is fairly constant over a wide range of collector-emitter voltages. However, in saturation, an important change takes place. A zoomed view of the output curves from 0 to 0.4 Volts Vce is shown below:
The change in gradient means that a change HFE has taken place. The purple trace shows a base current, Ib of 62uA. At Vce = 0.1 Volts the collector current has now fallen sharply to 2.5mA. The dc beta is now HFE = 2.5/0.062 = 40. Current gain in saturation will also vary with the amount of collector-emitter voltage (i.e. the amount of saturation) and transistor type. Because of this the bias circuit should be designed to work with the minimum value of HFE for any given transistor, and a rule of thumb is to assume of current gain of only 20x.
For higher current loads, it is important that the transistor remains in the saturation region. The graph below, shows collector current on a logarithmic scale, plotted against collector-emitter voltage.
For small base currents below 1mA the current gain is 20. However at 10mA base current the current gain drops to just 5. One way to design for a power transistor is allow 5x more base current than you actually need, to make sure the device remains saturated.
In saturation, a heat-sink is rarely required as little power is developed in the transistor. However in a power supply or other circuit where a transistor may be required to control large variations in current and voltage then significant power may be developed. If the power dissipation of the device is exceeded then it will be destroyed. In practice allow for the worst combination of currents and voltages and calculate accordingly.