V-I Characteristics of BJT
The current-voltage (I-V) characteristics of a BJT can be represented by a graph that shows the relationship between the collector current (IC) and the collector-emitter voltage (VCE) for a given base current (IB). The graph is typically divided into three regions:
Cut-off region: In this region, the emitter-base junction is reverse-biased and the collector-base junction is forward-biased. However, the forward bias on the collector-base junction is not large enough to overcome the built-in potential barrier, so very little current flows through the transistor.
Active region: In this region, the emitter-base junction is forward-biased and the collector-base junction is also forward-biased. The collector current in this region is proportional to the base current and is given by the equation IC = βIB, where β is the current gain of the transistor.
Saturation region: In this region, both the emitter-base junction and the collector-base junction are heavily forward-biased. The collector current in this region is no longer proportional to the base current and is instead limited by the external circuit.
The following graph shows the typical I-V characteristics of a BJT:
The graph shows that the collector current increases as the collector-emitter voltage increases in the active region. However, the collector current reaches a maximum value in the saturation region and cannot be increased further.
The I-V characteristics of a BJT can be used to design electronic circuits. For example, the active region is typically used for amplifiers, while the saturation region is typically used for switches.
Here is an example of how to use the I-V characteristics of a BJT to design an amplifier circuit:
Suppose we want to design an amplifier circuit that has a gain of 10. We can use a BJT to amplify the signal by choosing a transistor with a current gain of β = 10. The following circuit diagram shows a simple common emitter amplifier circuit:
In this circuit, the input signal is applied to the base of the transistor and the amplified output signal is taken from the collector of the transistor. The load resistor, RL, is used to set the gain of the amplifier.
The gain of the amplifier is given by the equation Av = -βRL/RE, where Av is the voltage gain of the amplifier, β is the current gain of the transistor, RL is the load resistor, and RE is the emitter resistor.