In a junction transistor, the Emitter (E), Base (B) and Collector (C) are the only three terminals available. Hence, in any circuit, one of these terminals has to be common to both input and output connections. Therefore, the junction transistor can be connected in either of these configurations:
- CE or Common Emitter
- CB or Common Base
- CC or Common Collector
Among these, the junction transistor is most widely used in the Common Emitter configuration. Also, the n-p-n Silicon transistors are used more commonly than the p-n-p transistors. Hence, we will look at the characteristics and configurations of an n-p-n Silicon Junction Transistor in a CE configuration.
Characteristics of a CE Junction Transistor
In Common Emitter (CE) configuration, the emitter is the common terminal. Hence, the input is between the base and the emitter while the output is between the collector and the emitter. Two terms that you must remember:
- Input characteristic – the variation of the base current (IB) with the base-emitter voltage (VBE)
- Output characteristic – the variation of the collector current (IC) with the collector-emitter voltage (VCE)
It is observed that the output characteristics are controlled by the input characteristics. Hence, the collector current changes with the base current. Let’s study them with the help of a circuit diagram shown below:
Studying the Input Characteristics
Also, a curve is plotted between the base current (IB) and the base-emitter voltage (VBE) to study the input characteristics of the junction transistor in CE configuration. The collector-emitter voltage (VCE) is kept at a fixed value to study the relation between IB and VBE.
Since we intend to study the input characteristics when the transistor is in an active state, VCE is maintained at a large value. The value chosen is large enough to ensure reverse biasing of the base-collector junction. For a Silicon transistor, VCE = 0.6-0.7 V. Also,
VCE = VCB + VBE
Hence, VCE has to be maintained at a value much larger than 0.7 V. The approximate range of voltage is between 3 and 20 V. An increase in the value of VCE appears as an increase in the value of VCB. Hence, we get almost identical curves for various values of VCE. Also, determining one input characteristic is sufficient to understand curve as shown below:
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Studying the Output Characteristics
To study the output characteristics, let’s plot a curve is between the Collector current (IC) and the collector-emitter voltage (VCE). Also, keep the base current (IB) at a steady value.
Now, if the base-emitter current (VBE) is increased by a small amount, you can observe an increase in hole current from the emitter and electron current from the base regions. Hence, IB and IC increase proportionally. Or, if IB increases, IC increases too. So, keeping IB constant and plotting IC against VCE, you can make the following observations:
For every value of IB, the plot of IC versus VCE displays one output characteristic.
Calculation of Important AC Parameters of Junction Transistors
Now, let’s calculate some important ac parameters of transistors using the linear segments of the input and output characteristics.
Input Resistance (ri)
Input resistance (ri) is the ratio of change in the base-emitter voltage (ΔVBE) to the subsequent change in base current (ΔIB) when the collector-emitter voltage (VCE) is kept constant.
ri = (ΔVBE/ ΔIB)VCE
This is ac resistance or dynamic resistance and which is obvious in the input characteristics since its value varies with the operating current in the transistor.
Output Resistance (ro)
Output resistance (ro) is the ratio of change in the collector-emitter voltage (ΔVCE) to the change in the collector current (ΔIC) when the base current (IB) is kept constant.
ro = (ΔVCE/ ΔIC)IB
A close look at the output characteristics reveals that initially, IC increases linearly for every small change in the value of VCE. The reason is simple – the base-collector junction is not reverse biased and the transistor is not active. On the contrary, the transistor is in a saturation state.
Hence, the current is controlled by the supply voltage (VCC) in this part of the characteristic. In this state VCC = VCE. As VCE increases and reaches a value which is higher than that required to reverse bias the junction, IC increases marginally with increasing VCE. This reciprocal of the slope of the linear part of the output characteristics offers the value of ro.
The bias of the base-collector junction primarily controls the output resistance of the transistor. Also, the output resistance (ro) is very high. This is because of the reverse-bias state of the diode. Hence, you observe that initially, the resistance is very low when the transistor is in a saturation state.
Current Amplification Factor (β)
Current Amplification Factor (β) is the ratio of change in the collector current (IC) to the change in base current (IB) when the collector-emitter voltage (VCE) is kept constant. Also, the transistor is in an active state. Now, the small signal current gain is
βac = (ΔIC/ ΔIB)VCE
This has a large value. On the other hand, if we take a simple ratio of IC and IB, we get βdc of the junction transistor.
βdc = IC/IB
It is important to note that IC and IB increase almost linearly. In simple words, if IB=0, then IC=0. Hence, the values of βac and βdc are nearly equal.
Solved Examples for You
Question: What are the input and output characteristics of a CE Junction Transistor?
Solution: In a Common Emitter configuration of a Junction Transistor, the emitter is the common terminal. Input is between the base and emitter. The output is between the collector and emitter. Input characteristics are the variation of base current (IB) with the base-emitter voltage (VBE). Output characteristics are the variation of collector current (IC) with the collector-emitter voltage (VCE).