CBSE NOTES CLASS 12 PHYSICS
CHAPTER 14 SEMICONDUCTORS
Transistor in common emitter configuration
When the emitter is the common terminal for the two power supplies whose other terminals are connected to base and collector, it is called common emitter configuration. The power supplies are named as follows.
VBB - power supply between base and emitter
VCC – power supply between collector and emitter
Transistor as an amplifier
The transistor works as an amplifier, when its emitter-base junction is forward biased and the base-collector junction is reverse biased. This state of transistor is called active state.
Explanation (emitter-base junction forward biased and base-collector junction reverse biased)
The heavily doped emitter has a high concentration of majority carriers, which will be holes in a p-n-p transistor and electrons in an n-p-n transistor.
These majority carriers enter the base region in large numbers. The base is thin and lightly doped, hence has very few majority carriers. Since base in a p-n-p is n-type, the majority carriers in the base are electrons. The large number of holes entering the base from the emitter, outnumber the small number of electrons there. The base collector-junction is reverse biased, hence the holes which are the minority carriers at the junction, easily cross the junction and enter the collector. The holes in the base could move either towards the base terminal to combine with the electrons entering from outside or cross the junction to enter into the collector and reach the collector terminal. The base is made thin so that most of the holes cross the junction instead of moving to the base terminal.
Now since emitter-base is forward biased, a large current enters the emitter-base junction, but most of it is diverted to adjacent reverse-biased base-collector junction and the current coming out of the base becomes a very small fraction of the current that entered the junction.
Let us assume that the current entering into the emitter from outside is equal to the emitter current IE, the current emerging from the base terminal is IB and that from collector terminal is IC.
If we represent the hole current and the electron current crossing the forward biased junction by Ih and Ie respectively then the total current in a forward biased diode is the sum IE = Ih + Ie.
The base current IB << IE, because most of IE goes to the collector instead of coming out of the base terminal.
Applying Kirchhoff’s law, we can see,
IE = IC + IB and also IC ≈ IE
We can describe the paths followed by the majority and minority carriers in a n-p-n is exactly the same as that for the p-n-p transistor. In this case the electrons are the majority carriers supplied by the n-type emitter region. They cross the thin p-base region and are able to reach the collector to give the collector current, IC .
- Hence in the active state of the transistor the emitter-base junction acts as a low resistance while the base collector acts as a high resistance.
Common emitter transistor characteristics
When a transistor is used in CE configuration, the input is between the base and the emitter and the output is between the collector and the emitter. The variation of the base current IB with the base-emitter voltage VBE is called the input characteristic.
The variation of the collector current IC with the collector-emitter voltage VCE is called the output characteristic.
The output characteristics are controlled by the input characteristics, i.e., the collector current changes with the base current.
To study the input characteristics of the transistor in CE configuration, a curve is plotted between the base current IB against the base-emitter voltage VBE. The collector-emitter voltage VCE is kept fixed while studying the dependence of IB on VBE.
The collector-emitter voltage VCE is kept large enough to make the base collector junction reverse biased.
We can see that, VCE = VCB + VBE
For Si transistor VBE is 0.6 to 0.7 V, VCE must be sufficiently larger than 0.7 V. Since the transistor is operated as an amplifier over large range of VCE, the reverse bias across the base-collector junction is high most of the time.
The input characteristics may be obtained for VCE somewhere in the range of 3 V to 20 V. Since the increase in VCE appears as increase in VCB, its effect on IB is negligible. As a consequence, input characteristics for various values of VCE will give almost identical curves.
Hence, it is enough to determine only one input characteristics.
The output characteristic is obtained by observing the variation of IC as VCE is varied keeping IB constant.
If VBE is increased by a small amount, both hole current from the emitter region and the electron current from the base region will increase. As a consequence both IB and IC will increase proportionately. This shows that when IB increases IC also increases. The plot of IC versus VCE for different fixed values of IB gives one output characteristic. So there will be different output characteristics corresponding to different values of IB.
The linear segments of both the input and output characteristics can be used to calculate some important ac parameters of transistors.
- Input resistance of transistor (ri): This is defined as the ratio of change in base-emitter voltage (ΔVBE) to the resulting change in base current (ΔIB) at constant collector-emitter voltage (VCE). This is dynamic (ac resistance) and its value varies with the operating current in the transistor:
- Output resistance of transistor (ro): This is defined as the ratio of change in collector-emitter voltage (ΔVCE) to the change in collector current (ΔIC) at a constant base current IB.
The output characteristics show that initially for very small values of VCE, IC increases almost linearly. This happens because the base-collector junction is not reverse biased and the transistor is not in active state. In fact, the transistor is in the saturation state and the current is controlled by the supply voltage VCC (=VCE) in this part of the characteristic. When VCE is more than that required to reverse bias the base-collector junction, IC increases very little with VCE. The reciprocal of the slope of the linear part of the output characteristic gives the values of ro.
The output resistance of the transistor is mainly controlled by the bias of the base-collector junction. The high magnitude of the output resistance (of the order of 100 kΩ) is due to the reverse-biased state of this diode. This is also the reason why the resistance at the initial part of the characteristic, when the transistor is in saturation state, is very low.
- Current amplification factor (β): This is defined as the ratio of the change in collector current to the change in base current at a constant collector-emitter voltage (VCE) when the transistor is in active state.
This is also known as small signal current gain and its value is very large.
The ratio of IC and IB is called βdc of the transistor.
- Since IC increases with IB almost linearly and IC = 0 when IB = 0, the values of both βdc and βac are nearly equal. So, for most calculations βdc can be used. Both βac and βdc vary with VCE and IB (or IC) slightly.