What is Common Emitter?
In this circuit arrangement, input is applied between base and emitter and output is taken from the
collector and emitter. Here, emitter of the transistor is common to both input and output circuits and hence the name common emitter connection. Fig. 8.16 (i) shows common emitter npn transistor circuit whereas Fig. 8.16 (ii) shows common emitter pnp transistor circuit.
- Base current amplification factor ( β). In common emitter connection, input current is IB
and output current is IC
The ratio of change in collector current (ΔIC) to the change in base current (ΔIB) is known as base current amplification factor i.e.
In almost any transistor, less than 5% of emitter current flows as the base current. Therefore, the value of β is generally greater than 20. Usually, its value ranges from 20 to 500. This type of connection is frequently used as it gives appreciable current gain as well as voltage gain. Relation between β and α. A simple relation exists between β and α. This can be derived as follows :
It is clear that as α approaches unity, β approaches infinity. In other words, the current gain in
common emitter connection is very high. It is due to this reason that this circuit arrangement is used in about 90 to 95 percent of all transistor applications.
Expression for collector current
In common emitter circuit, IB is the input current and IC is the output current
Concept of ICEO. In CE configuration, a small collector current flows even when the base current is zero [See Fig. 8.17 (i)]. This is the collector cut off current (i.e. the collector current that flows when base is open) and is denoted by ICEO. The value of ICEO is much larger than ICBO.
Measurement of Leakage Current in Common Emitter
A very small leakage current flows in all transistor circuits. However, in most cases, it is quite small
and can be neglected.
(i) Circuit for ICEO test. Fig. 8.18 shows the circuit for measuring ICEO. Since base is open (IB = 0), the transistor is in cut off. Ideally, IC = 0 but actually there is a small current from collector to emitter due to minority carriers. It is called ICEO (collector-to-emitter current with base open). This current is usually in the nA range for silicon. A faulty transistor will often have excessive leakage current.
(ii) Circuit for ICBO test. Fig. 8.19 shows the circuit for measuring ICBO. Since the emitter is open (IE = 0), there is a small current from collector to base. This is called ICBO (collector-to-base current with emitter open). This current is due to the movement of minority carriers across base collector junction. The value of ICBO is also small. If in measurement, ICBO is excessive, then there is a possibility that collector-base is shorted.
Example 8.8. Find the value of β if (i) α = 0.9 (ii) α = 0.98 (iii) α = 0.99.
Example 8.9. Calculate IE in a transistor for which β = 50 and IB = 20 µA.
Example 8.10. Find the α rating of the transistor shown in Fig. 8.20. Hence determine the value of IC using both α and β rating of the transistor.
Solution. Fig. 8.20 shows the conditions of the problem
Example 8.11. For a transistor, β = 45 and voltage drop across 1kΩ which is connected in the collector circuit is 1 volt. Find the base current for common emitter connection.
Solution. Fig. 8.21 shows the required common emitter connection. The voltage drop across RC (= 1 kΩ) is 1volt.
Example 8.12. A transistor is connected in common emitter (CE) configuration in which collector supply is 8V and the voltage drop across resistance RC connected in the collector circuit is 0.5V. The value of RC = 800 Ω. If α = 0.96,
(i) collector-emitter voltage
(ii) base current
Solution. Fig. 8.22 shows the required common emitter connection with various values.
Example 8.13. An n-p-n transistor at room temperature has its emitter disconnected. A voltage of 5V is applied between collector and base. With collector positive, a current of 0.2 µA flows. When the base is disconnected and the same voltage is applied between collector and emitter, the current is found to be 20 µA. Find α, IE and IB when collector current is 1mA.
Solution. When the emitter circuit is open [See Fig. 8.23 (i)], the collector-base junction is
reverse biased. A small leakage current ICBO flows due to minority carriers.
Example 8.14. The collector leakage current in a transistor is 300 µA in CE arrangement. If now
the transistor is connected in CB arrangement, what will be the leakage current? Given that β = 120
Example 8.15. For a certain transistor, IB = 20 µA; IC = 2 mA and β = 80. Calculate ICBO..
Example 8.16. Using diagrams, explain the correctness of the relation ICEO = (β + 1) ICBO.
Solution. The leakage current ICBO is the current that flows through the base-collector junction
when emitter is open as shown is Fig. 8.24. When the transistor is in CE arrangement, the *base
current (i.e. ICBO) is multiplied by β in the collector as shown in Fig. 8.25.
Example 8.17 Determine VCB in the transistor * circuit shown in Fig. 8.26 (i). The transistor is
of silicon and has β = 150.
Example 8.18. In a transistor, IB = 68 µA, IE = 30 mA and β = 440. Determine the α rating of the transistor. Then determine the value of IC using both the α rating and β rating of the transistor.
Example 8.19. A transistor has the following ratings : IC (max) = 500 mA and βmax = 300.
Determine the maximum allowable value of IB for the device.
For this transistor, if the base current is allowed to exceed 1.67 mA, the collector current will
exceed its maximum rating of 500 mA and the transistor will probably be destroyed.
Example 8.20. Fig. 8.27 shows the open circuit failures in a transistor. What will be the circuit
behaviour in each case ?
Solution. *Fig 8.27 shows the open circuit failures in a transistor. We shall discuss the circuit behaviour in each case.
Fig. 8.27 (i) shows an open emitter failure in a transistor. Since the collector diode is not forward biased, it is OFF and there can be neither collector current nor base current. Therefore, there will be no voltage drops across either resistor and the voltage at the base and at the collector leads of the transistor will be 12V.
Fig. 8.27 (ii) shows an open base failure in a transistor. Since the base is open, there can be no base current so that the transistor is in cut-off. Therefore, all the transistor currents are 0A. In this case, the base and collector voltages will both be at 12V
Note. It may be noted that an open failure at either the base or emitter will produce similar
Fig. 8.27 (iii) shows an open collector failure in a transistor. In this case, the emitter diode is still ON, so we expect to see 0.7V at the base. However, we will see 12V at the collector because there is no collector current.
Example 8.21. Fig. 8.28 shows the short circuit failures in a transistor. What will be the circuit
behaviour in each case ?
Solution. Fig. 8.28 shows the short circuit failures in a transistor. We shall discuss the circuit
behaviour in each case.
Fig. 8.28 (i) shows a short between collector and emitter. The emitter diode is still forward biased, so we expect to see 0.7V at the base. Since the collector is shorted to the emitter, VC= VE = 0V.
Base -emitter short
Fig 8.28 (ii) shows a short between base and emitter. Since the base is now directly connected to ground, VB = 0. Therefore, the current through RB will be diverted to ground and there is no current to forward bias the emitter diode. As a result, the transistor will be cutoff and there is no collector current. So we will expect the collector voltage to be 12V.
Fig. 8.28 (iii) shows a short between the collector and the base. In this case, the emitter diode is still forward biased so VB = 0.7V. Now, however, because the collector is shorted to the base, VC = VB = 0.7V.
Note. The collector-emitter short is probably the most common type of fault in a transistor. It is
because the collector current (IC) and collector-emitter voltage (VCE) are responsible for the major
part of the power dissipation in the transistor. As we shall see (See Art. 8.23), the power dissipation in a transistor is mainly due to IC and VCE (i.e. PD = VCE IC). Therefore, the transistor chip between the collector and the emitter is most likely to melt first.
Characteristics of Common Emitter Connection
The important characteristics of this circuit arrangement are the input characteristics and output
It is the curve between base current IB and base-emitter voltage VBE at constant collector-emitter voltage VCE. The input characteristics of a CE connection can be determined by the circuit shown in Fig. 8.29. Keeping VCE constant (say at 10 V), note the base current IB for various values of VBE. Then plot the readings obtained on the graph, taking IB along yaxis and VBE along x-axis. This gives the input characteristic at VCE = 10V as shown in Fig. 8.30. Following a similar procedure, a family of input characteristics can be drawn.
The following points may be noted from the characteristics :
(i) The characteristic resembles that of a forward biased diode curve. This is expected since the base-emitter section of transistor is a diode and it is forward biased.
(ii) As compared to CB arrangement, IB increases less rapidly with VBE. Therefore, input resistance of a CE circuit is higher than that of CB circuit.
It is the ratio of change in base-emitter voltage (ΔVBE) to the change in base current (ΔIB) at constant VCE i.e.
The value of input resistance for a CE circuit is of the order of a few hundred ohms.
It is the curve between collector current IC and collector-emitter voltage VCE at constant base current IB. The output characteristics of a CE circuit can be drawn with the help of the circuit shown in Fig. 8.29. Keeping the base current IB fixed at some value say, 5 µA, note the collector current IC
for various values of VCE. Then plot the readings on a graph, taking IC along y-axis and VCE along x-axis. This gives the output characteristic at IB = 5 µA as shown in Fig. 8.31 (i). The test can be repeated for IB = 10 µA to obtain the new output characteristic as shown in Fig. 8.31 (ii). Following similar procedure, a family of output characteristics can be drawn as shown in Fig. 8.31 (iii).
The following points may be noted from the characteristics:
(i) The collector current IC varies with VCE for VCE between 0 and 1V only. After this, collector
current becomes almost constant and independent of VCE. This value of VCE upto which collector current IC changes with VCE is called the knee voltage (Vknee). The transistors are always operated in the region above knee voltage.
(ii) Above knee voltage, IC is almost constant. However, a small increase in IC with increasing VCE is caused by the collector depletion layer getting wider and capturing a few more majority carriers before electron-hole combinations occur in the base area.
(iii) For any value of VCE above knee voltage, collector current IC is approximately equal toβ × IB
.Output resistance. It is the ratio of change in collector-emitter voltage (ΔVCE) to the change in
collector current (ΔIC) at constant IB i.e.
It may be noted that whereas the output characteristics of CB circuit are horizontal, they have noticeable slope for the CE circuit. Therefore, the output resistance of a CE circuit is less than that of CB circuit. Its value is of the order of 50 kΩ.
Common Collector Connection
In this circuit arrangement, input is applied between base and collector while output is taken between the emitter and collector. Here, collector of the transistor is common to both input and output circuits and hence the name common collector connection. Fig. 8.32 (i) shows common collector npn transistor circuit whereas Fig. 8.32 (ii) shows common collector pnp circuit.
Current amplification factor γ
In common collector circuit, input current is the base current IB and output current is the emitter current IE. Therefore, current amplification in this circuit arrangement can be defined as under :
The ratio of change in emitter current (ΔIE) to the change in base current (ΔIB) is known as current amplification factor in common collector (CC) arrangement i.e.
This circuit provides about the same current gain as the common emitter circuit as ΔIE~ ΔIC
However, its voltage gain is always less than 1.
The common collector circuit has very high input resistance (about 750 kΩ) and very low output resistance (about 25 Ω). Due to this reason, the voltage gain provided by this circuit is always less than 1. Therefore, this circuit arrangement is seldom used for amplification. However, due to relatively high input resistance and low output resistance, this circuit is primarily used for impedance matching i.e. for driving a low impedance load from a high impedance source.
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Reference: Principles Of Electronics By V K Mehta And Rohit Mehta