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What are the characteristics of a common emitter configuration?

      

What are the characteristics of a common emitter configuration?

  

Answers


Jim
1. Input characteristic: 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. 4.29. Keeping VCE constant (say at 10V), 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. 4.30. 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.
Input resistance. It is the ratio of change in base-emitter voltage (Change in VBE) to the change in base current (Change in IB) at constant VCE i.e.
comm11582019821.png
- The value of input resistance for a CE circuit is of the order of a few hundred ohms.

2. Output characteristic: 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. 4.29. 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. 4.31 (i). The test can be repeated for IB = 10 µA to obtain the new output characteristic as shown in Fig. 4.31 (ii). Following similar procedure, a family of output characteristics can be drawn as shown in Fig. 4.31 (iii).
comm1a1582019821.png
- 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, the collector current IC is approximately equal to ß × IB.
- Output resistance. It is the ratio of change in collector-emitter voltage (change in VCE) to the change in collector current (change in IC) at constant IB i.e.
comm21582019821.png
- 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 kilo-ohms.
jim items answered the question on November 16, 2018 at 18:10


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