![]() In this chapter we will primarily be using n-type transistors (NPN, NMOS) in the example circuits. The remaining terminal is what is thus common to both input and output. The easiest way to determine if a device is connected as common emitter/source, common collector/drain, or common base/gate is to examine where the input signal enters and the output signal leaves. This leads to the names common emitter, etc. This means one of the transistor terminals must be common to both the input and output circuits. Representing the basic amplifier as a two port network as in figure 9.1, there would need to be two input and two output terminals for a total of four. Output Resistance: The ratio of the variation in collector-emitter voltage to the collector-emitter current is known at collector currents at a constant base current I B is called output resistance r o.The transistor, as we have seen in the previous chapter, is a three-terminal device. In the active region I C = βI B, a small current I C is not zero, and it is equal to reverse leakage current I CEO. In the saturation region, the collector current becomes independent and free from the input current I B The collector-base junction of the transistor always in forward bias and work saturate. When the V CE falls, the I C also decreases rapidly. The value of the collector current I C increases with the increase in V CE at constant voltage I B, the value β of also increases. The output resistance of the common base connection is more than that of CE connection. The slope of the curve is quite more than the output characteristic of CB configuration. In the active region, the collector current increases slightly as collector-emitter V CE current increases. ![]() The characteristic curve for the typical NPN transistor in CE configuration is shown in the figure below. In CE configuration the curve draws between collector current I C and collector-emitter voltage V CE at a constant base current I B is called output characteristic. Input Resistance: The ratio of change in base-emitter voltage V BE to the change in base current ∆I B at constant collector-emitter voltage V CE is known as input resistance, i.e., ![]() The effect of CE does not cause large deviation on the curves, and hence the effect of a change in V CE on the input characteristic is ignored. Thus the input resistance of the CE configuration is comparatively higher that of CB configuration. The base current I B increases with the increases in the emitter-base voltage V BE. The curve for common base configuration is similar to a forward diode characteristic. The curve for different value of collector-base current is shown in the figure below. For drawing the input characteristic the reading of base currents is taken through the ammeter on emitter voltage V BE at constant collector-emitter current. The curve plotted between base current I B and the base-emitter voltage V EB is called Input characteristics curve. On the basis of these readings, the input and output curve plotted on the curve. For the various setting, the current and voltage are taken from the milliammeters and voltmeter. And the collector to emitter voltage varied by adjusting the potentiometer R 2. ![]() ![]() The base to emitter voltage varies by adjusting the potentiometer R 1. The characteristic of the common emitter transistor circuit is shown in the figure below. Substitute the value ΔI B in equations (1), we get,Ĭharacteristics of Common emitter (CE) Configuration The collector current is current to the emitter, and this current is abbreviated as I CEO that means collector- emitter current with the base open. If the base current is open (i.e., I B = 0). In CE configuration, the input current I B and the output current I C are related by the equation shown below. In other words, the current gain in a common emitter configuration is very high, and because of this reason, the common emitter arrangement circuit is used in all the transistor applications. The above equation shows that the when the α reaches to unity, then the β reaches to infinity. Substituting the value of ΔI E in equation (1), we get, ![]()
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