Common Emitter Configuration

Output Characteristic:

The output characteristic is curve between V_CE  and I_C  for the various values of I_B. For the constant value of IB and is shown in the figure 3. For the fixed value of I_B, I_C is not varying much dependent on V_CE but slopes are greater than characteristic of CE. The output characteristics can be divided into 3 parts.

Fig. 

(1) Active Region:

In this region the collector junction is reverse biased and the emitter junction is forward biased. It is the area to right of VCE = 0.5 V and above I_B= 0. In this area the transistor current responds very sensitively to I_B. If the transistor is to be used as an amplifier, it should operate in this region.

If α_dc  is truly constant then I_C  would be independent of V_CE. But due to early effect,  α_dc increases by 0.1% (0.001) for example from 0.995 to 0.996 as V_CE  increases from a few volts to 10V. Then bdc increases from 0.995 / (1-0.995) = 200 to 0.996 / (1-0.996) = 250 or about 25 per cent. This shows that small change in the a reflects large change in b. Thus the curves are subjected to large variations for same type of the transistors.

(2) Cut Off:

Cut off in a transistor can be given by I_B = 0, I_C= I_CO. A transistor is not at cut off if base current is simply reduced to zero under this condition,

I_C = I_E= I_CO / ( 1-α_dc) = I_CEO

The actual collector current with the base open is designated as I_CEO. As even in neighborhood of cut off, a dc can be as large as 0.9 for Ge, then I_C=10 I_CO, at zero base current. Accordingly in order to cut off transistor it is not sufficient to reduce IB to zero, but it is also necessary to reverse bias the emitter junction a little. It is found that the reverse voltage of 0.1 V is sufficient for the cut off a transistor. In Si, the a dc is nearly equal to zero, hence, I_C = I_CO. Hence even with I_B= 0, I_C= I_E= I_CO so that transistor is quite close to cut off. The cut off means I_E = 0, I_C = I_CO, I_B = -I_C = -I_CO, and V_BE is a reverse voltage whose magnitude is of order of 0.1 V for Ge and 0 V for Si.

Reverse Collector Saturation Current I_CBO:

In the physical transistor when the emitter current is reduced to zero, then collector current is called as I_CBO (nearly equal to I_CO). Reverse collector saturation current I_CBO also varies with avalanche multiplication, temperature, and variability of each sample. Consider the circuit shown in the fifure 4. V_BB is reverse voltage applied to reduce emitter current to zero. I_E = 0,      I_B = -I_CBO

If we need, V_BE = - 0.1 V Then  - V_BB + I_CBO R_B < - 0.1 V

Figure 4

If R_B = 100 K, I_CBO = 100 m A, then V_BB should be 10.1 Volts. Hence transistor should be capable to withstand this reverse voltage before the breakdown voltage exceeds.

(3).Saturation Region:

In this region both diodes are forward biased by at least cut in voltage. As the voltage V_BE and V_BC across a forward is approximately 0.7 V therefore, V_CE = V_CB + V_BE = - V_BC + V_BE is few tenths of volts. Hence the saturation region is quite close to zero voltage axis, here all the current quickly reduces to zero. In this region transistor collector current is approximately can be given by V_CC / R _C  and independent of base current. The normal transistor action is last and it behaves like a small ohmic resistance.

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