COMMON EMITTER CONFIGURATION

This is also called as GROUND-EMITTER configuration. The input current and the output voltage are taken as the independent variables, whereas the input voltage and output current are the dependent variables.

we may write,

Vbe= f(Vce,Ib)............1

Ic= f(Vce,Ib).................2

In the above equations, first equation will represent input characteristic curves and second equation represents the family of output characteristic curves. 

CE configuration

INPUT & OUTPUT CHARACTERISTICS:

P-N-P TRANSISTOR CE configuration

In the above graphs the fig(a) shows output characteristics and fig(b) shows input characteristics.
Common Emitter Curves:

The common emitter configuration of BJT is shown in fig. 1.

Fig. 1

In C.E. configuration the emitter is made common to the input and output. It is also referred to as grounded emitter configuration. It is most commonly used configuration. In this, base current and output voltages are taken as impendent parameters and input voltage and output current as dependent parameters

V_BE = f₁ ( I_B, V_CE )
I_C = f₂( I_B, V_CE )

Input Characteristic:

The curve between I_B and V_BE for different values of V_CE are shown in fig. 2. Since the base emitter junction of a transistor is a diode, therefore the characteristic is similar to diode one. With higher values of V_CE collector gathers slightly more electrons and therefore base current reduces. Normally this effect is neglected. (Early effect). When collector is shorted with emitter then the input characteristic is the characteristic of a forward biased diode when V_BE is zero and I_Bis also zero.

In fig(a), For fixed value of I_B, I_C is not varying much dependent on V_CE but slopes are greater than CE characteristic. The output characteristics can again be divided into three parts.

Fig. 3

(1) Active Region:

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

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

(2) Cut Off:

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

I_C = I_E= I_CO / ( 1-α_dc) = I_CEO

The actual collector current with base open is designated as I_CEO. Since even in the neighborhood of cut off, a _dc may be as large as 0.9 for Ge, then I_C=10 I_CO(approximately), at zero base current. Accordingly in order to cut off transistor it is not enough to reduce I_B to zero, but it is necessary to reverse bias the emitter junction slightly. It is found that reverse voltage of 0.1 V is sufficient for cut off a transistor. In Si, the a _dc is very nearly equal to zero, therefore, I_C = I_CO. Hence even with I_B= 0, I_C= I_E= I_CO so that transistor is very close to cut off.

In summary, 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 the order of 0.1 V for Ge and 0 V for Si.

Reverse Collector Saturation Current I_CBO:

When in a physical transistor emitter current is reduced to zero, then the collector current is known as I_CBO (approximately equal to I_CO). Reverse collector saturation current I_CBO also varies with temperature, avalanche multiplication and variability from sample to sample. Consider the circuit shown in fig. 4. V_BB is the reverse voltage applied to reduce the emitter current to zero.

I_E = 0,          I_B = -I_CBO

If we require, V_BE = - 0.1 V

Then  - V_BB + I_CBO R_B < - 0.1 V

Fig. 4

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

(3).Saturation Region:

In this region both the diodes are forward biased by at least cut in voltage. Since 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 also few tenths of volts. Hence saturation region is very close to zero voltage axis, where all the current rapidly reduces to zero. In this region the transistor collector current is approximately given by V_CC / R _C and independent of base current. Normal transistor action is last and it acts like a small ohmic resistance.

 Large Signal Current Gain β_dc :-

The ratio I_c / I_B is defined as transfer ratio or large signal current gain b_dc

Where I_C is the collector current and I_B is the base current. The b_dc is an indication if how well the transistor works. The typical value of b_dc varies from 50 to 300.

In terms of h parameters, b _dc is known as dc current gain and in designated h_fE ( b _dc = h_fE). Knowing the maximum collector current and b_dc the minimum base current can be found which will be needed to saturate the transistor.

This expression of b_dc is defined neglecting reverse leakage current (I_CO).

Taking reverse leakage current (I_CO) into account, the expression for the b_dc can be obtained as follows:

b_dc in terms of a_dc is given by

Since, I_CO = I_CBO

Cut off of a transistor means I_E = 0, then I_C= I_CBO and I_B = - I_CBO. Therefore, the above expression b_dc gives the collector current increment to the base current change form cut off to I_B and hence it represents the large signal current gain of all common emitter transistor.