Common Base Amplifier

 
The common base amplifier circuit is given  in the figure 1.  The  V_EE source forward biases emitter diode and V_CC  source reverse biased collector diode. The AC source vin is connected to  emitter through a coupling capacitor such that it blocks DC. This ac voltage produces small variation in currents and voltages. The load resistance R_L is connected to collector through coupling capacitor also so the variation in collector base voltage will be observed across R_L.

The DC equivalent circuit can be obtained by  reducing  all the AC  sources  to  zero Figure 1 and  opening  all the capacitors. The dc collector current is same as I_E  and V_CB can be given by

V_CB = V_CC - I_C R_C.

The current and voltage fix Q point. The AC equivalent circuit can be obtained by reducing all the DC sources to zero and shorting all the coupling capacitors. r'e represents AC resistance of the diode as shown in the figure 2.

Figure 2

Figure 3, shows diode curve relating I_E  and V_BE. In absence of the AC signal, the transistor operates at Q point. When the AC signal is applied, the emitter current and voltage also change. If signal is small, the operating point swings sinusoidally about Q point.

Fig .3

If AC signal is small, the points A and B are near to Q, and arc A B can be approximated by a straight line and diode appears to be the resistance which is given by

If input signal is small, input voltage and current will be sinusoidal in nature but if the input voltage is large then the current will no longer be sinusoidal due to the non linearity of diode curve. The emitter current is elongated on positive half cycle and compressed on the negative half cycle. Hence output will also be distorted.

r'e  is the ratio of ΔVBE and Δ IE and its value depends upon location of the Q. Higher up the Q point small will be value of r' e because same change in VBE produces the large change in I_E. The slope of curve at Q decides the value of r'e. From the calculation it can be proved that

r'e = 25mV / IE

Proof-

Generally the current through a diode can be given by

Where q is the charge on electron, V is drop across diode, T is temperature and K is a constant.

On differentiating with respect to V, we get,

   
The value of (q / KT) at temperature 25°C is approximately 40. Therefore,  

or,     

To a close approximation small changes in the collector current equal the small changes in emitter current. In AC equivalent circuit, the current =i_C' is shown upward as if =i_e' increases, then =i_C' also increases in same direction.

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