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.

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