A.C. Emitter Resistance

A.C. Emitter Resistance-The ac or dynamic resistance of emitter-base junction diode of a transistor is called ac emitter resistance. It is defined as the change in base-emitter voltage divided by change in corresponding emitter current [See Fig. 1]

 i.e. R_ac =V_BE/V_E I For instance, suppose an ac base voltage change of 1 mV produces an ac emitter current change of 50 µA. Then emitter diode has an ac resistance of

 R_ac =1mV/50 µA=20 Ω

Formula For AC Emitter Resistance

It can be show mathematically that the ac resistance of emitter diode is give by ;

R_ac= 25mV/ I_E

where I_E = dc emitter current (= V_E/R_E) at Q point

Note the significance of this formula. A.C. Emitter Resistance-It implies that ac emitter resistance can be finnd simply by substituting the quiescent value of emitter current into the equation. There is no need to have the characteristics available. It is important to keep in mind that this formula is accurate only for small signal operation. It is a usual practice to represent ac emitter resistance by r_e′.

∴ r_e ′ = 25mV/ I_E

The subscript e indicates emitter. The lower case r is use to indicate an ac resistance. The prime shows that it is an internal resistance.

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