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ThevEmCir

ThevEmCir - Thvenin Emitter Circuit The Thvenin equivalent...

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Thévenin Emitter Circuit The Thévenin equivalent circuit seen looking into the emitter is useful in calculating the response of common-collector stages. It consists of a voltage source v e ( oc ) in series with a resistor r ie from the emitter node to signal ground. Fig. 1(a) shows the BJT symbol with a Thévenin source connected to the base. The resistor R tc represents the external load resistance in series with the collector. With the emitter open circuited, we denote the emitter voltage by v e ( oc ) . The voltage source in the Thévenin emitter circuit has this value. To solve for it, we use the simpli fi ed T model in Fig. 1(b). Figure 1: (a) BJT with Thévenin source connected to the base. (b) Simpli fi ed T model circuit for calculating v e ( oc ) . The current i 0 e can be solved for by superposition of the sources v tb and αi 0 e . It is given by i 0 e = v tb r 0 e + r 0 + R tc + αi 0 e R tc r 0 e + r 0 + R tc (1) This can be solved for i 0 e to obtain i 0 e = v tb r 0 e + r 0 + (1 α ) R tc = v tb r 0 e + r 0 + R tc / (1 + β ) (2) The open-circuit emitter voltage is given by v e ( oc ) = v tb i 0 e r 0 e = v

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