Chapter Vb

# Chapter Vb - ELEN-325 Introduction to Electronic Circuits...

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ELEN-325. Introduction to Electronic Circuits Jose Silva-Martinez - 1 - Confidential 3/25/2009 5.8. Common-collector amplifier. Circuit description. The common-collector amplifier is used for coupling circuits with small driving capabilities with heavy loads. The voltage gain of the amplifier is less than but close to unity; the current gain is however nearly 1+ β . Due to this property, the amplifier can be considered as a power amplifier and is often used for driving low load impedances such as speakers, motors, etc. The circuit’s schematic is shown in Figure 5.17. VCC v i v E R B1 R B2 v B R E C B R S R L V 0 C L Fig. 5.17. Common-collector configuration: signal is injected at the base terminal and output is at the emitter terminal. For DC analysis, the AC input signal is short circuited, and the capacitors are replaced by open circuits. The equivalent circuit is shown in Fig. 5.18a. Resistors RB1 and RB2 together with the effect of VCC can be replaced by an equivalent circuit as depicted in figure 5.18b. The equivalent voltage VB and RB are VCC R R VCC R R R V 1 B B 2 B 1 B 2 B BB ± ² ³ ³ ´ µ = ± ² ³ ³ ´ µ + = (5.30) 2 B 1 B 2 B 1 B B R R R R R + = (5.31) The base and emitter currents are related by the following expression CQ E B EQ E BQ B BB I R 1 R 7 . 0 I R 7 . 0 I R V ± ² ³ ³ ´ µ ± ² ³ ³ ´ µ β β + + β + = + + = (5.32) and the equation for the transistor’s output yields, CQ E CEQ EQ E CEQ I R V I R V VCC α + = + = (5.33)

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ELEN-325. Introduction to Electronic Circuits Jose Silva-Martinez - 2 - Confidential 3/25/2009 VCC R B1 R B2 v B R E I CQ I BQ VCC V CEQ R B R E I CQ I BQ I EQ V BB + - + - (a) (b) Fig. 5.18. a) Common-collector equivalent circuit for DC analysis, and b) modified equivalent circuit. The AC equivalent circuit is obtained for Fig. 5.17 by making VCC=0 and replacing the transistor by its small signal model. It is easier to find the solution of the circuit if the T-model is used, as depicted in Figure 5.19a. Similarly to the common-emitter configuration, the small signal transfer function presents two poles due to the coupling capacitors C B and C L , and the resistors associated with them. At the medium frequency band where the capacitors can be considered as short circuits, the simplified equivalent circuit shown in Fig. 5.19b results. v i v e R B R E C B R S R L V 0 C L r e i e α i e i b v i R B R E1 IIR L R S v 0 r e i e α i e i b v b z b z i v b a) (b) Fig. 5.19. Small signal model for the common-collector amplifier: a) including the AC coupling capacitors, model valid for low and medium frequencies, and b) small signal model valid for medium frequencies only; the poles introduce by the capacitors are ignored.
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## This note was uploaded on 01/01/2012 for the course ECEN 325 taught by Professor Silva during the Spring '08 term at Texas A&M.

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Chapter Vb - ELEN-325 Introduction to Electronic Circuits...

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