Microelectronic Circuit Design, 3E McGraw-Hill Chapter 13 Small-Signal Modeling and Linear Amplification Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock

Microelectronic Circuit Design, 3E McGraw-Hill BJT Amplifier BJT is biased in active region by dc voltage source V BE. Q-point is set at (I C, V CE ) = (1.5 mA, 5 V) with I B = 15  A. Total base-emitter voltage is: Collector-emitter voltage is: This is the load line equation.

Microelectronic Circuit Design, 3E McGraw-Hill BJT Amplifier (cont.) 8 mV peak change in v BE gives 5  A change in i B and 0.5 mA change in i C. 0.5 mA change in i C produces a 1.65 V change in v CE. If changes in operating currents and voltages are small enough, then i C and v CE waveforms are undistorted replicas of input signal. Small voltage change at base causes large voltage change at collector. Voltage gain is given by: Minus sign indicates phase shift between input and output signals.

Microelectronic Circuit Design, 3E McGraw-Hill MOSFET Amplifier MOSFET is biased in active region by dc voltage source V GS. Q-point is set at (I D, V DS ) = (1.56 mA, 4.8 V) with V GS = 3.5 V. Total gate-source voltage is: 1 V p-p change in v GS gives 1.25 mA p-p change in i D and 4 V p-p change in v DS.

Microelectronic Circuit Design, 3E McGraw-Hill Coupling and Bypass Capacitors AC coupling through capacitors is used to inject ac input signal and extract output signal without disturbing Q-point Capacitors provide negligible impedance at frequencies of interest and provide open circuits at dc. C 1 and C 3 are large-valued coupling capacitors or dc blocking capacitors whose reactance at the signal frequency is designed to be negligible. C 2 is a bypass capacitor that provides a low impedance path for ac current from emitter to ground, thereby removing R E (required for good Q-point stability) from the circuit when ac signals are considered.

Microelectronic Circuit Design, 3E McGraw-Hill dc and ac Analysis DC analysis: –Find dc equivalent circuit by replacing all capacitors by open circuits and inductors by short circuits. –Find Q-point from dc equivalent circuit by using appropriate large- signal transistor model. AC analysis: –Find ac equivalent circuit by replacing all capacitors by short circuits, inductors by open circuits, dc voltage sources by ground connections and dc current sources by open circuits. –Replace transistor by small-signal model –Use small-signal ac equivalent to analyze ac characteristics of amplifier. –Combine end results of dc and ac analysis to yield total voltages and currents in the network.

Microelectronic Circuit Design, 3E McGraw-Hill dc Equivalent for BJT Amplifier All capacitors in original amplifier circuits are replaced by open circuits, disconnecting v I, R I, and R 3 from circuit.

Microelectronic Circuit Design, 3E McGraw-Hill ac Equivalent for BJT Amplifier