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Chapter 4 Small-Signal Modeling and Linear Amplification

Chapter 4 Small-Signal Modeling and Linear Amplification. Chapter Goals. Understanding of concepts related to: Transistors as linear amplifiers dc and ac equivalent circuits Use of coupling and bypass capacitors and inductors to modify dc and ac equivalent circuits

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Chapter 4 Small-Signal Modeling and Linear Amplification

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  1. Chapter 4Small-Signal Modeling and Linear Amplification

  2. Chapter Goals Understanding of concepts related to: • Transistors as linear amplifiers • dc and ac equivalent circuits • Use of coupling and bypass capacitors and inductors to modify dc and ac equivalent circuits • Small-signal voltages and currents • Small-signal models for BJT transistors • Amplifier characteristics such as voltage gain, input and output resistances and linear signal range • Rule-of-thumb estimates for voltage gain of common-emitter and common-source amplifiers. Chap13 - 2

  3. Introduction to Amplifiers • BJT is an excellent amplifier when biased in forward-active region • In these regions, transistors can provide high voltage, current and power gains. • Bias is provided to stabilize the operating point in desired operation region. • Q-point also determines • Small-signal parameters of transistor • Voltage gain, input resistance, output resistance • Maximum input and output signal amplitudes • Power consumption Chap13 - 3

  4. To obtained linear amplifier, the ac current and voltages must a small enough To insure a linear relationship between the ac signals. To meet this objective The Time-varying signals are assume to be small signals.

  5. BJT Amplifier BJT is biased in active region by dc voltage source VBE. Q-point is set at (IC, VCE)=(1.5 mA, 5 V) with IB = 15 mA. Total base-emitter voltage is: Collector-emitter voltage is: This gives the load line

  6. BJT Amplifier (contd.) If changes in operating currents and voltages are small enough, then IC and VCE 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 1800 phase shift between input and output signals. 8 mV peak change in vBE gives 5 mA change in iB and 0.5 mA change in iC. 0.5 mA change in iC gives 1.65 V change in vCE .

  7. 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. C1 and C3 are large coupling capacitors or dc blocking capacitors, their reactance at signal frequency is negligible. C2 is bypass capacitor, provides low impedance path for ac current from emitter to ground, removing RE (required for good Q-point stability) from circuit when ac signals are considered.

  8. DC Equivalent for BJT Amplifier • All capacitors in original amplifier circuits are replaced by open circuits, disconnecting vI, RI, and R3 from circuit.

  9. AC Equivalent for BJT Amplifier • Find ac equivalent circuit by replacing all capacitors by short circuits,

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