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Chapter 13 Small-Signal Modeling and Linear Amplification

Chapter 13 Small-Signal Modeling and Linear Amplification. Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock. DC and AC Equivalents for MOSFET Amplifier. dc equivalent. Full circuit. ac equivalent. Simplified ac equivalent. Small-Signal Operation of Diode.

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Chapter 13 Small-Signal Modeling and Linear Amplification

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

  2. DC and AC Equivalentsfor MOSFET Amplifier dc equivalent Full circuit ac equivalent Simplified ac equivalent Microelectronic Circuit Design, 3E McGraw-Hill

  3. Small-Signal Operation of Diode • The slope of the diode characteristic at the Q-point is called the diode conductance and is given by: • gd is small but non-zero for ID = 0 because slope of diode equation is nonzero at the origin. • Diode resistance is given by: Microelectronic Circuit Design, 3E McGraw-Hill

  4. Small-Signal Operation of Diode (cont.) Subtracting ID from both sides of the equation, For id to be a linear function of signal voltage vd , This represents the requirement for small-signal operation of the diode. Microelectronic Circuit Design, 3E McGraw-Hill

  5. Current-Controlled Attenuator Magnitude of ac voltage vo developed across diode can be controlled by value of dc bias current applied to diode. From ac equivalent circuit, From dc equivalent circuit ID =I, For RI = 1 kW, IS = 10-15 A, If I = 0, vo = vi, magnitude of vi is limited to only 5 mV. If I = 100 mA, input signal is attenuated by a factor of 5, and vi can have a magnitude of 25 mV. Microelectronic Circuit Design, 3E McGraw-Hill

  6. Small-Signal Model of BJT Using 2-port y-parameter network, The port variables can represent either time-varying part of total voltages and currents or small changes in them away from Q-point values. bo is the small-signal common-emitter current gain of the BJT. Microelectronic Circuit Design, 3E McGraw-Hill

  7. Hybrid-Pi Model of BJT Transconductance: • The hybrid-pi small-signal model is the intrinsic representation of the BJT. • Small-signal parameters are controlled by the Q-point and are independent of geometry of the BJT Input resistance: Output resistance: Microelectronic Circuit Design, 3E McGraw-Hill

  8. Small-Signal Current Gain and Amplification Factor of BJT Amplification factor is given by: For VCE << VA, mF represents maximum voltage gain individual BJT can provide and doesn’t change with operating point. bo > bF for iC < IM , and bo < bF for iC > IM , however, bF and bo are assumed to be equal. Microelectronic Circuit Design, 3E McGraw-Hill

  9. Equivalent Forms of Small-Signal Model for BJT • Voltage -controlled current source gmvbe can be transformed into current-controlled current source, • Basic relationship ic = bib is useful in both dc and ac analysis when BJT is in forward-active region. Microelectronic Circuit Design, 3E McGraw-Hill

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