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Understanding Single-Transistor Amplifiers in Microelectronic Circuit Design

Explore signal injection and extraction in BJT and FET circuits, amplifier families, inverting amplifiers, followers, and follower circuit characteristics in microelectronic circuit design.

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Understanding Single-Transistor Amplifiers in Microelectronic Circuit Design

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  1. Chapter 14Single-Transistors Amplifiers Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 1

  2. Signal Injection and Extraction: BJT • In forward-active region, • To cause change in current, vBE = vB - vE must be changed. Base or emitter terminals are used to inject signal because even if Early voltage is considered, collector voltage has negligible effect on terminal currents. • Substantial changes in collector or emitter currents can create large voltage drops across collector and emitter resistors and collector or emitter can be used to extract output. Since iB is a factor of bF smaller than iC or iE currents, base terminal is not used to extract output. Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 2

  3. Signal Injection and Extraction: FET • In pinch-off region, • To cause change in current, vGS = vG - vS must be changed. Gate or source terminals are used to inject signal because even with channel-length modulation, drain voltage has negligible effect on terminal currents. • Substantial changes in drain or source currents can create large voltage drops across drain and source resistors and drain or source can be used to extract output. Since iG is always zero, gate terminal is not used to extract output. Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 3

  4. Amplifier Families • Constraints for signal injection and extraction yield three families of amplifiers • Common-Emitter (C-E)/Common- Source (C-S) • Common-Base (C-B)/Common- Gate (C-G) • Common-Collector (C-C)/Common- Drain (C-D) • All circuit examples here use the four-resistor bias circuits to establish Q-point of the various amplifiers • Coupling and bypass capacitors are used to change the ac equivalent circuits. Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 4

  5. Inverting Amplifiers: Common-Emitter (C-E) and Common-Source (C-S) Circuits AC equivalent for C-E Amplifier AC equivalent for C-S Amplifier Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 5

  6. Followers: Common-Collector (C-C) and Common-Drain (C-D) Circuits AC equivalent for C-C Amplifier AC equivalent for C-D Amplifier Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 6

  7. Inverting Amplifiers: Common-Base (C-B) and Common-Gate (C-G) Circuits AC equivalent for C-B Amplifier AC equivalent for C-G Amplifier Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 7

  8. Inverting Amplifiers: Summary • C-E and C-S amplifiers have similar voltage gains. • C-S amplifier provides extremely high input resistance but that of C-E is also substantial due to the mfRE term. • Output resistance of C-E amplifier is much higher than that of C-S amplifier as mf is much larger for BJT than for FET. • Input signal range of C-E amplifier is also higher than that of C-S amplifier. • Current gains of both are identical to those of individual transistors. • Following transformation is used to simplify circuit analysis by absorbing RE (or RS ) into the transistor (For FET, current gain and input resistance are infinite). Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 8

  9. Follower Circuits: Common-Collector and Common-Drain Amplifiers AC equivalent for C-C Amplifier AC equivalent for C-D Amplifier Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 9

  10. Follower Circuits: Terminal Voltage Gain For C-S Amplifier, take limit of voltage gain of C-E amplifier as and In most C-C and C-D amplifiers, Output voltage follows input voltage, hence theses circuits are called followers. BJT gain is closer to unity than FET. Mostly, ro can be neglected as gain<< mf Neglecting ro, Assuming Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 10

  11. Follower Circuits: Input Signal Range For small-signal operation, magnitude of vbe developed across rp in small-signal model must be less than 5 mV. If , vb can be increased beyond 5 mV limit.Since only small portion of input signal appears across base-emitter or gate-source terminals, followers can be used with relatively large input signals without violating small-signal limits. In case of FET, magnitude of vgs must be less than 0.2(VGS - VTN). Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 11

  12. Follower Circuits: Input Resistance and Overall Voltage Gain Overall voltage gain is For C-S Amplifier, Input resistance looking into the base terminal is given by For C-S Amplifier, Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 12

  13. Follower Circuits: Voltage Gain Calculations (Example) • Problem: Find overall voltage gain. • Given data: Q-point values and values for RI, R1, R2, R4, R7 ,for both BJT and FET. • Assumptions: Small-signal operating conditions. • Analysis: For C-C Amplifier, Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 13

  14. Follower Circuits: Voltage Gain Calculations (Example cont.) • Analysis: For C-D Amplifier, Microelectronic Circuit Design, 3E McGraw-Hill Chap 14 - 14

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