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

2. Microelectronic Circuit Design, 3E McGraw-Hill Signal Injection and Extraction: BJT In forward-active region, To cause change in current, v BE = v B - v E 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 i B is a factor of  F smaller than i C or i E currents, base terminal is not used to extract output. Chap 14 - 2

3. Microelectronic Circuit Design, 3E McGraw-Hill Signal Injection and Extraction: FET In pinch-off region, To cause change in current, v GS = v G - v S 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 i G is always zero, gate terminal is not used to extract output. Chap 14 - 3

4. Microelectronic Circuit Design, 3E McGraw-Hill 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. Chap 14 - 4

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

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

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

8. Microelectronic Circuit Design, 3E McGraw-Hill 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  f R E term. Output resistance of C-E amplifier is much higher than that of C-S amplifier as  f 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 R E (or R S ) into the transistor (For FET, current gain and input resistance are infinite). Chap 14 - 8

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

10. Microelectronic Circuit Design, 3E McGraw-Hill Follower Circuits: Terminal Voltage Gain Neglecting r o, Assuming 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, r o can be neglected as gain<<  f Chap 14 - 10

11. Microelectronic Circuit Design, 3E McGraw-Hill Follower Circuits: Input Signal Range For small-signal operation, magnitude of v be developed across r  in small- signal model must be less than 5 mV. If, v b 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 v gs must be less than 0.2(V GS - V TN ). Chap 14 - 11

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

13. Microelectronic Circuit Design, 3E McGraw-Hill Follower Circuits: Voltage Gain Calculations (Example) Problem: Find overall voltage gain. Given data: Q-point values and values for R I, R 1, R 2, R 4, R 7,for both BJT and FET. Assumptions: Small-signal operating conditions. Analysis: For C-C Amplifier, Chap 14 - 13

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