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

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

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

4. Microelectronic Circuit Design, 3E
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

5. Microelectronic Circuit Design, 3E
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
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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

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

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 mf RE 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

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

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

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

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

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