1. 6/8/2016Faculty of Engineering Cairo University Chap 14 - 1 Lecture 2 Single-Transistor Amplifiers Dr. Ahmed Nader Adapted from presentation by Richard C. Jaeger Travis N. Blalock

2. 6/8/2016Faculty of Engineering Cairo University Chapter Goals Detailed study of three broad classes of amplifiers –Inverting amplifiers- that provide high voltage gain with a 180 0 phase shift (common-emitter and common-source configurations) –Followers- that provide nearly unity gain similar to op amp voltage follower (common-collector and common-drain configurations) –Noninverting amplifiers- that provide high voltage gain with no phase shift (common-base and common-gate configurations). Detailed design –Voltage gain & Current gain –Input voltage range –Input and output resistances –Coupling and bypass capacitor design and lower cutoff frequency for each type of amplifier. Chap 14 - 2

3. 6/8/2016Faculty of Engineering Cairo University 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 - 3

4. 6/8/2016Faculty of Engineering Cairo University 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 - 4

5. 6/8/2016Faculty of Engineering Cairo University Amplifier Families Constraints for signal injection and extraction yield three families of amplifiers –Common-Emitter(C-E) / Common-Source(C-S) –Common-Collector(C-C) / Common-Drain(C-D) –Common-Base(C-B) / Common-Gate(C-G) 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 - 5

6. 6/8/2016Faculty of Engineering Cairo University 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 - 6

7. 6/8/2016Faculty of Engineering Cairo University Followers: Common-Collector (C-C) and Common-Drain (C-D) Circuits AC equivalent for C-C AmplifierAC equivalent for C-D Amplifier Chap 14 - 7 Some Redundant Components!

8. 6/8/2016Faculty of Engineering Cairo University 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 - 8

9. 6/8/2016Faculty of Engineering Cairo University Inverting Amplifiers: C-E and C-S Amplifier Review Chap 14 - 9

10. 6/8/2016Faculty of Engineering Cairo University 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-S amplifier is higher than that of C-E amplifier. Chap 14 - 10

11. 6/8/2016Faculty of Engineering Cairo University Follower Circuits: Common-Collector and Common-Drain Amplifiers AC equivalent for C-C AmplifierAC equivalent for C-D Amplifier Chap 14 - 11

12. 6/8/2016Faculty of Engineering Cairo University 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 - 12 What about body effect?

13. 6/8/2016Faculty of Engineering Cairo University 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 - 13

14. 6/8/2016Faculty of Engineering Cairo University 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 - 14

15. 6/8/2016Faculty of Engineering Cairo University 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 - 15

16. 6/8/2016Faculty of Engineering Cairo University Follower Circuits: Voltage Gain Calculations (Example cont.) Analysis: For C-D Amplifier, Chap 14 - 16

17. 6/8/2016Faculty of Engineering Cairo University Follower Circuits: Output Resistance In case of FET, Thus equivalent resistance looking into emitter or source of a transistor is approximately 1/ g m. Current is injected into emitter of BJT. Current  o i coming out of collector must be supported by v eb =  o i/g m, given by first term. i b =-i/  o +1creates voltage drop in R th given by second term Chap 14 - 17

18. 6/8/2016Faculty of Engineering Cairo University Follower Circuits: Output Resistance (Example) Problem: Find output resistance. 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. Small -signal values are known. Analysis: For C-C Amplifier, For C-D Amplifier, Chap 14 - 18

19. 6/8/2016Faculty of Engineering Cairo University Follower Circuits: Current Gain Terminal current gain is the ratio of the current delivered to the load resistor to the current being supplied from the Thevenin source. Chap 14 - 19

20. 6/8/2016Faculty of Engineering Cairo University Follower Circuits: Summary Both C-C and C-D amplifiers have voltage gains approaching unity. C-D amplifier provides extremely high input resistance because of infinite resistance looking into gate terminal of FET as compared to C- C amplifier. Output resistance of C-C amplifier is much lower than the C-D amplifier due to higher transconductance of BJT than an FET for given operating current. Both C-C and C-D amplifiers can handle relatively large input signal levels.. Current gains of FET is inherently infinite, whereas that of BJT is limited by its finite value of  o. Chap 14 - 20

21. 6/8/2016Faculty of Engineering Cairo University Follower Circuits: Summary (cont.) Chap 14 - 21

22. 6/8/2016Faculty of Engineering Cairo University Noninverting Amplifiers: Common-Base and Common-Gate Circuits AC equivalent for C-E AmplifierAC equivalent for C-S Amplifier Chap 14 - 22

23. 6/8/2016Faculty of Engineering Cairo University C-B and C-G Amplifiers: Terminal Voltage Gain and Input Resistance Polarities of v be and dependent current source g m v be are both reversed, signal source is transformed to its Norton equivalent r o is neglected. For C-S Amplifier, take limit of voltage gain of C-E amplifier as Chap 14 - 23 What if r o is not neglected?

24. 6/8/2016Faculty of Engineering Cairo University C-B and C-G Amplifiers: Overall Voltage Gain Overall voltage gain is For C-S Amplifier, For R 6 >> R I, For (very low R I ), This is the upper bound on gain. For, r o can be neglected as gain<<  f Chap 14 - 24

25. 6/8/2016Faculty of Engineering Cairo University C-B and C-G Amplifiers: Input Signal Range For small-signal operation, In case of FET, Relative size of g m and R I determine signal-handling limits. …for R 6 >> R I. Chap 14 - 25

26. 6/8/2016Faculty of Engineering Cairo University C-B and C-G Amplifiers: Voltage Gain Calculations (Example) Problem: Find overall voltage gain. Given data: Q-point values and values for R 1, R 2, R 3, R 7,for both BJT and FET, R I =2 k , R 4 =12 k . Assumptions: Small-signal operating conditions. Analysis: For C-E Amplifier, For C-S Amplifier, Chap 14 - 26

27. 6/8/2016Faculty of Engineering Cairo University Desired resistance is that looking into collector with base grounded and resistor R th in emitter. The redrawn equivalent circuit is same as that for C-E amplifier except resistance in base is zero and resistance in emitter is relabeled as R th. And for the FET C-G amp C-B and C-G Amplifiers: Output Resistance Chap 14 - 27

28. 6/8/2016Faculty of Engineering Cairo University Noninverting Amplifiers: Current Gain Terminal current gain is the ratio of the current delivered to the load resistor to the current being supplied to the base terminal. Chap 14 - 28

29. 6/8/2016Faculty of Engineering Cairo University C-B and C-G Amplifiers: Summary C-B and C-G amplifiers have similar voltage and current gains. Numerical differences occur due to difference in parameter values of BJT and FET at similar operating points. C-B amplifier can achieve high output resistance due to higher amplification factor of BJT. C-B amplifier can more easily reach low levels of output resistance due to higher transconductance of BJT for a given operating current. Input signal range of C-G amplifier is inherently larger than that of C-B amplifier. Chap 14 - 29

30. 6/8/2016Faculty of Engineering Cairo University C-B and C-G Amplifiers: Summary (cont.) Chap 14 - 30

31. 6/8/2016Faculty of Engineering Cairo University Simplified Characteristics of BJT Single-Stage Amplifiers Chap 14 - 31

32. 6/8/2016Faculty of Engineering Cairo University Simplified Characteristics of BJT Single-Stage Amplifiers Chap 14 - 32

33. 6/8/2016Faculty of Engineering Cairo University Simplified Characteristics of FET Single-Stage Amplifiers Chap 14 - 33

34. 6/8/2016Faculty of Engineering Cairo University Simplified Characteristics of FET Single-Stage Amplifiers Chap 14 - 34

35. 6/8/2016Faculty of Engineering Cairo University Selecting Amplifier Configuration A single-transistor amplifier with a gain of 80 dB and input resistance of 100 k . –A v = 10 80/20 = 10,000. For even best BJTs, gain<  f = 40V A = 40(150) = 6000 and FET typically has much lower intrinsic gain. Hence such large gain can’t be achieved by single-transistor amplifier. A single-transistor amplifier with gain of 52 dB, input resistance of 250 k . –A v = 10 52/20 = 400. Since we need large gain and relatively large input resistance, we can use C-E amplifier. A v = 20V CC, so, V CC =20 V. which is small but acceptable. For FET, even with small gate overdrive, V DD =100 V which is too large Chap 14 - 35

36. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design Since impedance of a capacitor increases with decreasing frequency, coupling and bypass capacitors reduce amplifier gain at low frequencies. To choose capacitor values, short-circuit time constant method is used: each capacitor is considered separately with all other capacitors replaced by short circuits. To neglect a capacitor, the magnitude of capacitive impedance must be much smaller than the equivalent resistance appearing at its terminals. Chap 14 - 36

37. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design: C-E and C-S Amplifiers For C-E amplifier, For C-S amplifier, For coupling capacitor C 1, For coupling capacitor C 3,  is chosen to be lowest frequency for which midband operation is needed in given application. Chap 14 - 37

38. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design: C-E and C-S Amplifiers (contd.) In this case, we can neglect impedances of capacitors C 1 and C 3, the find the equivalent resistance looking up into emitter or source of amplifier. Chap 14 - 38

39. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design: C-E and C-S Amplifiers (Example) Problem: Choose values of coupling and bypass capacitors. Given data: f = 1000Hz, values of all resistors and input and output resistances for both C-E and C-S amplifiers. Analysis: For C-E amplifier: For C-S amplifier: Chap 14 - 39

40. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design: C-C and C-D Amplifiers For C-E amplifier, For C-S amplifier, For coupling capacitor C 1, For coupling capacitor C 3, Chap 14 - 40

41. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design: C-C and C-D Amplifiers (Example) Problem: Choose values of coupling and bypass capacitors. Given data: f = 1000Hz, values of all resistors and input and output resistances for both C-E and C-S amplifiers. Analysis: For C-C amplifier: For C-D amplifier: Chap 14 - 41

42. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design: C-B and C-G Amplifiers For C-E amplifier, For C-S amplifier, For coupling capacitor C 1, For coupling capacitor C 3, Chap 14 - 42

43. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design: C-B and C-G Amplifiers (contd.) In this case, we can neglect impedances of capacitors C 1 and C 3, the find the equivalent resistance looking up into emitter or source of amplifier. Chap 14 - 43

44. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design: C-B and C-G Amplifiers (Example) Problem: Choose values of coupling and bypass capacitors. Given data: f = 1000Hz, values of all resistors and input and output resistances for both C-E and C-S amplifiers. Analysis: For C-B amplifier: Chap 14 - 44

45. 6/8/2016Faculty of Engineering Cairo University Coupling and Bypass Capacitor Design: C- B and C-G Amplifiers (Example contd.) For C-G amplifier: Chap 14 - 45