Chapter 13 Output Stages and Power Amplifiers

Transcription

1 Chapter 13 Output Stages and Power Amplifiers 13.1 General Considerations 13.2 Emitter Follower as Power Amplifier 13.3 Push-Pull Stage 13.4 Improved Push-Pull Stage 13.5 Large-Signal Considerations 13.6 Short Circuit Protection 13.7 Heat Dissipation 13.8 Efficiency 13.9 Power Amplifier Classes 1

2 Why Power Amplifiers? Drive a load with high power. Cell phone needs 1W of power at the antenna. Audio system needs tens to hundreds Watts of power. Ordinary Voltage/Current amplifiers are not equipped for such applications 2

3 Chapter Outline 3

4 Power Amplifier Characteristics Experiences small load resistance. Delivers large current levels. Requires large voltage swings. Draws a large amount of power from supply. Dissipates a large amount of power, therefore gets hot. 4

5 Power Amplifier Performance Metrics Linearity Power Efficiency Voltage Rating 5

6 Types of Amplifiers

7 Emitter Follower Large-Signal Behavior I As V in increases V out also follows and Q 1 provides more current. 7

8 Emitter Follower Large-Signal Behavior II However as V in decreases, V out also decreases, shutting off Q 1 and resulting in a constant V out. 8

9 Example: Emitter Follower V 1 V V I V V = 0.5V V 211mV out in T ln + 1 = out RL IS in out I V V I I R ( ) C1 in = T ln + C1 1 L IS I 0.01I V 390mV C1 1 in 9

10 Linearity of an Emitter Follower As Vin decreases the output waveform will be clipped, introducing nonlinearity in I/O characteristics. 10

11 Push-Pull Stage As V in increases, Q 1 is on and pushes a current into R L. As V in decreases, Q 2 is on and pulls a current out of R L. 11

12 I/O Characteristics for Large V in V out =V in -V BE1 for large +V in V out =V in + V BE2 for large -V in For positive V in, Q 1 shifts the output down and for negative V in, Q 2 shifts the output up. 12

13 Overall I/O Characteristics of Push-Pull Stage However, for small V in, there is a dead zone (both Q 1 and Q 2 are off) in the I/O characteristic, resulting in gross nonlinearity. 13

14 Small-Signal Gain of Push-Pull Stage The push-pull stage exhibits a gain that tends to unity when either Q1 or Q2 is on. When Vin is very small, the gain drops to zero. 14

15 Sinusoidal Response of Push-Pull Stage For large Vin, the output follows the input with a fixed DC offset, however as Vin becomes small the output drops to zero and causes Crossover Distortion. 15

16 Improved Push-Pull Stage V B =V BE1 + V BE2 With a battery of V B inserted between the bases of Q 1 and Q 2, the dead zone is eliminated. 16

17 Implementation of V B Since V B =V BE1 + V BE2, a natural choice would be two diodes in series. I 1 in figure (b) is used to bias the diodes and Q 1. 17

18 Example: Current Flow I I = I I + I in 1 B1 B2 I in If V out =0 & β 1 =β 2 >>1 => I B1 =I B2 18

19 Example: Current Flow II V D1 V BE V out V in If I 1 =I 2 & I B1 I B2 I in =0 when V out =0 19

20 Addition of CE Stage A CE stage (Q 4 ) is added to provide voltage gain from the input to the bases of Q 1 and Q 2. 20

21 Bias Point Analysis V A =0 V out =0 I C1 =[I S,Q1 /I S,D1 ] [I C3 ] For bias point analysis, the circuit can be simplified to the one on the right, which resembles a current mirror. The relationship of I C1 and I Q3 is shown above. 21

22 Small-Signal Analysis A V =-g m4 (r π1 r π2 )(g m1 +g m2 )R L Assuming 2r D is small and (g m1 +g m2 )R L is much greater than 1, the circuit has a voltage gain shown above. 22

23 그림을표시할수없습니다. Output Resistance Analysis R out 1 ro3 ro4 + g + g ( g + g )( r r ) m1 m2 m1 m2 π1 π2 If β is low, the second term of the output resistance will rise, which will be problematic when driving a small resistance. 23

24 Example: Biasing CE A V =5 Output Stage A V =0.8 R L =8Ω β npn = 2β pnp =100 I C1 I C2 g g 1 + g = 2 Ω m1 m2 m1 m2 C1 C2 π1 π2 C3 C4 ( 4 ) g Ω I I 6.5mA r r = 133Ω 1 I I 195µ A 24

25 Problem of Base Current 195 µa of base current in Q 1 can only support 19.5 ma of collector current, insufficient for high current operation (hundreds of ma). 25

26 Modification of the PNP Emitter Follower R out 1 ( β + 1) g 2 m3 Instead of having a single PNP as the emitter-follower, it is now combined with an NPN (Q 2 ), providing a lower output resistance. 26

27 Example: Input Resistance 1 iin = vin vin rπ 3 RL + r = β ( β + 1) R + r in 3 2 L π 3 R L 1 ( β + 1) g 2 m3 27

28 Additional Bias Current I 1 is added to the base of Q 2 to provide an additional bias current to Q 3 so the capacitance at the base of Q 2 can be charged/discharged quickly. 28

29 Example: Minimum V in Min V in 0 V out V EB2 Min V in V BE2 V out V EB3 +V BE2 29

30 HiFi Design Class B Amplifier with an OP amp connected in a negativefeedback loop to reduce cross over distortion

31 HiFi Design Using negative feedback, linearity is improved, providing higher fidelity. 31

32 Short-Circuit Protection Q s and r are used to steal some base current away from Q 1 when the output is accidentally shorted to ground, preventing short-circuit damage. 32

33 MOSFET Output Stage CMOS Class AB Output Stage. Biasing diodes are implemented with diode connected MOSFETS Q 1 and Q 2.

34 CMOS Inverter Output Stage CMOS Inverter for Wide Output Swing Negative Feedback path to reduce the output impedance Hi-Fi is also realized.

35 Emitter Follower Power Rating Pav = I 1 VCC V 2 P P = TV av,max 1 CC Maximum power dissipated across Q 1 occurs in the absence of a signal. 35

36 Example: Power Dissipation Avg Power Dissipated in I 1 1 T P 1( ) I1 = I V sin 0 p ωt VEE dt T P = IV I1 1 EE 36

37 Push-Pull Stage Power Rating P av VP VCC V = R π 4 L P P 2 VCC av,max = 2 π RL Maximum power occurs between V p =0 and 4V cc /π. 37

38 Example: Push-Pull P av P av VP VCC = R π L V 4 P If V p = 4V CC /π P av =0 Impossible since V p cannot go above supply (V CC ) 38

39 Heat Sink Heat sink, provides large surface area to dissipate heat from the chip. 39

40 Thermal Runaway Mitigation ID1ID2 I I = I I I I C1 C2 SD, 1 SD, 2 SQ, 1 SQ, 2 Using diode biasing prevents thermal runaway since the currents in Q 1 and Q 2 will track those of D 1 and D 2 as long as theie I s s track with temperature. 40

41 Efficiency η = P out P out + P ckt η η EF EF = Emitter Follower Push-Pull Stage 2 P 2R 2 PP 2 VP 2RL + I1 ( 2VCC VP 2) VP 2RL + 2 I1 ( VCC / π VP 4) V π P = η VV I 1 =V P /R L 4V CC V L η = PP P CC 4 V 2 P 2R = I 1 =V P /R L L Efficiency is defined as the average power delivered to the load divided by the power drawn from the supply 41

42 Example: Efficiency Emitter Follower V P =V CC /2 η = 1 15 η = 1 4 Push-Pull I 1 =(V P /R L )/β V CC VP π + V P β 42

43 Power Amplifier Classes Class A: High linearity, low efficiency Class B: High efficiency, low linearity Class AB: Compromise between Class A and B 43