Chapter 13 Output Stages and Power Amplifiers. Why Power Amplifiers? Power Amplifier Characteristics. Chapter Outline. Drive a load with high power.

Transcription

1 Chapter 3 Output Stages and ower Amplifiers Why ower Amplifiers? Drive a load with high power. 3. General Considerations 3. Emitter Follower as ower Amplifier 3.3 ush-ull Stage 3.4 Improved ush-ull Stage 3.5 Large-Signal Considerations 3.6 Short Circuit rotection 3.7 Heat Dissipation 3.8 Efficiency 3.9 ower Amplifier Classes Cell phone needs W of power at the antenna. Audio system needs tens to hundreds Watts of power. Ordinary Voltage/Current amplifiers are not equipped for such applications Chapter Outline ower 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. 3 4

2 ower Amplifier erformance Metrics Emitter Follower Large-Signal Behavior I Linearity ower Efficiency Voltage Rating As V in increases V out also follows and Q provides more current. 5 6 Emitter Follower Large-Signal Behavior II Example: Emitter Follower V V V I V V = 0.5V V mv out in T ln + = out RL IS in out I V V I I R ( ) C in = T ln + C L IS I 0.0I V 390mV C in However as V in decreases, V out also decreases, shutting off Q and resulting in a constant V out. 7 8

3 Linearity of an Emitter Follower ush-ull Stage As Vin decreases the output waveform will be clipped, introducing nonlinearity in I/O characteristics. As V in increases, Q is on and pushes a current into R L. As V in decreases, Q is on and pulls a current out of R L. 9 0 I/O Characteristics for Large V in Overall I/O Characteristics of ush-ull Stage V out =V in -V BE for large +V in V out =V in + V BE for large -V in For positive V in, Q shifts the output down and for negative V in, Q shifts the output up. However, for small V in, there is a dead zone (both Q and Q are off) in the I/O characteristic, resulting in gross nonlinearity.

4 Small-Signal Gain of ush-ull Stage Sinusoidal Response of ush-ull Stage The push-pull stage exhibits a gain that tends to unity when either Q or Q is on. When Vin is very small, the gain drops to zero. 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. 3 4 Improved ush-ull Stage Implementation of V B V B =V BE + V BE With a battery of V B inserted between the bases of Q and Q, the dead zone is eliminated. 5 Since V B =V BE + V BE, a natural choice would be two diodes in series. I in figure (b) is used to bias the diodes and Q. 6

5 Example: Current Flow I Example: Current Flow II I = I I + I in B B V D V BE V out V in I in If V out =0 & β =β >> => I B =I B If I =I & I B I B I in =0 when V out =0 7 8 Addition of CE Stage Bias oint Analysis V A =0 V out =0 I C =[I S,Q /I S,D ] [I C3 ] A CE stage (Q 4 ) is added to provide voltage gain from the input to the bases of Q and Q. 9 For bias point analysis, the circuit can be simplified to the one on the right, which resembles a current mirror. The relationship of I C and I Q3 is shown above. 0

6 Small-Signal Analysis Output Resistance Analysis A V =-g m4 (r π r π )(g m +g m )R L Assuming r D is small and (g m +g m )R L is much greater than, the circuit has a voltage gain shown above. R out ro 3 ro 4 + g + g ( g + g )( r r ) m m m m π π If β is low, the second term of the output resistance will rise, which will be problematic when driving a small resistance. Example: Biasing roblem of Base Current CE A V =5 Output Stage A V =0.8 R L =8Ω β npn = β pnp =00 I C I C g g + g = Ω m m m m C C π π C3 C4 ( 4 ) g Ω I I 6.5mA r r = 33Ω I I 95µ A 3 95 µa of base current in Q can only support 9.5 ma of collector current, insufficient for high current operation (hundreds of ma). 4

7 Modification of the N Emitter Follower Example: Input Resistance R out g ( β + ) m3 Instead of having a single N as the emitter-follower, it is now combined with an NN (Q ), providing a lower output resistance. 5 iin = vin vin rπ 3 RL + r = β ( β + ) R + r in 3 L π 3 R L ( β + ) g m3 6 Additional Bias Current Example: Minimum V in I is added to the base of Q to provide an additional bias current to Q 3 so the capacitance at the base of Q can be charged/discharged quickly. Min V in 0 V out V EB Min V in V BE V out V EB3 +V BE 7 8

8 HiFi Design Short-Circuit rotection Using negative feedback, linearity is improved, providing higher fidelity. 9 Q s and r are used to steal some base current away from Q when the output is accidentally shorted to ground, preventing short-circuit damage. 30 Emitter Follower ower Rating Example: ower Dissipation av = I V CC av,max CC Maximum power dissipated across Q occurs in the absence of a signal. = TV 3 Avg ower Dissipated in I T I = I ( V sin ) 0 p t VEE dt T ω = I V I EE 3

9 ush-ull Stage ower Rating Example: ush-ull av VCC av = R π 4 L VCC av,max = π RL Maximum power occurs between V p =0 and 4V cc /π. 33 av VCC = R π 4 L Impossible since V p cannot go above supply (V CC ) If V p = 4V CC /π av =0 34 Heat Sink Thermal Runaway Mitigation IDI D I I = I I I I C C S, D S, D S, Q S, Q Heat sink, provides large surface area to dissipate heat from the chip. 35 Using diode biasing prevents thermal runaway since the currents in Q and Q will track those of D and D as long as theie I s s track with temperature. 36

10 Efficiency Example: Efficiency Emitter Follower CC out η = + out Efficiency is defined as the average power delivered to the load divided by the power drawn from the supply ckt ush-ull Stage V RL RL ηef = η = RL + I ( VCC ) RL + I ( VCC / π 4) V π ηef = η 4V I =V /R = VCC I L 4 =V /R L 37 Emitter Follower V =V CC / η = 5 η = 4 V ush-ull I =(V /R L )/β CC π + V 38 β ower Amplifier Classes Class A: High linearity, low efficiency Class B: High efficiency, low linearity Class AB: Compromise between Class A and B 39