Chapter 14 Output Stages and Power Amplifiers

 14.1 General Considerations  14.2 Emitter Follower as Power Amplifier  14.3 Push-Pull Stage  14.4 Improved Push-Pull Stage  14.5 Large-Signal Considerations  14.6 Short Circuit Protection  14.7 Heat Dissipation  14.8 Efficiency  14.9 Power Amplifier Classes

1 Why Power Amplifiers?

 Drive a load with high power.

 Cellular phones need 1W of power at the antenna.

 Audio systems deliver tens to hundreds of watts of power.

 Ordinary Voltage/Current amplifiers are not suitable for such applications

CH 13 Output Stages and Power Amplifiers 2 Chapter Outline

CH 13 Output Stages and Power Amplifiers 3 Power Amplifier Characteristics

 Experiences low 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”.

CH 13 Output Stages and Power Amplifiers 4 Power Amplifier Performance Metrics

 Distortion - Linearity

 Power Efficiency

 Voltage Rating – Transistor Breakdown voltage

CH 13 Output Stages and Power Amplifiers 5 Emitter Follower Large-Signal Behavior I

Av R L R L1 g m 

For RL  8 ,

1/gImC 0.8  , thus,  32.5 mA

 As Vin increases Vout also follows and Q1 provides more current.

 For Vin=4.8V, Vout=4.0V, IL=500mA, IE1=532.5mA

CH 13 Output Stages and Power Amplifiers 6 Emitter Follower Large-Signal Behavior II

 However, as Vin decreases, Vout also decreases, shutting off Q1 and resulting in a constant Vout.

 For Vin=0.8V, Vout=0V, IL=0mA, IE1=32.5mA  For Vin=0.7V, Vout=-0.1V, IL=12.5mA, IE1=20mA  When all current (32.5mA) flows to the load, Vout=-0.26V

CH 13 Output Stages and Power Amplifiers 7 Example 14.1: Emitter Follower

 Vin=0.5V, Vout=?

Vout VVVIIin BE1  out,  1  C 1 RL

VVIIBE11 Tln( C / S )  Vout 1 VVIVin Tln  1  out RILS

For VVin  0.5

Vout  211 mV by a few iterations

CH 13 Output Stages and Power Amplifiers 8 Example 14.1: Emitter Follower

 For what Vin, Q1 carries only 1% of I1?

IC1 VVIIRinTCL ln  11 IS

For II0.01C11

Vin 390 mV

CH 13 Output Stages and Power Amplifiers 9 Linearity of an Emitter Follower

 As Vin decreases the output waveform will be clipped, introducing nonlinearity in I/O characteristics.

CH 13 Output Stages and Power Amplifiers 10 Push-Pull Stage

 As Vin increases, Q1 is on and pushes current into RL.

 As Vin decreases, Q2 is on and pulls current out of RL.

CH 13 Output Stages and Power Amplifiers 11 Example 14.2: I/O Characteristics for Large Vin

Vout = Vin-VBE1 for large +Vin

Vout = Vin+|VBE2| for large -Vin

 For positive Vin, Q1 shifts the output down and for negative Vin, Q2 shifts the output up.

CH 13 Output Stages and Power Amplifiers 12 Overall I/O Characteristics of Push-Pull Stage

 However, for small Vin, there is a “dead zone” (both Q1 and Q2 are off) in the I/O characteristic, resulting in gross nonlinearity.

CH 13 Output Stages and Power Amplifiers 13 Example 14.3: 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.

CH 13 Output Stages and Power Amplifiers 14 Example 14.4: 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.”

CH 13 Output Stages and Power Amplifiers 15 Improved Push-Pull Stage

VB = VBE1 + |VBE2|

 With a battery of VB inserted between the bases of Q1 and Q2, the dead zone is eliminated.

CH 13 Output Stages and Power Amplifiers 16 Implementation of VB

 Since VB=VBE1+|VBE2|, a natural choice would be two diodes in series.

 I1 in figure (b) is used to bias the diodes and Q1. CH 13 Output Stages and Power Amplifiers 17 Example 14.6: Current Flow I

IIIIinBB112

Iin

Usually IIVB12 B unless out 0

IVin flows even when out = 0.

CH 13 Output Stages and Power Amplifiers 18 Example 14.8: Current Flow II

VVDBE1  → VVoutin

IVIIin0 when out  0 if 12 

CH 13 Output Stages and Power Amplifiers 19 Addition of CE Stage

 A CE stage (predriver) is added to provide voltage gain from

the input to the bases of Q1 and Q2. CH 13 Output Stages and Power Amplifiers 20 Bias Point Analysis

Vout=0 VA=0

 For bias point analysis for Vout=0, the circuit can be simplified to the one on the right, which resembles a current mirror.

ISQ,1 IICC13 ISD,1 CH 13 Output Stages and Power Amplifiers 21 Small-Signal Analysis

 Assuming 2rD is small and (gm1+gm2)RL is much greater than 1,

Av  g m4 r 1 r 2  (  1) R L  g r r  r r g  g 1 R m4  1  2   1  2 m 1 m 2   L

 gm4 r 1 r 2 g m 1  g m 2  R L

CH 13 Output Stages and Power Amplifiers 22 Example 14.9: Output Resistance Analysis

1 rrOO34|| Rout  gm1 gg mm 21()( g m 2 || r 1 ) r 2

 If β is low, the second term of the output resistance will rise, which will be problematic when driving a small resistance. CH 13 Output Stages and Power Amplifiers 23 Example14.10: Biasing

 Compute the required bias current.

Predriver (CE stage):AV  5 125uA OutputStage:ARVL 0.8 for  8  60uA 65uA 2  100, II  6.5mA npn pnp C12 C 6.5mA 135uA 11 195uA gm1 g m 2 2   g m 1  g m 2  4 

Thus, IICC12 6.5 mA

rr12|| 400  || 200   133 

gm4 r 1|| r 2  1  g m 1  g m 2   R L  4 1 gImC44 133    195 μA

CH 13 Output Stages and Power Amplifiers 24 Problem of Base Current

 195 µA of base current in Q1 can only support 19.5 mA of collector current, insufficient for high current operation (500 mA for 4 V on 8 ). CH 13 Output Stages and Power Amplifiers 25 Modification of the PNP Emitter Follower

1 Rout  231 gm  Instead of having a single PNP as the emitter-follower, it is now

combined with an NPN (Q2), providing a lower output resistance. CH 13 Output Stages and Power Amplifiers 26 Example 14.11: Input Resistance

v R Comparing with the standard EF, out  L v 1 11 in R  rout  L 1 231 gm 1 231 gm 231 gm r 3 r 3

CH 13 Output Stages and Power Amplifiers 27 Example 14.11: Input Resistance

  1 RL iin v in v in r 3 1 RL  231 gm

rin3( 2  1) R L  r 3

CH 13 Output Stages and Power Amplifiers 28 Additional Bias Current

 I1 is added to the base of Q2 to provide an additional bias current to Q3 so the capacitance at the base of Q2 can be charged/discharged quickly. Additional pole at the base of Q2. CH 13 Output Stages and Power Amplifiers 29 Example 14.12: Minimum Vin

MinVin  0 Min VVinBE 2

VVout EB2 VVVoutEBBE32

CH 13 Output Stages and Power Amplifiers 30 Power Amplifier Classes

Class A: High linearity, low efficiency

Class B: High efficiency, low linearity

Class AB: Compromise between Class A and B

CH 13 Output Stages and Power Amplifiers 31 HiFi Design

 As Vout becomes more positive, gm rises and Av comes closer to unity, resulting in nonlinearity.  Using negative feedback, linearity is improved, providing higher fidelity.

CH 13 Output Stages and Power Amplifiers 32 Short-Circuit Protection

 Qs and r are used to “steal” some base current away from Q1 when the output is accidentally shorted to ground, preventing short-circuit damage. CH 13 Output Stages and Power Amplifiers 33 Power transistors

 Package and heat sink

Small-signal Transistor Power Transistor in Heat Sink Power Transistor Inside Emitter Follower Power Rating (Class A)

Pull down is done by a current source with huge current!

1 T P I V dt avT 0 C CE 1 T Vtsin I P  V  Vsin t dt 0 1 CCP TRL 2 VVVPPP IVIVI1 CCCC   1  if 1  RRLL2

 Maximum power dissipated across Q1 occurs in the absence of a signal. CH 13 Output Stages and Power Amplifiers 35 Example 14.13: Power Dissipation

 Avg Power Dissipated in the Current Source I1

1 T PIIpEE11 Vt V dt sin  T 0

 IV1  EE

CH 13 Output Stages and Power Amplifiers 36 Push-Pull Stage Power Rating

1 T /2 PIVdt  av, NPNCCET 0 1 T /2 Vtsin  VVtdt sin  P 0 CCP TRL 2 VVVCCPCC VVVPPP    RRRLLL 44

 No power for half of the period.

CH 13 Output Stages and Power Amplifiers 37 Push-Pull Stage Power Rating

1 T /2 PIVdt  av, NPNCCET 0 1 T /2 Vtsin  VVtdt sin  P 0 CCP TRL 2 VVVCCPCC VVVPPP    RRRLLL 44

 No power for half of the period.

 Maximum power occurs between Vp=0 and 4Vcc/π.

2 VVVCC P CC PVav,max 2 when P  2   RL 

CH 13 Output Stages and Power Amplifiers 38 Push-Pull Stage Power Rating

1 T PIVdt  av, PNPCCET T /2 1 T Vtsin   Vtsin Vdt  P T /2 PEE  TRL 2 VVVVVVEE PPPEEP    RRRLLL 44

PPVVav,, PNPav NPNCCEE  when

 Maximum power occurs between Vp=0 and 4VCC/π. 2 VVVCC P CC PVav,max 2 when P  2   RL  CH 13 Output Stages and Power Amplifiers 39 Heat Sink

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

CH 13 Output Stages and Power Amplifiers 40 Thermal Runaway Mitigation

IIDD12 VVVVDDTT12 lnln IIS, DS 1, D 2

IIDD12  VT ln IISDSD, 1 , 2

IICC12 VVVVBEBETT12 lnln IIS, QS 1, Q 2

IICC12  VT ln IISQSQ, 1 , 2

With the same VT , II II DD12 CC12 IIIIS, D 1 S , DS 2, Q 1 S , Q 2

 Using diode biasing prevents thermal runaway since the

currents in Q1 and Q2 will track those of D1 and D2 as long as their Is’s track with temperature. CH 13 Output Stages and Power Amplifiers 41 Efficiency

 Efficiency is defined as the average power delivered to the load divided by the power drawn from the supply

Power Delivered to Load P  out Power Drawn From Supply Voltage PPoutckt

Emitter Follower (Class A)

2 VRPL2 EF  2 VP22 R LCC I11 V PEE  VI V

VVPP  if  IV and1 V EE CC 4VRCCL

 Maximum efficiency for EF is 25%.

CH 13 Output Stages and Power Amplifiers 42 Example 14.15: Efficiency of EF

 EF designed for full swing operates with half swing.

VP VCC With IVV1  = and EE  CC RRLL 2 VRPL2 EF  2 VRIVVIVP22 L11 CC  P   EE VR2 2  PL 2 VVCCCC VRVVVPLCCPCC22    RRLL Thus, 1  6.7% EF VV /2 P CC 15

CH 13 Output Stages and Power Amplifiers 43 Efficiency

Push-Pull Stage (Class A or AB)

2 VP 2RL PP  2 VVVPPP 2 VCC  24RRLL  V  P 4 VCC

 Maximum efficiency for PP is 78.5%.

CH 13 Output Stages and Power Amplifiers 44 Example 14.16: Efficiency incl. Predriver

IVR1   PL  2 VP 2R   L 2VVP CC VP ()VVCC EE RRLL 1 V  P 4 VVCCCC 11 V  P 11 4  VCC 

CH 13 Output Stages and Power Amplifiers 45