CHAPTER 13 OUTPUT STAGES AND POWER AMPLIFIERS
Chapter Outline
13.1 Classification of Output Stages
13.2 Class A Output Stage
13.3 Class B Output Stage
13.4 Class AB Output Stage
13.5 Biasing the Class AB Circuit
13.6 CMOS Class AB Output
p Stages
g
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13.1 CLASSIFICATIONS OF OUTPUT STAGES
Output Stages
 Class A output stage:
 Bias current is greater than the magnitude of the signal current
 Conduction angle is 360
 Class B output stage:
 Biased at zero dc current
 Conduction angle is 180
 Another transistor conducts during the alternate half-cycle
 Class AB output stage:
 An
A iintermediate
t
di t class
l between
b t
A andd B
 Biased at a nonzero dc current much smaller than the peak current of the signal
 Conduction angle is greater than 180 but much smaller than 360
 Two transistors are used and currents are combined at the load
 Class C output stage:
 Conduction angle is smaller than 180
 The current is passed through a parallel LC network to obtain the output signal
 Class A, B and AB are used as output stage of op amps
 Class AB amplifiers are preferred for audio power amplifier
 Class C amplifiers are usually used at higher frequencies
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 Collector current waveforms for the transistors operating in different classes
 The classification also applies for output stages with MOSFETs
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13.2 CLASS A OUTPUT STAGE
Transfer Characteristics
 Q1 biased with a constant current I supplied by Q2
vO  vI  vBE1
vO max  VCC  VCE1sat
vO min   IRL or vO min  VCC  VCE 2 sat
I
| VCC  VCE 2 sat |
RL
Si
Signal
l Waveforms
W f
 The output swing from VCC to VCC for I = VCC/RL
 The instantaneous power dissipation in Q1: PD1 = vCE1iC1
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Power Dissipation
 Power dissipation for RL = VCC/I:
 The maximum instantaneous power dissipation in Q1 is VCCI
 This is equal to the power dissipation in Q1 with no input signal
applied (quiescent power dissipation)
 The transistor Q1 much be able to withstand a continuous power
dissipation of VCCI
 Power dissipation for unloaded case:
 Maximum power dissipation occurs when vO = VCC
 The maximum power dissipation in Q1 is 2VCCI
 Power dissipation for an output short circuit:
 A positive input may lead to an infinite load current
 The output stages are usually equipped with short-circuit protection to guard against such a situation
 Power dissipation in Q2:
 Q2 conducts a constant current I
 Maximum voltage across the collector and the emitter is 2VCC
 Maximum instantaneous power dissipation in Q2 is 2VCCI
 A more significant quantity for design purposes is the average power dissipation of VCCI
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Power Conversion Efficiency
 The power conversion efficiency is defined as   PL(load power) /PS(supply power)

 The load power (PL) with an sinusoid output with a peak value of Vo is


(Vo / 2 ) 2 1 Vo2

PL 
RL
2 RL
 The total average supply power is PS = 2VCCI
 The conversion efficiency is given by



1 Vo2
1  Vo  Vo


 
4 IRLVCC 4  IRL  VCC




 Maximum efficiency (25%) is obtained when Vo  VCC  IRL
 Class A output stage is rarely used in high-power applications
 The efficiency achieved in practice is usually in the range of 10% to 20%
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13.3 CLASS B OUTPUT STAGE
Circuit Operation




Both transistors are cut off when vI is zero  vO is zero
One of the transistor turns on as vI exceeds 0.5 V  vO follows vI
The circuit operates in a push-pull fashion
The class B stage is biased at zero current and conducts only when
the input signal is present
Transfer Characteristic
 There exists a range of input centered around zero
where both QN and QP are off
 The transfer characteristic shows a dead band
which results in the crossover distortion at the output
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Power Conversion Efficiency
 The average
load power by neglecting the cross-over distortion is

PL 
1 Vo2
2 RL

 The current drawn from each supply consists of half-sine waves of peak amplitude Vo / RL
 The average power
drawn from each of the two power supply is

PS   PS  
1 Vo
VCC
 RL
 The total
 supply power is
PS 
2 Vo
VCC
 RL
 The
Th efficiency
ffi i
is
i given
i
by
b

 Vo
4 VCC
 Maximum efficiency is obtained when the output swing is maximized ( VCC):


4
 78.5%
 The maximum
average power available from a class B stage is

PL 
1 Vo2
2 RL
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Power Dissipation
 The quiescent power dissipation of the class B stage is zero (unlike class A)
 The average power dissipation of the class B stage is given by PD = PS  PL


2 Vo
1 Vo2
VCC 
PD 
 RL
2 RL
 QN and QP must be capable of safely dissipating half of PD
 PD depends on the output swing and the worst-case power dissipation is given by

Vo
PD max

2

VCC  PD max
2
2 VCC

 RL
 The maximum power dissipation of QN and QP occurs at  = 50%:
PDN max  PDP max
2
1 VCC

 RL
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Reducing Crossover Distortion
 The distortion can be reduced by employing a high-gain op amp and overall negative feedback
 The 0.7V dead band is reduced by a factor of the dc gain of the op amp
 The slew rate limitation of the op amp may cause the alternate turning on and off to be noticable
Single-Supply Operation
 The class B stage can be operated from a single supply
 The load is capacitivelu coupled
 The derivations are directly applicable with supply of 2VCC
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13.4 CLASS AB OUTPUT STAGE
Circuit Operation
 Cross-over distortion can be eliminated by biasing QN and QP at a small nonzero current
 The bias current iN = iP = IQ = Isexp(VBB/2VT)
 When vI goes positive by a certain amount:
vO  vI  VBB / 2  vBEN
vBEN  vBEP  VBB
VT ln
I
i
iN
 VT ln P  2VT ln Q  iN iP  I Q2  iN2  iN iL  I Q2  0
IS
IS
IS
 The load current is supplied by QN which acts as the output emitter follower
 QP will be conducting a current that decreases as vO increases (negligible for large vO)
 QP acts as the output emitter follower when vI goes negative
 The power properties are almost identical to those derived for the class B stage
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Output Resistance
 The output resistance is estimated by assuming the source supply vI ideal
Rout  reN || reP 
VT VT
V
||
 T
i N iP i N  i P
 The output resistance remains approximately constant in the region around vI = 0
 The output resistance decreases at larger load currents
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13.5 BIASING THE CLASS AB CIRCUIT
Biasing Using Diodes
 The bias voltage VBB is generated by passing a constant current IBIAS through a pair of diodes
 The diodes need not to be large devices
 Quiescent current IQ in QN and QP will be IQ = nIBIAS where n is the ratio of the areas of the emitter
junction of the BJT and the junction area of the diodes
 IBN increases from IQ/N to IL/N for a positive vO
 IBIAS has to be greater than the IBN for maximum IL case
 The ratio n cannot be a large number as n = IQ/IBIAS
 This biasing arrangement provides thermal stabilization of the quiescent current in the output stage
 Collect current increases with temperature
p
for a fixed VBE
 Heat from power dissipation increases with current
 Positive feedback may cause thermal runaway
 VBB decreases at the same rate of VBEN + VEBP
 Thermal runaway is alleviated with close thermal contact
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Biasing Using the Voltage Multiplier
 The class AB stage can be biased by VBE multiplier
I R  VBE1 / R1
VBB  VBE1 (1  R2 / R1 )
 The value of VBE1 is determined by the portion of IBIAS that flows through the collector of Q1
I C1  I BIAS  I R
I C1
I
VBE1  VT ln C1  VT ln
I BIAS  I R
IS
 The quiescent current can be adjusted by the resistance value
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13.6 CMOS CLASS AB OUTPUT STAGES
The Classical Configuration
 Circuit operation


1
1

VGG  VGS1  VSG 2  Vtn  | Vtp |  2 I BIAS 

 k n (W / L)1

k p (W / L) 2 



1
1

VGG  VGSN  VSGP  Vtn  | Vtp |  2 I Q 

 k n (W / L) n
p (W / L) p 
k


 1 / k  (W / L) 1 / k p (W / L) 2
1
n
I Q  I BIAS 

 1 / kn (W / L) n 1 / k p (W / L) p





2
 For the case Q1 and Q2 are matched and QP and QN are matched:
I Q  I BIAS
(W / L) n
(W / L)1
 A drawback of the CMOS class AB circuit is the restricted range of output voltage swing
vO max  VDD  VOV |BIAS Vtn  vOVN
vO min  VSS  VOV | I  | Vtp |  | vOVP |
where
vOVN is the overdrive voltage of QN when it is supplying iLmax
and
|vOVP| is the overdrive voltage of QP when sinking the maximum negative value of iL
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An Alternative Circuit Using Common-Source Transistors
 The allowable output range can be increased by replacing the source followers with a pair of
complementary transistors in the common-source configuration
 QP supplies the load current when vO is positive, allowing an output as high as VDD  |vOVP|
 QN sinks the load current when vO is negative, allowing an output as low as VSS + vOVN
 The disadvantage is its high output resistance Rout = ron||rop
 Negative feedback (error amplifiers) is employed to reduce the output resistance
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Output Resistance
 The output resistance is derived by two half circuits: Rout  Routn || Routp
 The analysis techniques for feedback (shunt-series feedback) is utilized:
  1 and A 
vo
 g mp ( rop || RL )
vi
 The open-loop output resistance: Ro  RL || rop
 The output resistance with feedback: Rof  Ro /(1  A)  ( RL || rop ) /[1  g m ( RL || rop )]
 The output resistance excluding RL: Routp  1 /(1 / Rof  1 / RL )  rop ||
 Overall output resistance: Rout  1 /  ( g mp  g mn )
1
1

g mp g mp
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The Voltage Transfer Characteristics
1
2
2
 For the case where QN and QP are matched: I Q  kVOV
 Drain currents: iDN

v v
 I Q 1   O I
VOV

2

 and

iDP

v v
 I Q 1   O I
VOV




2
 Load current: iL  iDP  iDN

 Output voltage: vO  vI /1 

 Gain error: vO  vI  

VOV 
VOV 
 v I 1 
 4I R 
4I Q RL 
Q L 

VOV
VOV

4I Q RL
2g m RL
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