Cross-Over Distortion
The non-zero “turn-on” voltage of a transistor
causes cross-over distortion in a class B
output stage.
vout
Ideal
response
Approximate
transistor
response.
0
VBE
vin
Eliminating Cross-Over Distortion
vout
NPN response
for vB =
vIN+0.7
NPN
response
vin
PNP
response
PNP
response for
vB = vIN-0.7
Class AB Output Stage
Eg. Positive half cycle:
vB1  vin  Vbias
If vB1  VBE
vout  vB1  VBE  vin  Vbias  VBE
If Vbias  VBE
vout  vin
Practical Class AB Stages



In practice, there isn’t an exact “turn-on”
voltage (VBE).
Vbias is set slightly high so that there is a nonzero quiescent collector current.
Each transistor will now conduct for slightly
more than 180° - i.e. Class AB operation.
Class AB Efficiency



Slightly more power is dissipated using a
class AB stage compared with a class B due
to the non-zero quiescent collector current.
In a well designed circuit, this extra power
should be insignificant so the class B
efficiency calculations are still valid.
I.e. maximum efficiency = 78 %.
Thermal Effects
The quiescent collector current depends on VBE and also
on the junction temperature. So, in designing the biasing
network, thermal effects must be considered.
 qVBE 
I C  I S exp 
 kT 
but
VBE  0.6 V
 qVG 
I S  I G exp 
 kT 
VG  1.2 V
Net result is that if VBE is fixed, IC rises exponentially
with temperature.
Collector Current [mA] (VBE=0.5 V)
Thermal Effects
1.2
0.8
0.4
0
20
30
40
Temperature [°C]
50
60
Thermal Runaway
Collector Current Flows, so
power is dissipated
Temperature rises
Collector current rises
Power dissipation
increases
Suppressing Thermal Runaway




Fit a bigger heatsink.
Use series emitter-resistors.
Use a temperature dependent bias voltage.
The latter two are preferred methods. Both
introduce negative feedback.
Emitter Resistors
2Vbias  VBE 1  VEB 2  2VRE
By symmetry:
VBE 1  VEB 2  Vbias  VRE
 Vbias  I C RE
So, if IC rises, VBE falls and
IC is reduced.
Note RE should be small
compared with RL to minimise
power wasted.
Bias Voltage – The VBE Multiplier
Base current is negligible, so:
VBE
I R1  I R 2 
R2
VBE R1
 VR1  I R 2 R1 
R2
VBE
Vbias  VR1  VR 2
Vbias
 R1 
VBE R1

 VBE  VBE 1  
R2
 R2 
 R1 
 0.51  
 R2 
VBE Multiplier – Temperature Effects



If junction temperature rises but IC stays the
same, VBE must fall causing Vbias to fall also.
Negative thermal feedback achieved if the
transistor is in close contact with the output
devices.
Especially suitable for integrated circuits
where close thermal contact is guaranteed.
Design Example – (i) RE
Let RL = 16 W and Amax = 12 V.
(Also assume Vout = 0 through
d.c. feedback).
VE1(max)  VB1(max)  VBE  15  0.7  14.3 V
RL
VE1 max 
 Amax
RE  RL
 14.3
16
 12
RE  16
 RE  3 W
Let RE  2 W
Design Example – (ii) Ibias
I bias  I B1(max) 
I C1(max)
 (min)

A(max)
RL  (min)
12

16  200
 I bias  3.75 mA
Let I bias  10 mA
NB. Ibias is set well above
minimum to ensure that a
significant current flows through
the VBE multiplier.
Design Example – (iii) Vbias
Peak output current = 0.75 A,
choose quiescent collector current
to be small by comparison, e.g.
I C1  I C 2  25 mA
Vbias  2VBE  2VRE
 2  0.7  2  0.025 2
Vbias  1.5 V
Design Example – (iii cont) Vbias
For constant bias voltage,
I B 3  I R1  I bias
so choose I R1  1 mA
VR 2  VBE  0.5  I R1 R2
 R2  500W
 R1 
Vbias  0.51    1.5
 R2 
 R1  2 R 2  1 kΩ
Class AB – Summary


Class AB achieves the efficiency of a class B
output stage but without cross-over
distortion.
Biasing arrangements are more complex,
however, as the threat of thermal runaway
must be eliminated.