ESE319 Introduction to Microelectronics
Class B Output Stage
●
●
●
Class B Operation
Multisim Simulations
Class B Power Efficiency
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
1
ESE319 Introduction to Microelectronics
Class B Amplifier Operation
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
2
ESE319 Introduction to Microelectronics
The Class B Output Stage
i CN
For vI > 0.7 V
V BEN =v I −v O
vO =v I −0.7V
i EN
For vI < -0.7 V
V EBP=v O −v I
vO =v I 0.7V
i EP
I. For vI > VBEN = 0.7 V, QN = ON, Qp = OFF.
a. i L =i EN ≈i CN
b. QN saturates if vO > VCC - VCE-sat.
c. QP is cut off for vI > -0.7 V.
The output voltage vO follows the input vI, less
the QN VBE drop, up to QN saturation.
II. For vI < VBEP = -0.7 V, QP = ON, QN = OFF.
i CP
0. For vI = 0 V, QN and QP cutoff.
vO = 0 V
a. i L =−i EP ≈−iCP
b. QP saturates if vO < - VCC + VCE-sat
c. QN is cut off for vI < 0.7 V.
The output voltage vO follows the input vI, plus
the QP VEB drop, down to QP saturation.
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
3
ESE319 Introduction to Microelectronics
Class B Amplifier VTC Plot
QN saturated
vo =v i −0.7 V
For:
0.7 V v i V CC −V CENsat 0.7V
-0.7 V
v o =0 V
For:
−0.7 V vi 0.7 V
0.7 V
vo =v i 0.7 V
For:
QP saturated
−V CC −V ECPsat −0.7 V v i −0.7 V
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
4
ESE319 Introduction to Microelectronics
Class B Crossover Distortion
Crossover distortion in audio power amps produces unpleasant
sounds.
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
5
ESE319 Introduction to Microelectronics
Multisim Simulation Class B VTC
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
6
ESE319 Introduction to Microelectronics
Multisim Simulation Class B VTC - cont.
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
7
ESE319 Introduction to Microelectronics
Multisim Simulation Class B VTC - cont.
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
8
ESE319 Introduction to Microelectronics
Multisim Simulation Class B Crossover Distortion
The “dead band” from VBEP = 0.7 V < vi < VBEN = 0.7 V causes crossover
distortion. This distortion is severe, particularly for low voltage signals.
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
9
ESE319 Introduction to Microelectronics
Class B Power Analysis
Simplifying Assumptions:
1. Ignore the dead-zone in vO.
2. Let vCEN-sat = vECP-sat = 0 V => maximum vO swing is −V CC ≤v o ≤V CC
The average power to the load resistor:
2
2
V o−rms 1 V o− peak
P L−av =V o−rms I o−rms =
=
RL
2 RL
The average power delivered by the positive +VCC power
supply over the positive half-cycle (current is zero from T/2 to T):
T
2
T
2
T
2
V o− peak
2
1
1
1
P DP−av = ∫ V CC i CN dt = ∫ V CC i L dt = ∫ V CC
sin 
t dt
T 0
T 0
T 0
RL
T
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
10
ESE319 Introduction to Microelectronics
Class B Power Analysis - cont.
T
2
V o− peak
2
1
P DP−av = V CC
sin

t dt
∫
T
RL 0
T
−1
Recalling from calculus ∫ sin ax dx=
cos ax 
a
t = T/2
V o− peak −T
2
1
P DP−av = V CC

cos 
t  t = 0
T
RL
T
2
1 V o− peak
2 T
2
P DP −av =−
V CC {cos   −cos  0}
R
T 2
T
2
L
-1
1
1 V o− peak
P DP −av =−
V CC {cos −cos 0}
2  RL
1 V o− peak
P DP −av =
V CC
 RL
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
11
ESE319 Introduction to Microelectronics
Class B Power Analysis - cont.
Power delivered by the negative –VCC power source is equal to that
for +VCC:
1 V o− peak
P DN −av =P DP−av =
V CC
 RL
Hence:
2 V o− peak
P D−av =P DP−av  P DN −av =
V CC
 RL
The power conversion efficiency of the class B amplifier (accurate for
|Vo-peak| > 0.7 V) is:
2
1 V o− peak
P L−av
2 RL
 V o− peak
=
=
=
P D−av 2 V o− peak
4 V CC
V CC
 RL
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
12
ESE319 Introduction to Microelectronics
Class B Power Analysis - cont.
Maximum Class B power conversion efficiency:
Since we assume −V CC ≤v o ≤V CC
 V o− peak

=
⇒max = =0.785 or 78.5 %
4 V CC
4
Recall for Class A: max =0.25 or 25 %
Power Dissipation:
Class B power dissipated in the transistors:
2
2 V o− peak
1 V o− peak
P Disp=P D−av −P L =
V CC −
2 RL
 RL
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
13
ESE319 Introduction to Microelectronics
Class B Power Analysis - cont.
Note that the power dissipation goes to zero as Vo-peak approaches
zero ( More precisely, it's zero if Vo-peak is less than 0.7 V):
2
2 V o− peak
1 V o− peak
P Disp=P D−av −P L−av =
V CC −
R
2 RL

L
To estimate maximum PDisp: set derivative of PDisP w.r.t. Vo-peak to zero, i.e.
d P Disp
2 V CC V o− peak
=
−
=0 =>
d V o− peak  R L
RL
2
V o− peak = V CC V CC

2
2
2
4 V CC 1 4 V CC 2 V CC
P Disp max = 2
−
= 2
2
 RL 2  RL  RL
½ PDisp(max) is dissipated in QN and ½ PDisp(max) is dissipated in QP.
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
14
ESE319 Introduction to Microelectronics
Class B Power Analysis Summary
Power from sources:
Power output to load:
Power Conversion
Efficiency:
2 V o− peak
P D−av =
V CC
 RL
2
V
1 o− peak
P L−av =
2 RL
 V o− peak
=
4 V CC
2
V
V
2 o− peak
1 o− peak
Transistor dissipation P Disp=
V CC −
2 RL
 RL
Q +Q :
N
P
Max dissipation at:
V o− peak =
2
V CC

2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
2
V
2 CC
P D−av max=
 RL
2
1 V CC
P L−av max=
2 RL

max = ≈0.785
4
2
2 V CC
P Disp max= 2
 RL
max −Disp =0.5
15
ESE319 Introduction to Microelectronics
Power Dissipation Function Plot
Let VCC = 12 V and R L =100 
P Disp
P Disp max =
2
2V CC
2
 RL
2
2 V o− peak
1 V o− peak
P Disp=
V CC −
 RL
2 RL
=0.29 W
PDisp(max) =
0.29 W
0.20 W
0.7 V
= 7.63 V
V o− peak
Not accurate for small Vo-peak.
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
16
ESE319 Introduction to Microelectronics
Power Conversion Efficiency vs.
Output Voltage Amplitude
Let VCC = 12 V and vo =vi −0.7V
0.8
max =0.785
0.7
0.6
 V o− peak
=
4 V CC
0.5
Efficiency  0.4
0.3
0.2
0.1
0.0
0
2
4
6
8
V o− peak Volts
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
10
12
17
ESE319 Introduction to Microelectronics
Class B Audio Amplifier Example
It is required to design a Class B output stage
to deliver an average power of 20 W to an
8  speaker load.
To avoid saturation of the transistors and nonlinear distortion, the power supply VCC is to be
5V greater than peak output voltage.
1. Determine the VCC required.
2. Determine peak current drawn from each
power supply.
3. Determine the total power drawn from the
power supplies.
4. Determine the power conversion efficiency.
5. Determine the max power each transistor
must be able to dissipate safely.
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
18
ESE319 Introduction to Microelectronics
Class B Audio Amplifier Example - cont.
SOLUTION:
1. Determine the VCC required.
P L−av =20 W and R L =8 
2
1 V o− peak
P L−av =
=20W ⇒V o− peak =  2 P L−av R L=  2∗20∗8=17.9V
2 RL
V CC =V o− peak 5 V =17.9V 5 V =23V
2. Determine peak current drawn from each power supply.
V o− peak 17.9V
I o− peak =
=
=2.24 A
RL
8
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
19
ESE319 Introduction to Microelectronics
Class B Audio Amplifier Example - cont.
SOLUTION cont.:
3. Determine the total power drawn from the power supplies.
2 V o− peak
2 17.9V
P D−av =
V CC =
23V =32.8 W
 RL
 8
4. Determine the power conversion efficiency.
 V o− peak  17.9V
=
=
=0.61 or 61 %
4 V CC
4 23V
5. Determine the max power each transistor must dissipate safely.
2
V
1
1 CC 1 23V 2
P DispN max = P DispP max = P Dispmax = 2
= 2
=6.7W
2
 RL  8 
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
20
ESE319 Introduction to Microelectronics
Multisim Simulation – Max Power Dissipation
vO =v I −0.7V
P DP−av
P L−av
1 V o− peak
V CC
 RL
= 0.265 W
P DP−av =
2
1 V o− peak
P L−av =
2 RL
= 0.240 W
Vi-peak = 7.63 V
P DN − av
vO =v I 0.7V
1 V o− peak
V CC
 RL
= 0.265 W
P DN −av =
 sim=
P L−av
0.2198
=
=0.45
P DP−av  P DN −av 0.24290.2449
theory =
P L−av
0.240
=
=0.45
P DP−av P DN −av 0.2650.265
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
21
ESE319 Introduction to Microelectronics
Multisim Simulation – Max Power
Conversion Efficiency
P L−av
P DP−av
Vi-peak = 12.7 V
P DN − av
 sim=
P L−av
0.6772
=
=0.79
P DP−av P DN −av 0.42310.4239
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
22
ESE319 Introduction to Microelectronics
Anticipating Class AB – Eliminating
Crossover Distortion
If we bias the bases of the emitter follower pair, we can
set both transistors on the verge of conduction – or have
them conduct a small bias current with zero signal input:
V BB
≈0.7 V.
2
Class AB is a compromise
between Class and Class B
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
23
ESE319 Introduction to Microelectronics
Summary
Class B advantages:
1. Much higher power conversion efficiency than class A
for large signal amplitudes.
2. Zero power dissipation with zero input.
Class B disadvantage:
Higher distortion than class A, especially at low input
amplitudes.
Class AB operation is a compromise mode:
1. Delivering better linearity than class B, with reduced
efficiency.
2. Delivering better efficiency than class A, with reduced
linearity.
2008 Kenneth R. Laker (based on P. V. Lopresti 2006) updated 05Nov08 KRL
24