Class AB Output Stage - Penn Engineering · Class AB Amplifier Operation – VTC cont. i P i N =I Q...
Transcript of Class AB Output Stage - Penn Engineering · Class AB Amplifier Operation – VTC cont. i P i N =I Q...
ESE319 Introduction to Microelectronics
1Kenneth R. Laker, updated 14Nov14 KRL
Class AB Output Stage● Class AB amplifier Operation● Multisim Simulation - VTC● Class AB amplifier biasing● Widlar current source● Multisim Simulation - Biasing
ESE319 Introduction to Microelectronics
2Kenneth R. Laker, updated 14Nov14 KRL
Class AB Operation
IQ
IQ
VB
vI
(set by VB)
ESE319 Introduction to Microelectronics
3Kenneth R. Laker, updated 14Nov14 KRL
Basic Class AB Amplifier Operation1. Bias Q
N and Q
P into slight conduction (fwd. act.)
when vI = 0: i
N = i
P.
2 Ideally QN and Q
P are:
a. Matched (unlikely with discrete transistors and challenging in IC).
b. Operate at same ambient temperature.
NOTE. This is base-voltage biasing with all its stability problems!
iL=iN−iP
3.For vi > 0: i
N > i
P i.e. Q
N most cond. (like Class B).
4.For vi < 0: i
P > i
N i.e. Q
P most cond. (like Class B).
V BB
ESE319 Introduction to Microelectronics
4Kenneth R. Laker, updated 14Nov14 KRL
Class AB VTC Plot
Ideally the two DC base voltage sources are matched and VBB/2 = 0.7 V.V BB /2≈0.7V
Ideally, zero cross-over distortion
ESE319 Introduction to Microelectronics
5Kenneth R. Laker, updated 14Nov14 KRL
Class AB VTC Simulation
VBB/2
VBB/2
VCC
-VCC
RL
RSig
Amplitude: 20 Vp
Frequency: 1 kHz
Looks like Class A VTC
ESE319 Introduction to Microelectronics
6Kenneth R. Laker, updated 14Nov14 KRL
Class AB VTC Simulation - cont.
V BB
2=0.1V
V BB
2=0.5V
V BB
2=0.7V
Amplitude: 2 Vp
Frequency: 1 kHz
Cross-over distortion
ESE319 Introduction to Microelectronics
7Kenneth R. Laker, updated 14Nov14 KRL
Class AB Amplifier Operation - cont.
I N=I P= I Q= I S eV BB
2V T
Output voltage for vi ≠ 0:
for vi0 vo=viV BB
2−v BEN ⇒ vo≈vi
Base-to base voltage is constant!vBENvEBP=V BB
for vi0 vo=vi−V BB
2v EBP⇒ vo≈v i
Bias (QN & Q
P matched):
iN=iPiL
vBEN=V BB
2v i−vO
vEBP=vO−−V BB
2v i
for vi ≠ 0
+vi
for all vi
iN iP= I Q2
Let us next show thatfor all v
i
V BB
ESE319 Introduction to Microelectronics
8Kenneth R. Laker, updated 14Nov14 KRL
Class AB Amplifier Operation - cont.
for v i0 vo=v iV BB
2−vBEN ⇒vBEN=vi−vo
V BB
2
for v i0 vo=v i−V BB
2vEBP⇒ vEBP=vo−v i
V BB
2
ADD
vBENv EBP=V BB for all vi
iN=I S evBEN
V T ⇒vBEN=V T ln iN
I S iP=I S evEBP
V T ⇒vEBP=V T ln iP
I S Using the currents
I N=I P= I Q= I S eV BB
2V T ⇒V BB=2V T ln I Q
I S V T ln iN
I S V T ln iP
I S =2V T ln I Q
I S for all vi
Note for Class B VBB
= 0
ESE319 Introduction to Microelectronics
9Kenneth R. Laker, updated 14Nov14 KRL
Class AB Amplifier Operation - cont.
V T ln iN
I S V T ln iP
I S =2V T ln I Q
I S V T ln iN iP
I S2 =2V T ln I Q
I S ln iN iP −ln I S
2 =2ln I Q−2ln I S
ln iN iP =ln I Q2
iN iP= I Q2
iN=iPiL
Constant base voltage condition:
from the previous slide
vBENvEBP=V BB =>
or iN iP= I Q2
ESE319 Introduction to Microelectronics
10Kenneth R. Laker, updated 14Nov14 KRL
Class AB Amplifier Operation – VTC cont.iP iN= I Q
2The constant base voltage condition
For example let IQ = 1 µA and iN = 10 mA.
iP=I Q2
iN=1⋅10−6
10⋅10−3=0.1mA= 1
100iN
The Class AB circuit, over most of its input signal range, operates as if either the Q
N or Q
P transistor is conducting and the Q
P or Q
N transistor is cut off.
For small values of vI both Q
N and Q
P conduct, and as v
I is increased or
decreased, the conduction of QN or Q
P dominates, respectively.
Using this approximation we see that a class AB amplifier acts much like a class B amplifier; but with a much reduced dead zone.
where IQ is typically small.
ESE319 Introduction to Microelectronics
11Kenneth R. Laker, updated 14Nov14 KRL
Class AB Power Conversion Efficiency & Power Dissipation Similar to Class B
Accurate for small Vo-peak
.
V o− peak
Let VCC = 12 V and RL=100
= 7.63 V0.7 V
PDisp(max) = 0.29 W
0.20 W
PDisp−B=2
V o−peak
RLV CC−
12
V o−peak2
RL
P Disp
PDisp max=2V CC
2
2RL
=0.29W
P Disp ≠ 0 when Vo-peak
= 0
ESE319 Introduction to Microelectronics
12Kenneth R. Laker, updated 14Nov14 KRL
Class AB Amplifier BiasingA straightforward biasing approach: D1 and D2 are diode-connectedtransistors identical to QN and QP, respectively.They form mirrors with the quiescentcurrents IQ set by matched R's:
I Q=2V CC−1.42R
=V CC−0.7
R
R=V CC−0.7
I Q
Recall: With mirrors, the ambient temperature for all transistors needs to be matched!
or:
QN
QP
+
-
VBB
IQ
IQ
IQ
IQ
D2
D1
current mirror
V BB=V CC−I Q R− I Q R−V CC
ESE319 Introduction to Microelectronics
13Kenneth R. Laker, updated 14Nov14 KRL
Widlar Current Source
I REF=V CC−V BE1
R=12V −0.7V
11.3k =1mA
V BE1=V T ln I REF
I S
I Q Re=V BE1−V BE2=V T ln I REF
I Q
IREF
VCC
IQ
IQ
VBE1
+ +- -V
BE2
R
Re
emitter degeneration
Q2 = QNI
Q = I
N
Note: Pages 543-546 in Sedra & Smith Text.
IN = bias current for Class AB amplifier NPN
Note Re ≥ 0 iff IQ ≤ IREF
V BE2=V T ln I Q
I S V BE1=V BE2I Q Re⇒V BE1−V BE2=I Q Re
V BE1−V BE2=V T ln I REF
I S
I S
I Q=V T ln
I REF
I Q
ESE319 Introduction to Microelectronics
14Kenneth R. Laker, updated 14Nov14 KRL
Widlar Current Source - cont.
I Q Re=V T ln I REF
I Q
RI
REF
IQ
VCC
If IQ specified and IREF chosen by designer:
Re=V T
I Qln I REF
I Q Example Let IQ = 10 µA & choose IREF = 10 mA, determine R and Re:
R=V CC−V BE1
I REF=12V−0.7V10mA
=1.13 k
Re=V T
I Qln I REF
I Q =0.025V10 A
ln 10m A10A
.=2500 ln 1000=17.27 k R=1.13 k Re=17.27 k
IQ
Re
R=V CC−V BE1
I REF
ESE319 Introduction to Microelectronics
15Kenneth R. Laker, updated 14Nov14 KRL
Widlar Current Mirror Small-Signal Analysis
r≫1/gm
.≈.
i x=g m viro=gm vv x−−v
rov=−r∥Re i x
i x=−g m r∥Re i xv x
ro−
r∥Re i x
ro
gm Re∥r≫1
Rout is greatly enhanced by adding emitter degeneration.
⇒Rout=v x
i x≈ro[ gmRe∥r]
.≈−gm r∥Re i xv x
r o
Rout
Re ReRe
ESE319 Introduction to Microelectronics
16Kenneth R. Laker, updated 14Nov14 KRL
iL
Class AB Current Biasing SimulationBias currents set at I
REF and I
Q by R and emitter resistor(s) R
e.
IREF I
QN
IQP
IL
PNP Widlar current mirror
NPN Widlar current mirror
RL=100 Ω
Amplitude: 0 Vp
Frequency: 1 kHz
Re=10 Ω
Re=10 Ω
R=2.8 kΩ
R=2.8 kΩ
iN
iL
I REF≈4mA
R=V CC−V BE1
I REF=
V CC−V EB3
I REF≈2.8k
Re=V T
I QNln I REF
I QN ≈9
I Q= I QN= I QP≈2mA
iL=iN−iPQ1
Q2
Q3
Q4
6.312mA
IREF
-39µA
2.322mA
ESE319 Introduction to Microelectronics
17Kenneth R. Laker, updated 14Nov14 KRL
ConclusionsADVANTAGE:
Class AB operation improves on Class B linearity.Power conversion efficiency similar to Class B
DISADVANTAGES:1. Emitter resistors absorb output power.2. Power dissipation for low signal levels higher than Class B.3. Temperature matching will be needed – more so. if emitter degeneration resistors are not used.