ESE319 Introduction to Microelectronics
1Kenneth R. Laker, update 15Oct13 KRL
Common Emitter BJT Amplifier DesignCurrent Mirror Design
ESE319 Introduction to Microelectronics
2Kenneth R. Laker, update 15Oct13 KRL
Design Example
ESE319 Introduction to Microelectronics
3Kenneth R. Laker, update 15Oct13 KRL
Design Step 1 (Set RB and R
E)
Choose an RB >> 10RS:
RB=5000
must be ≥ 10RB:
≈100
RE=10⋅5000100
=500
Nearest standard size*: 470 Ω
RE=470
1RE
*RCA Lab: http://www.ese.upenn.edu/rca/components/passive/listcomponents.html#resistors
vs
vO
ESE319 Introduction to Microelectronics
4Kenneth R. Laker, update 15Oct13 KRL
Design Step 2 (Set RC)
=100
RE=470
RB=R1∥R2=5 k
For a gain of Av = -10:
RC=10RE=4.7 k
470
Nearest standard size*:
RC=4.7k
SPEC: v s−max=0.1V pkvs
vO
For a gain of -10, the max ac collector voltage v
o-max swing is 1 V pk; hence,
the dc VRC
> 1 V.
ESE319 Introduction to Microelectronics
5Kenneth R. Laker, update 15Oct13 KRL
Design Step 3 (Set bias point neglecting IB)
=100
RE=470
RB=5k
RC=4.7k
V RC=4.7V≈
V CC
3Thus:
I C=4.7 /4700=1mA.
And (ignoring IB):
V B=V BG≈ I C RE0.7=0.470.7=1.17V
470
4.7 k
vs
vO
Recall vsig-max
= 0.1 V pk and |Av| = 10
We have plenty of room - choose the dc collector-resistor voltage conservatively to allow for bias point changes with temperature – let's use:
ESE319 Introduction to Microelectronics
6Kenneth R. Laker, update 15Oct13 KRL
Design Step 4 (Set R1 and R
2)
=100
RE=470
RB=5k
RC=4.7k
Recall:
RB=R1∥R2=R1
R2
R1R2=5 k
And:V B=
R2
R1R2V CC=1.17V
Or:R2
R1R2=
V B
V CC=1.1712
=0.098≈0.1
470
4.7 k
vs
vO
ESE319 Introduction to Microelectronics
7Kenneth R. Laker, update 15Oct13 KRL
Design Step 4 cont. (Set R1 and R
2)
R1∥R2=R1
R2
R1R2=R1⋅0.1=5k
Substituting:
R1=50 k
Standard size: R1=47k R2
47 kR2=0.1Finally:
0.9 R2=0.147k⇒R2=5222Standard size: R2=5.1 k
470
4.7 k
Revised RB:
RB=R1∥R2=47 k 5.1 k52.1 k
=4.6 k
=100
RE=470
RB=4.6 k
RC=4.7k
NOTE: 1RE≈47k ≥ 10RB=46k
vs
vo
ESE319 Introduction to Microelectronics
8Kenneth R. Laker, update 15Oct13 KRL
Design Step 5 (set Cin
) - Close to the Finish!
Estimate Rin:
RB∣∣rbg≈4.2 k
RB in parallel with rbg => RB dominates. Estimate as 4.2 k . Coupling capacitor reactance, then, should be about 420 Ω at f = fmin.
rbg=r1RE≈47.5 k
∣ 1j 2 f min C in∣=420
C in=1
420⋅220=1.19 ˙10−4
2≈19F
470
4.7 k47 k
5.1 k
Using the RCA Lab Component List
C in=10
2 f min420
Cin = 22 µF
I C=1mA=100r=2.5k
vs
voC in
RB∣∣rbg
ESE319 Introduction to Microelectronics
9Kenneth R. Laker, update 15Oct13 KRL
Final Design
4.7 k
470
47 k
5.1 k
22 µF
vs
voCin
ESE319 Introduction to Microelectronics
10Kenneth R. Laker, update 15Oct13 KRL
Multisim Simulation
20 Hz Gain
1 Khz Gain Actual |AV (f = 1 kHz)| = 9.3 < 10
Actual |AV (f = 20.7 Hz)| = 9.27 9.3.≈.
ESE319 Introduction to Microelectronics
11Kenneth R. Laker, update 15Oct13 KRL
Multisim Oscilloscope Plots
vs
vo
ESE319 Introduction to Microelectronics
12Kenneth R. Laker, update 15Oct13 KRL
Discussion1. We neglected re. Including the internal emitter resistance, the simulated gain becomes:
AV=−RC
REr e=− 4700
47025=−9.5
2. There is some attenuation of the signal voltage at the base. A more accurate calculation of the input attenuation:
vbg≈RB∥rbg
RB∥r rgRS= 42004250
v sig=0.988 vsigRB∥rbg=4.2 k ⇒
Multiplying the two quantities: Av=−9.5⋅0.988=−9.4
This fine-tuning of the estimate may not be all that helpful – since we will be using 5% components to build the circuit!
Close to 9.3!
ESE319 Introduction to Microelectronics
13Kenneth R. Laker, update 15Oct13 KRL
Common Emitter Amplifier - Current Source Biasing
VCC
IE
RC
ro
RE
RB
Rs
vs
C1
CE
1. If NO RE & C
E, what is the ap-
prox. mid-band voltage gain?
2. If RE & C
E, what is the approx.
mid-band voltage gain?
3. What is the purpose of C1?
vo
Qamp
ESE319 Introduction to Microelectronics
14Kenneth R. Laker, update 15Oct13 KRL
Common Emitter Amplifier - Current Source Biasing
VCC
IE
RC
ro
RE
RB
Rs
vs
C1
CE
1. If NO RE & C
E, what is the ap-
prox. mid-band voltage gain?
2. If RE & C
E, what is the approx.
mid-band voltage gain?
3. What is the purpose of C1?
vo vo
v s=−RC
r o≪1
vo
vs=−RC
RE
No important purpose.
Qamp
ESE319 Introduction to Microelectronics
15Kenneth R. Laker, update 15Oct13 KRL
vsig
RC
V CC
V EE
iE
vO
Q1
Q2
Qamp
iC
Rs
v s
RB
iBV BRref
Common Emitter Amplifier - Current Source Biasing
Rref
Rs
vs
I REF=V CC−V BE Qref
Rref
IREF
I REF
I REF=V CCV EE−V BE Qref
Rref
VBE(Qref)VBE(Qref)
vO
ESE319 Introduction to Microelectronics
16Kenneth R. Laker, update 15Oct13 KRL
Common Emitter Amplifier - Current Source Biasing
vsig
RC
Rref V CC
V EE
iE
vO
Q1
Q2
Qamp
iC
RS
v s
RB
iBV B
I E /1
1. The current mirror sets IE (IC).
2. RB serves the purpose to provide a high impedance looking into the base and IB = and V
B = - I
B R
B.
3. VB = 0.7V + V
E sets the emitter-to-
ground voltage VE and, hence, sets V
CE
for a given IC.
4. vs is the signal source.
ESE319 Introduction to Microelectronics
17Kenneth R. Laker, update 15Oct13 KRL
Bias Setting1. Since RB does not interfere with the bias, the signal source can “usually” be connected to the base without need for a blocking capacitor.
2. Choose RB “ large” compared to RS to avoid attenuating vs. But not too large!
3. Choose Rref to set IE.
RsRref
v s
RS
RB
RC
V CC
iE
vO
iC
Q1
Q2
Qamp
V EE
ESE319 Introduction to Microelectronics
18Kenneth R. Laker, update 15Oct13 KRL
Bias Setting - Continued
I REF=V CCV EE−0.7
Rref
Choose:I C≈ I E≈ I REF=1mA
Rref =23.310−3
=23.3 k
Choose standard size:(RCA Lab Comp List)
Rref =22 k
V CC=I REF Rref V BE Qref −V EEIREF
+-
VBE(Qref
)
22 k OhmRref
RC
RB
For dc bias set vs = 0
Q1
Q2
Qamp
ESE319 Introduction to Microelectronics
19Kenneth R. Laker, update 15Oct13 KRL
Bias Setting - CompletedWith the base through RB:
This implies that there is about an12.7 V drop to split across RC & VCE. Let's choose V
RC = 5.6 V & V
CE = 7.1V.
RC=V RC
I C=5.610−3
=5.6 k
Choose standard size:(RCA Lab Comp List)
RC=5.6 k
5.6 k Ohm
I B≈0
Neglect the base current through R
B
I b≈0
V CC=V RCV CE QampV E
Rref
RB
RC
V E=V EB Qamp≈−0.7V
V E
ESE319 Introduction to Microelectronics
20Kenneth R. Laker, update 15Oct13 KRL
Gain Setting1. Connect the source to the base.
2. Provide a path for the smallsignal emitter current.
3. Choose RE for the desired gain(Av = - RC/RE).
4. CE is nearly a short circuit for f ≥ f
min
Calculate CE s.t. at f ≥ f
min.
RS≪RB
5.6 k Ohm
v s
RS
RrefRC
RE
C E
RB
vo
∣ 1j 2 f C E∣≪RE
Q1 Q
2
Qamp
ESE319 Introduction to Microelectronics
21Kenneth R. Laker, update 15Oct13 KRL
Gain Setting - Continued
Choose the nearest standardsize resistors for RC and RE.
RE=RC
20=560020
≈270
Gain check:ib=
v s
RS1reRE
vo=−RC ic=−RC ib
∣Av∣=∣vo
v s∣≈ 1
RC
reRE≈5600295
=19
5.6 k OhmRs
ib
ie
Design for |AV| = 20:
270
Typical18 ≤ |AV| ≥ 22RS RC
RE
reie
ib
v s
vo
or 25.57 dB
ESE319 Introduction to Microelectronics
22Kenneth R. Laker, update 15Oct13 KRL
RE and C
E
∣ 1j 2 f C E∣≪RE⇒
12 f min C E
=RE
10⇒C E=
102 f min RE
≈300F @ fmin
= 20 Hz
CE = 300 µF too large; too conservative!
Re
Ce
RE
C E
RE
C Eie
vO vo
v s v s
Q1 Q
2
Qamp Q
amp
ac circuitoverall circuit with bias
ESE319 Introduction to Microelectronics
23Kenneth R. Laker, update 15Oct13 KRL
Complete the Design
If we choose next largest standard size (RCA Lab Comp List):
C E=33F => flow
= 12.5 Hz
CE = 300 µF too large!!
Less Conservative Design:
for flow
= 20 HzC E=1
2 f low RE≈30F
∣ 1j 2 f C E∣≪RE⇒
12 f min C E
=RE
10⇒C E=
102 f min RE
≈300F @ fmin
= 20 Hz
C E=1
2 f low REwhere f low≈ f low−3dB
ESE319 Introduction to Microelectronics
24Kenneth R. Laker, update 15Oct13 KRL
Multisim Bode Plots
20 Hz Gain
1 kHz Gain
CE = 33 µF
Actual |AV (f = 1 kHz)| = 25.52 dB (18.88)
Spec.18 ≤ |AV| ≥ 22
Actual |AV (f = 20.5 Hz)| = 22.63 dB
ESE319 Introduction to Microelectronics
25Kenneth R. Laker, update 15Oct13 KRL
ib
RB
ic
ib
dc
small ac @ midband
roDue to C
E
What if RE = 0 Ω ?
A¿
Av=vo
v s≈−RC
r e
ESE319 Introduction to Microelectronics
26Kenneth R. Laker, update 15Oct13 KRL
What About Interface to the Output Load (RL)?
RL
Is the above interface to RL OK?
ESE319 Introduction to Microelectronics
27Kenneth R. Laker, update 15Oct13 KRL
What About Interface to the Output Load (RL)?
RL
Is the above interface to RL OK? - No
1. Need CL to block dc bias on v
C, i.e. v
o is small signal only.
2. For ac (f ≥ fmin
)
C L
vov
C
RL' =RC∥RL Unless R
L >> R
C, R
L alters A
v.
ESE319 Introduction to Microelectronics
28Kenneth R. Laker, update 15Oct13 KRL
Multi-Stage Amplifier
vO1
vo1
vo2C
in1
Cin2
Rin2
>>RC1
Stage 1 Stage 2
Av=vo1
vs
vo2
vo1=
vo2
v s=−
RC1
RE−
RC2
RE
RC1
RC2
RL >>R
C2