#### Transcript Common Emitter Amplifier Design Rules VRE should be > 100 mV.

```Common Emitter Amplifier
Design Rules
VRE should be > 100 mV.
Design Procedure
• Decide on an IC that yield to proper
gm and rπ.
• Choose a proper ICRE, e.g. 200 mV.
• Determine Vx given IC and ICRE.
• Choose R1 and R2 to provide
necessary value of VX and establish
I1>>IB.
• Select an RC to place the transistor at
the edge of saturation.
Example 1
• Specification
– gm=19.2 mS→IC=0.5 mA
• Assume that VRE=200 mV.
– RE=0.2 V/IC=400 Ohms
• Calculate VBE
– VBE=VTln(IC/IS), IS=6.734x10-15 A→VBE=0.65 V
• Calculate VX=VBE+VRE=0.65+0.2V=0.85 V
Example 1(Cont.)
• IC=0.5 mA, β=150→ IB=3.33 uA
• I1>>IB. Let’s say that I1=40IB. →I1=133.3 uA
• Assume that VCC=12 V.
→R1+R2=VCC/I1→R1+R2=90 KOhms
• Vx=VBE+RE*IC=R2*VCC/(R1+R2)→R2=6.38
KOhm
• R1=(R1+R2)-R2=90 Kohms-6.38 Kohms=83.619
Kohms
• Place Q1 at the edge of Saturation: VCCRC*IC=VX→RC=22.30 KOhms
Comparison
Designed
Value
Simulation
IC
0.5 mA
0.463 mA
VBE
0.65 V
0.641 V
VX
0.85 V
0.828 V
IB
3.33 uA
3.83 uA
I1
133.3 uA
134 uA
VRE
200 mV
187 mV
I1/IB
40
34.98
Sensitivity to Component
Variation
Nom.
1%
5%
R3
(KOhm)
6.38
6.44
6.69
VBE (mV)
0.641
0.652
0.644
IB (uA)
3.83
3.94 uA 5.43
IC (mA)
463
uA
477 uA
521 uA
1% error in R3 leads to 3 % error in IC.
5% error in R3 leads to 12.5 % error in IC.
Increase VRE to 400 mV
Nom.
1%
5%
R3
(KOhm)
7.88
7.96
8.27
VBE (mV)
0.639
0.641
0.641
IB (uA)
3.90
3.99
4.55
IC
472
uA
483 uA
519
1% error in R3 leads to 2.3 % error in IC.
5% error in R3 leads to 9.9 % error in IC.
• As VRE increases, the circuit becomes
slightly less sensitive to Resistor variation
• But VCE also drops, increasing the
likely hood that the circuit can be
driven into saturation.
What if we drive the base with a
small signal?
Input and Output
Vin, m=1 mV
Vout, m=46 mV
Replace the transistor by its
small signal equivalent circuit
(EQ 5.157)
Comparision:
EQ 5.157: 49.33
and Gain
VRE
RE
AV
0
0
436.50
0.1
200
89.27
0.2
400
49.33
0.3
600
33.89
0.4
800
25.7
Idea: Apply degeneration to the
biasing, but not to the signal!
Zc at 1 KHz: 159.2 mOhms
Av=349
Output/Input Impedance
It is desirable to maximize the input impedance
and minimize the output impedance of the amplifier.
Measurement of Input/Output
Imepdance
Open, because the output
Is not connected to
any external source
1.
2.
3.
Disable the effect
of any input voltage
Source.
Apply a test voltage (Vx)
Measure the resulting current (IX)
Calculate Vx/IX
DC and Small-Signal Analysis
2-step analysis:
1. DC analysis
2. Small signal analysis
(Premise: the change in IC due to the signal must remain small)
Summary of Impedances Seen
at Terminals of a Transistor
(Into the base)
(Into the collector)
(Into the emitter)
Input Impedance
• Derivation of Input Impedance of
Degenerated CE Stage
• Input Resistance with no emitter
resistance
• Input Impedance with Base Resistance
• Input Impedance with Bias Resistors
included
Input Impedance of the
Degenerated CE Stage
Interpretation: Any impedance tied between
the emitter and ground is multiplied by (Beta+1)
when seen from the base.
Input Resistance without
Emitter Degeneration Resistor
Rin=rπ
Input Impedance Including the
Biasing Resistors
(EQ 5.226)
Input Resistance with RB in
Series
Output Impedance
• Derivation of Output Impedance with
Emitter Degeneration Resistance
• Output Impedance without Emitter
Degeneration Resistance
Output Impedance
Derivation
Without Emitter Degeneration
Rout=ro
Output Impedance
(If Early Effect is negligible)
Rout=RC
Gain Modification
• Gain of a Degenerated CommonEmitter Amplifier
• Without Emitter Degeneration
• Gain with a base resistance
• Gain with biasing resistors
Emitter Degeneration
Without Emitter Degeneration
Gain with a base resistance
General CE Stage
PNP CE Amplifier
Calculation
Voltage Gain
Analytical: 13.80
Example 2: Multistage
Amplifier
Multistage Amplifier
Calculation