dc Bias Point Calculationsee321/spring99/LECT/lect14... · 1999-03-05 · Lecture 14-8 SPICE •...
Transcript of dc Bias Point Calculationsee321/spring99/LECT/lect14... · 1999-03-05 · Lecture 14-8 SPICE •...
Lecture 14-1
dc Bias Point Calculations
• Instead of supplying two dc voltages, the VBE is generally established by a voltage divider using the VCC supply
• But then the ac input cannot be attached as shown. Why?
VCC
RL
R2
R1 RE
RC
vin
Lecture 14-2
Decoupling
• What is the advantage of adding decoupling to this circuit?
VCC
RL
R2
R1
RC
vin
Lecture 14-3
Output Decoupling
• What would be the ideal values for these capacitors?
• How do we select the capacitor values?
VCC
RL
R2
R1
RC
vin
Lecture 14-4
PNP Amplifiers
• dc bias voltages are negative
-10V
RL
R2
R1
RC
vin
Lecture 14-5
PNP Amplifiers
• Which can also be designed by shifting all of the dc voltages by the same potential:
10V
RL R2
R1
vin RC
Lecture 14-6
Decoupling and Biasing
• Decoupling also allows us to specify the effective base resistance, RBB, independent of Vin
VCC
RL
R2
R1
RC
vin
VCC
RL
RBB
RC
vin
VBB
Lecture 14-7
Decoupling and Biasing
• We’d like to have VBB >> VBE so that the currents are not overly dependent on VBE. Why is this dependency undesirable?
• But what is the problem with a large VBB?
VCC
RL
RBB
RC
vin
VBB
Lecture 14-8
SPICE
• Discrete amplifier design rules of thumb: select bias point so that the expected ac signal can swing from max to min without clipping at the supply or forcing the transistor into saturation
dc solution
Q1Custom
R2333E3Ω
C2
1E-6F
+ -
RC1400Ω
+ 10VVCC
Ib
0.000 pA +
-
VOUTAC
0.000 pV
C111E-6F+ -
IC
2.209 mA
+ SINVSSIN
+
-
VIN
794.702 mV
R1143E3Ω
R1950E3Ω
+
-
VOUT
6.908 V
10k Hz10mV
Lecture 14-9
SPICE
• The 1µF capacitors are nearly perfect at decoupling the input and output dc signal components at this 10kHz frequency
time0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms
-2
0
2
4
6
8
V
VOUTAC VOUT VINAC*20
Lecture 14-
SPICE
• Decoupling is evident from the frequency response
• Decoupling would cause distortion at low frequency
frequencye2 e3 e4 e5 e6 e7 e8 e9
36
37
38
39
40
41
42
DB(VOUTAC)
Lecture 14-
SPICE
• For smaller decoupling C’s (10nF) we will see some distortion even at 10kHz
time0.0 0.1 0.2 0.3 0.4 0.5 0.6 ms
-1
0
1
2
3
4
5
6
7
8
V
VOUTAC VOUT VINAC*20
Lecture 14-
SPICE
• Frequency response with 10nF decoupling C’s
frequencye2 e3 e4 e5 e6 e7 e8 e9
-20
-10
0
10
20
30
40
50
DB(VOUTAC)
Lecture 14-
Controlling Parameter Variations
• The current gain, β,varies with temperature
• Recombination greatly depends on temperature
• Recombination also depends on the injection level (magnitude of current)
• An emitter resistor, RE is used as negative feedback to reduce the β-variation effect on IC
VCC
RL
R2
R1
RC
vin RE
Lecture 14-
Controlling Parameter Variations
• Make RBB small enough that the base voltage is roughly independent of base current so that RE feedback can be most effective
• But why not too small?
VCC
RL
R2
R1
RC
vin RE
• If base voltage is nearly constant, an increase in current due to β-variation will tend to decrease VBE because of RE
Lecture 14-
Universal Amplifier Configuration
• Dual supplies for generality, makes base biasing simpler
VCC
RB
RC
RE
• Use decoupling for input and output signals
• But what does the capacitor at the emitter do?
-VEE
Lecture 14-
Decoupling Capacitors
• Decoupling C’s are generally used only for discrete design since it is difficult to build a large capacitor on a chip
• Direct coupled amplifiers are generally used on ICs, but they are similar in form to the amplifiers that we will study here
VCC
RB
RC
RE
-VEE
Lecture 14-
Common Emitter Amplifier
• Popular discrete-component amplifier configuration
VCC
RB
RC
RE
-VEE
C1
C2
C3
RL
vs
BypassCapacitor
RS
Lecture 14-
Common Emitter Design Example
VCC
RC
RE
-VEE
C2
C3
RL
vs
• If there is no dc component in vin, then we don’t need C1or RB with -VEE.
RS
Lecture 14-
Common Emitter Design ExampleVCC=15v
RC
RE
-VEE=-15v
vs
RS=2.5k
Lecture 14-
Common Emitter Design Example
Lecture 14-
Common Emitter Design Example
Lecture 14-
SPICE dc Analysis
Q1Custom
C11E-6F+ -
RC10E3Ω
C21E-6F+ -
+ 15VVCC
+
-
VOUTAC
0.000 pV
+ -15VVEE
RB2500Ω
IC
983.289 µA
+ SINVSSIN
+
-
VIN
-24.582 mV
RE14300Ω
RL5E3Ω
+
-
VOUT
5.167 V
+
-
VINAC
0.000 pV
Lecture 14-
SPICE ac Analysis
frequencye2 e3 e4 e5 e6 e7 e8 e9
0
20
40
DB(VOUTAC)