the mosfet amplifier - common source...the mosfet amplifier - common source •the output is...
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The MOSFET Amplifier - COMMON SOURCE
• The output is measured at the drain terminal
• The gain is negative value
• Three types of common source
– source grounded
– with source resistor, RS
– with bypass capacitor, CS
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Common Source - Source Grounded
● A Basic Common-Source Configuration:
Assume that the transistor is biased in the saturation region by resistors R1 and R2, and the signal frequency is sufficiently large for the coupling capacitor to act essentially as a short circuit.
+vgs
-
gmvgs
vs
vs
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EXAMPLE
The transistor parameters are:VTN = 0.8V, Kn = 0.2mA/V2
and = 0.
VDD = 5V
RD = 10 k520 k
320 k
0.5 k
Voltage Divider biasing:
Change to Thevenin Equivalent
RTH = 198 k
VTH = 1.905 V
vs
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5. Confirm your assumption: VDS > VDSsat, our assumption that the transistor is in saturation region is correct
2. Assume the transistor is biased in the saturation region, the drain current:
3. Use KVL at DS loop
IDRD + VDS – VDD = 0
VDS = VDD – IDRD = 2.56 V
1. Calculate the value of VGS
VGS – VTH = 0
VGS = 1.905 V
mAI
VVKI
D
TNGSnD
244.0)8.0905.1(2.0 2
2
4. Calculate VDSsat = VGS – VTN = 1.905 – 0.8 = 1.105 V
gm = 0.442 mA/V
DC ANALYSIS
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OUTPUT SIDE
1. Get the equivalent resistance at the output side, RO
2. Get the vo equation where vo = - gm vbeRO
INPUT SIDE
3. Calculate Ri
4. Get vbe in terms of vi
OUTPUT SIDE
1. Equivalent resistance at the output side, RO
2. Get the vo equation where vo = - gm vgs RO
____________________________________________
INPUT SIDE
3. Get vgs in terms of vi
COMMON EMITTER GROUNDED
COMMON SOURCE GROUNDED
vs
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OUTPUT SIDE
1. Equivalent resistance at the output side, RO = ro||RD
2. Get the vo equation where vo = - gm vgs RO = - gm vgs (ro||RD)
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INPUT SIDE
3. Get vgs in terms of vi vgs = vi
4. Av vi = vo = - gm vgs (ro||RD) open circuit voltage
Av vi = - gm vi (ro||RD)
Av = - gm (ro||RD) open circuit voltage gain
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1. The output resistance, Ro = RD
2. The output voltage:
vo = - gmvgs (Ro) = - gmvgs (10) = -4.42 vgs
3. Get vi in terms of vgs
vgs = vi
RTH
198.1 k
0.5 k
RD = 10 k
0.442 vgs+
vgs
-
+vi
-
vs
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Av vi = vo open circuit voltage
Avvi = -4.42 vgs = -4.42 vi
Av = -4.42 open circuit voltage gain
Equation of vo : vo = - gmvgs (Ro) = - gmvgs (10) = -4.42 vgs
vi = vgs
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To find new voltage gain, vo/vs with input signal voltage source, vs
vi in terms of vs use voltage divider: vi = [ Ri / ( Ri + Rs )] * vs = 0.9975 vs
RSi = 0.5 kΩ
vS vo
10 k
vo = Avvi because there is no load resistor
vo = -4.42 (0.9975 vs)
vo/vs = -4.41
Ri = RTH = 198.1 kΩ