In lab:
Circuit Diagram:
DC analysis
S# Measured Calculated % Error
1 ID = 15.01 pA ID = 14.90pA 0.666
2 IS = 15.01 pA IS = 14.90pA 0.655
3 IG = 0 A IG = 0 A 0A
4 VGS = -150nV VGS = -150nV 0V
5 VDS = 15V VDS = 15V 0V
Table 1
Post-Lab
Question 1
Design PCB layout of Figure 16 in OrCAD PCB Editor.
SCHEMATIC CIRCUIT DIAGRAM
PCB:
Question 2
What’s the significance of CS amplifier?
A common-source amplifier is one of three basic single-stage field-effect transistor (FET) amplifier
topologies, typically used as a voltage or transconductance amplifier. The common-source (CS) amplifier
may be viewed as a transconductance amplifier or as a voltage amplifier. The FET device's output
resistance typically is not high enough for a reasonable transconductance amplifier, nor low enough for
a decent voltage amplifier.
If we place a resistor between the source terminal and ground, it basically stops the drain current from
changing too much and in result, keeps the Operating point almost same even in the presence of huge
temperature variations
Question 3
List the biasing configurations for the common source amplifier and
explain them in detail with proper circuit diagrams
The common-source (CS) amplifier may be viewed as a transconductance amplifier or as a voltage
amplifier.
As a transconductance amplifier, the input voltage is seen as modulating the current going to the load.
As a voltage amplifier, input voltage modulates the amount of current flowing through the FET, changing
the voltage across the output resistance according to Ohm's law. However, the FET device's output
resistance typically is not high enough for a reasonable transconductance amplifier nor low enough for a
decent voltage amplifier.
There are 5 biasing configurations for common source amplifier:
1. Basic N-channel JFET common source circuit.
2. Basic N-channel JFET common source circuit with source degeneration.
3. Basic N-channel MOSFET common source amplifier with active load I D.
4. Small Signal Circuit for N-channel MOSFET common source amplifier.
Figure 4 shows the corresponding small-signal circuit when a load resistor RL is added at the output node
and a Thévenin driver of applied voltage VA and series resistance RA is added at the input node. The
limitation on bandwidth in this circuit stems from the coupling of parasitic transistor
capacitance Cgd between gate and drain and the series resistance of the source RA. (There are other
parasitic capacitances, but they are neglected here as they have only a secondary effect on bandwidth.)
5. Small Signal Circuit for N-channel MOSFET common source amplifier using Miller’s theorem to
introduce Miller’s capacitance.
Using Miller's theorem, the circuit of Figure 4 is transformed to that of Figure 5, which shows the Miller
capacitance CM on the input side of the circuit. The size of CM is decided by equating the current in the
input circuit of Figure 5 through the Miller capacitance.
These two currents are the same, making the two circuits have the same input behavior. Usually the
frequency dependence of the gain vD / vG is unimportant for frequencies even somewhat above the
corner frequency of the amplifier, which means a low-frequency hybrid-pi model is accurate for
determining vD / vG. This evaluation is Miller's approximation and provides the estimate (just set the
capacitances to zero in Figure 5):
Question 4
Explain the effect of source resistance on the voltage gain and output
impedance of the amplifier
1 . We can use the value of Rs to control the magnitude of the signal vgs and thus ensure that vgs does
not become too large and cause unacceptably high nonlinear distortion. This is the first benefit of
including resistor Rs.
2. Rs cause the useful bandwidth of the amplifier to be extended. The mechanism by which Rs causes
such improvement in amplifier performance is that of negative feedback. The price paid for these
improvements is a reduction in voltage gain.
3. Including Rs, results in a gain reduction by the factor (1 + gmRs). This factor is called the amount of
Feedback and it determines the magnitude of performance improvements and the reduction in gain.
4. Rs reduce the variability in ID. The action of Rs that reduces the variability of ID is just a variation in
ID. Because of its action in reducing the gain, Rs is called source degeneration resistance.
Question 5
Explain the effects of Bypass capacitor on voltage gain
The bypass capacitor makes the source resistor look almost like a short circuit to the input AC signal. As
a result, the full signal appears across the gate and source.
Without the bypass capacitor, some of the input signal is lost across the source-to-ground resistor.
With the bypass capacitor, almost the entire input signal appears across the gate and source.
Only the difference between the gate voltage and the source voltage is amplified.
The capacitor is nearly invisible to the DC bias circuit but looks almost like a short to the AC gain circuit.
Performance Viva
Total/15
(10 Marks) (5 Marks )
Performance /6
Results /3
Critical Analysis /1
Comments
Critical Analysis:
In this lab I learned about how to operate a MOSFET and compute its gain.
I also learned different biasing techniques through which we can improve gain of a circuit
and analyzed MOSFET’s operation under different temperatures.
In DC analysis, I calculated the currents and voltages across the transistor.
I saw the effect of source resistor on gain. It decreases Id current and hence gain is also
decreased.
The bypass capacitor is used to force signal currents around the circuit elements which
gives a low impedance level path at the frequency.
