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Objective of the Experiment:
Biasing a NMOS transistor, using it in a common-source amplifier configuration and measuring
amplification, and to studying effect of the source resistor are the objectives of this MOSFET
lab.
Significance of the Experiment:
In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor (FET)
amplifier topologies, typically used as voltage or transconductance amplifier. The easiest way to tell if a FET
is common source, common drain, or common gate is to examine where the signal enters and leaves. The
remaining terminal is what is known as “common". In this example, the signal enters the gate, and exits the
drain. The only terminal remaining is the source. This is common-source FET circuit. 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 (ideally infinite), nor low enough for a decent voltage
amplifier (ideally zero). Another major drawback is the amplifier’s limited high-frequency response.
Theory:
The biasing is done by fixing the gate voltage with a voltage divider and also by using a source
resistor RS. The source resistor gives negative feedback and stabilizes the bias current as function
of temperature variation and transistor characteristic. The source resistor such that voltage VS at
source terminal is about 1/3rd to 1/5th of VDD . The resistance RD is chosen such that the signal at
the drain has a relatively large output sweep. An important characteristic of a transistor is its
transconductance gm . It is a measure of the rate of change of output current with respect to input
voltage Vgs Amplifier input resistance RG=RG1||RG2 and output resistance Rout=rO||RD .
Experimental Setup:
1. CD4007 MOSFET
2. Resistor (10KΩ = 3pcs, 48KΩ = 1pc, 6.84K= 1pcs)
3. Capacitor (0.1µF=1pcs, 1µF=1pcs, 33nF=1pcs)
4. DMM etc.
Circuit Diagram:
Fig1: Circuit for measurement of the common source voltage amplifier characteristics.
Experimental Procedure:
First of all we have to setup the circuit and check the DC biasing. Then we have to apply a 40mV
(p-p) input voltage of 20 kHz and observe the output signal. We have to determine the gain from
the ratio if output and input. After that we have to measure the phase difference between input
and output signal. After that we have to measure output for different cases and have to record the
output, as well as the gain. Finally we have to determine the upper and lower cutoff frequency
and the bandwidth.
Measurement and Data:
Part 01:
VD =9.17V
VG =6.65V
VS=4.05V
ID =0.58mA
Pot=38.1K
Part 02:
step 2.
Vin=40mV; Vout=270mV; Avo=6.75V/V
step 3.
phase difference =180º.
input and output are out of phase.
step 4.
Vin=1.19V; Vout=7.76 V; AV=6.5 V/V.
step 5.
Vin=1.19V; Vout= 0.996V; AV=0.837V/V.
step 06:
Vin=40mV; Vout= 149mV; AV=3.72V/V.
step 07:
fmid=5.26 kHz ; Vin=40mV; Vout=156.8mV.
step 08:
flower=290Hz; Vin=40mV; Vout=110.8mV; Φ=112º
step 09:
fupper=126 kHz; Vin=40mV; Vout=110.8mV; Φ=125º
step 10.
Amplifier bandwidth BW= (fupper- flower) = 125.7 KHz
BW*Amid=492.7kHz
Answer to the question no 01:
For the presence of the Cc2 with drain, the DC voltage component is present at the drain but has
been removed from the output voltage at Vout node, because we have measured output at that
node which is connected with a capacitor. This capacitor removed the DC part of voltage. The
output terminal has only response for the ac signal.
Answer to the question no 02:
Drain to bulk short eliminates back gate effect on threshold voltage.
Body effect occurs in a MOSFET when the source is not tied to the substrate (which is always
connected to the most negative power supply in the integrated circuit for n-channel devices and
to the most positive for p-channel devices). Thus the substrate (body) will be at signal ground,
but since the source is not, a signal voltage vbs develops between the body (B) and the source
(S). The substrate then acts as a “second gate” or a backgate for the MOSFET. Thus the signal
Vbs gives rise to a drain-current component, which we shall write as gmbvbs, where gmb is the
body transconductance, defined as, gm = gmb(∂ i/∂ v)
Recalling that Id depends on Vbs through the dependence of Vt on VBS, we can show that gmb
= χgm
Figure: Small-signal, equivalent model in which the source is not connected to body.
In many applications the source terminal is connected to the substrate (or body) terminal B,
Which results in the pn junction between the substrate and the induced channel having a
constant zero (cutoff) bias. The effect of VSB on the channel can be most conveniently
represented as a change in the threshold voltage Vt. Specifically, it has been shown that
increasing the reverse substrate bias voltage VSB results in an increase in Vt according to the
relationship.
Key findings:
I. The output voltage changes with the change of gate voltage.
II. The inputs draw current.
III. BJT suffers from several non-ideal effects.
References:
1. Microelectronic circuit, 5th edition, Oxford University Press, New York, NY, 2009,
PP.323-325.
2. .http://en.wikipedia.org/wiki/bodyeffect
3. http://www.circuitstoday.com/boddyeffectanalysis
Conclusion:
In this experiment, we have observed the amplifier characteristics of NMOS transistor. Here we
used it as a common-source voltage amplifier. We have found the gain in different conditions.
We also determined bandwidth higher cut off frequency, lower cut off frequency and lots more.
.