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ECEN Lab 11: MOSFET Amplifier Configurations
Objective
The purpose of this lab is to examine the properties of the MOS amplifier configurations. DC operating point, voltage gain, and input and output impedances of common-source and common-drain topologies will be studied.
Introduction
MOSFET DC Biasing
Figures 1(a) and (b) show typical resistive biasing circuits for NMOS and PMOS transistors, respectively.
Figure 4: (a) PMOS Common-Source Configuration (b) DC equivalent (c) AC equivalent
Typical design specifications for the common-source configuration includes:
0-to-peak unclipped output voltage swing: V^o
Voltage gain: Av = Vo,ac
Vi
Input and output resistances: Ri and Ro THD at the maximum output level
Based on the typical specifications, design procedure for the common-source amplifiers in Figs. 3 and 4 can be given as follows:
To avoid clipping or distortion due to possible variation of Vt , VRD may be chosen slightly less than the value given in the equation above.
• Calculate Vov = 2VRD , then ID = k0 W V 2 .
jAv j2 L ov
• Calculate RD = VRD and RS = VRS
IDID
• Find RG 1 and RG 2 such that VRG 2 = VRS +jVt j+Vov and Rid = RG 1kRG 2, which yields
RG1 =
Rid VDD
RG2 =
RG 1Rid
VRS + jVt j + Vov
RG 1 Rid
where Rid is the desired input resistance.
Common-Drain (Source Follower) Configuration
Figures 5 and 6 show the common-drain configurations (also known as source follower) for NMOS and PMOS transistors, respectively. DC analysis of this configuration can be performed using the same equations given in (1) and (2), whereas AC analysis yields
Av =
Vo,ac
=
RS
Vi
1
+ RS
gm
Ri = RG 1 k RG 2
Ro = RS k
1
(7)
gm
VDD
RG1 RG1
VI VO
RG2 RS RG2
VDD
RI
VO,DC VI
RS
RG
RO
VO,AC
RS
(a) (b) (c)
Figure 5: (a) NMOS Common-Drain (Source Follower) Configuration (b) DC equivalent (c) AC equivalent
VDD
RG2
RS
RG2
VO
VI
RG1
RG1
VDD
RS
RS
VO,DC
RI
VO,AC
RO
VI RG
(a) (b) (c)
Figure 6: (a) PMOS Common-Drain (Source Follower) Configuration (b) DC equivalent (c) AC equivalent
In typical multi-stage amplifiers, source follower is directly connected to a gain stage, such as a common-source amplifier, without the extra biasing resistors RG 1 and RG 2. Therefore, DC voltage levels in a source follower is typically dependent on the previous amplifier stage.
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Calculations
1. Using the 2N7000G transistor, design the common-source amplifier in Fig. 3(a) with the following specifica-tions:
Supply Voltage, VDD
5V
^
1V
0-to-Peak Output Swing, Vo
Voltage Gain, jAv j
25
Input Resistance, Ri
10k
THD for 5kHz 1V (0-to-peak) Sine Wave Output Voltage, Vo
5%
Show your design procedure and all your calculations.
2. Using 2N7000G and the same RG 1, RG 2 and RS values from your common-source amplifier, calculate Av , Ri and Ro for the source follower in Fig. 5.
Simulations
For all simulations, provide screenshots showing the schematics and the plots with the simulated values prop-erly labeled.
1. Draw the common-source amplifier schematic in Fig. 3(a) using the calculated component values and 2N7000G transistor.
(a) Perform DC operating point or interactive simulation to obtain the DC solution for VRG 2, VRS , VRD ,
Vo,dc and ID .
(b) Perform AC simulation to obtain Av and Ri .
(c) Apply a 5kHz 40mV sine wave signal to the input Vi and obtain the time-domain waveforms for the input and output voltages using transient simulation. Perform Fourier simulation to measure the total harmonic distortion (THD) on the output waveform.
(d) Increase the input amplitude to measure the clipping levels at the output voltage Vo .
2. Draw the source follower schematic in Fig. 5(a) using the calculated component values and 2N7000G transis-tor.
(a) Perform DC operating point or interactive simulation to obtain the DC solution for VRG 2, VRS and ID .
(b) Perform AC simulation to obtain Av , Ri and Ro .
(c) Apply a 5kHz 0.8V sine wave signal to the input Vi and obtain the time-domain waveforms for the input and output voltages using transient simulation. Perform Fourier simulation to measure the total harmonic distortion (THD) on the output waveform.
Measurements
For all measurements, provide screenshots showing the plots with the measured values properly labeled.
1. Build the common-source amplifier in Fig. 3(a) using the simulated component values and 2N7000G transistor.
(a) Measure the DC values for VRG 2, VRS , VRD , Vo,dc and IC using the voltmeter or scope.
(b) Measure Av and Ri using the network analyzer.
(c) Apply a 5kHz 40mV sine wave signal to the input Vi and obtain the time-domain waveforms for the input and output voltages using the scope. Measure the total harmonic distortion (THD) on the output waveform using the spectrum analyzer.
(d) Increase the input amplitude to measure the clipping levels at the output voltage Vo using the scope.
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2. Build the source follower circuit in Fig. 5(a) using the simulated component values and 2N7000G transistor.
(a) Measure the DC values for VRG 2, VRS and ID using the voltmeter or scope.
(b) Measure Av , Ri and Ro using the network analyzer.
(c) Apply a 5kHz 0.8V sine wave signal to the input Vi and obtain the time-domain waveforms for the input and output voltages using the scope. Measure the total harmonic distortion (THD) on the output waveform using the spectrum analyzer.
Report
1. Include calculations, schematics, simulation plots, and measurement plots.
2. Prepare a table showing simulated and measured results.
3. Compare the results and comment on the differences.
Demonstration
1. Build the common-source amplifier in Fig. 3(a) and the source follower in Fig. 5(a) on your breadboard and bring it to your lab session.
2. Your name and UIN must be written on the side of your breadboard.
3. Submit your report to your TA at the beginning of your lab session.
4. For the common-source amplifier in Fig. 3(a):
Measure Av and Ri using the network analyzer.
Apply a 5kHz 40mV sine wave input and show the time-domain output voltage using the scope.
With the 5kHz 40mV sine wave input, measure the THD at the output using the spectrum analyzer.
5. For the source follower in Fig. 5(a):
Apply a 5kHz 0.8V sine wave input, and show the time-domain waveforms at the input and the output using the scope.
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