ECE354: Electronic Circuits University of Toronto 2019

Lab 1: Common-source Amplifiers

Introduction
The common-source amplifier is one of the basic amplifiers in CMOS analog circuits. Because
of its very high input impedance, relatively high gain, low noise, speed, and simplicity, common-
source amplifiers find different applications from sensor signal amplification to RF low-noise am-
plification. Good understanding of this amplifier is essential not only for good design of one but
also for analysis of other advanced circuits such as differential amplifiers which you see later in
this course.
In this lab, you are to design, simulate, and implement NMOS- and PMOS-based common-
source amplifiers with a resistive load shown in Figure 1. They will be designed for different
requirements such as gain, swing and supply voltage.

VDD VDD

RD Vi
Vo
Vo

Vi
RD

(a) (b)

Figure 1: (a) NMOS and (b) PMOS common-source amplifiers.

Preparation
Go through the following preparation steps.
1. Take a look at the two different design approaches given below for the NMOS common-
source amplifier shown in Figure 1a.
Design for the maximum output swing with an arbitrary ID : For a given ID and known
device parameters
pVov can be approximated by using the transistor large signal model active
equation (Vov = 2ID /µCoxW /L). The maximum swing possible is VDD −Vov . In order to
use the maximum swing, the output node , VO , should be placed in the middle of the swing

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range: VO = (VDD +Vov )/2. This also requires ID RD = (VDD −Vov )/2 and thus RD = (VDD −
Vov )/2ID . Since gm = 2ID /Vov , Av = −gm RD = (−2ID /Vov ) · (VDD − Vov )/2ID = −(VDD −
V ov)/Vov assuming RD << ro .
Design procedure for the maximum gain with an arbitrary ID and output swing: For a
given ID and known device parametersp Vov can be approximated by using the transistor large
signal model active equation (Vov = 2ID /µCoxW /L). Since gm = 2ID /Vov and RD = (VDD −
VO )/ID , small signal gain can be written as Av = −gm RD = (−2ID /Vov ) · (VDD − VO )/ID =
−2(VDD − VO )/Vov assuming RD << ro . For the maximum gain possible, VO should be
minimum, leading to VO = Vov +Vswing /2.
2. Design common-source amplifiers for the criteria shown in Table 1. Perform hand analysis
to fill in the blanks in Table 1 using the device parameters shown in Table 2. Be careful
the design procedures given above are for NMOS common-source amplifier. You may need
some modifications in the equations for the PMOS common-source amplifier.
3. Perform a DC operating point simulation for the amplifiers designed above. Note that the
transistor models used in simulation are far more complicated and accurate than the simple
square law used for hand analysis. Therefore, some deviation from the hand analysis comes
with no surprises.
4. Perform a DC sweep to plot Vo , ID , and dVo /dVi (= Av ) versus Vi in the same plot window.
Vi should be swept from 0 V to VDD .
5. Label and comment on the plots to clearly show the the small-signal gain (Av ), Vi , and output
swing for the ID specified in Table 1.
6. Perform a transient simulation to show Vo versus time. Use a sinusoidal voltage source at
1 kHz with 10-mVpp amplitude as the input source. Make sure that the input and thus the
output are biased at the voltages found in the previous step. Verify the small-signal gain
found in the previous step.
7. Organize the results for presentation to your TA in a report. You should also bring your
LTSpice simulation files with a memory stick for presentation to your TA. Please name your
files in the following format: lab1 ... surname1 surname2. Computers are provided in the
laboratory room.

Lab - Part I: LTSpice Simulation Challenge

The first part of this lab will be an LTSpice simulation challenge that will be announced at the
beginning of the lab session by your TA. You will work in groups of two and have 50 minutes to
finish. Completing the preparation part of the lab and general knowledge of the course should be
enough to finalize this part.

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Table 1: Hand analysis table

VDD (V) Type Gain Swing (Vpp ) Vov (V) ID (A) gm (A/V) Vo (V) RD (Ω) Av (V/V)
5.0 NMOS - max 1m
5.0 PMOS - max 0.5 m
1.2 NMOS max 0.2 0.5 m

Table 2: NMOS and PMOS device parameters

Type Device VT (V) µn/pCoxW /L (mA/V2 )
NMOS ALD1101 0.71 4.49
PMOS ALD1102 -0.65 -2.10

Lab - Part II: Common-Souce Amplifier Implementation

Do the following for the first two common-source amplifiers designed by hand analysis. A mini-
mum parts list for this lab is shown in Table 3. Resistors will be supplied in the laboratory.

1. Measuring Vo versus Vi
Repeat the following for the first two common-source amplifiers designed in the preparation.

1. Implement the common-source amplifier on the breadboard. Connect a 50-Ω resistor across
the input and the ground as shown in Figure 2 . This resistor is important for the voltage
reading of the signal generator to be correct. Most of the signal generators have a 50-Ω out-
put impedance and the voltage reading is correct only if its load is 50 Ω. You will need this
50-Ω termination many times in future labs when you use a signal generator although
it won’t be explicitly shown in lab manuals.

2. Configure the signal generator for a triangular wave with 0 V to VDD swing at 100 Hz. Make
sure to do this step without connecting the transistor to the signal generator but with the 50-Ω
resistor connected, as an excessive gate voltage can permanently damage the transistor.

3. Connect the input signal to the transistor and measure the input (Vi ) and output (Vo ) simul-
taneously using both of the input channels of the oscilloscope. Use the input signal as the

Table 3: Minimum parts list

Part Description Quantity
ALD1101 NMOS transistor 1
ALD1102 PMOS transistor 1
- 10-kΩ multi-turn potentiometer 1

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VDD VDD

RD Vi
Vo 50 Ω Vo

Vi
RD
50 Ω

(a) (b)

Figure 2: (a) NMOS and (b) PMOS common-source amplifiers with a 50-Ω input impedance.

trigger source. Adjust the horizontal scale to display roughly two periods of the triangu-
lar wave and adjust the vertical scale of each input to maximize the displayed signal swing
without clipping. Make sure that Vi is swinging 0 V to VDD .

4. Enable the XY plot mode of the oscilloscope to plot Vo versus Vi . Determine and record
the input and output bias voltage that meets the ID requirement in Table 1, and calculate the
small-signal gain around that point. Sketch a Vo versus Vi curve and label important points.
How does this compare with the simulation and hand analysis?

5. Organize the results for presentation to your TA.

2. Sinusoid testing
Repeat the following for the first two common-source amplifiers designed in the preparation.

1. Disconnect the signal source from the circuit, and configure the signal source for 1-kHz
10-mVpp sinusoid.

2. Place the input bias circuit on the breadboard as shown in Figure 3, and tune the potentiome-
ter for the input bias voltage found in the previous step. This input bias circuit is used
many times in future labs so make sure you feel comfortable with it.

3. Connect the signal source to the breadboard and show both the input and output on the
oscilloscope. How does the gain compare with hand analysis and simulation?

4. Organize the results for presentation to your TA.

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VDD VDD
Large VDD Input bias
RD
Vi capacitor 10 kΩ circuit
Vo VDD
POT Large Vo
50 Ω Vi capacitor 10 kΩ
Input bias POT
circuit 50 Ω RD

(a) (b)

Figure 3: (a) NMOS and (b) PMOS common-source amplifiers with an input bias circuit.

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