differential amplifier design project report

The goal is to design a differential input stage with the specifications mentioned in Table 2. The
parameters regarding the MOS transistors shall be given only after signing the Non Disclosure
Agreement. The students need to calculate the parameters such as the dimensions of the MOS
transistors to fulfill the required specs.
Please note that the designed circuit should be as simple as possible, just to meet the specifica-tions
mentioned in Table 2, for relieving the complexity on the design part since the motive here
is to get familiarity with the design tools. Students are however left free to explore new things but
keep in mind that everything should be wrapped up in the end of the exercise, so take so much
extra burden what you can manage.
IMPORTANT: The input stage designed here will most likely be used in Analog CMOS 2 De-sign
Project. The Project in Analog CMOS 2 carries 35% weight in the final grade.
Specified Value / Range Variable Description
Minimum Typical Maximum
/Used *
Vdd High Supply Voltage 2.7V 3.3 V
Vss Low Supply Voltage Gnd
CL Load Capacitance 2 pF
Av DC Gain 80
? t Unit Gain Frequency 20 MHz
Ibias Bias Current 10 µA
Ivdd Current Consumption 1mA
PM Phase Margin 80 °
VCM Common Mode Voltage range 1.3V
SR Slew Rate 15V/µs
Table 2: Specifications for the Design Exercise. * Last column to be filled with obtained results
by the group.
Remarks on the Specifications:
Vdd: Choose either of the values, 2.7V, or 3V or, 3.3V for your design. You need not vary or
sweep this in your simulations.
CL: 2pF load is to be simulated. However, you may add additional load capacitance if you need
more compensation (to improve the Phase Margin) for your circuit. Just take care that you in-crease
the bias current to maintain the Slew Rate.
Av: The DC gain should not be less that 80 (38dB) over entire Common-Mode Voltage range.
? t: The Unity Gain Frequency should be at least 20MHz.
Ibias: This does not mean that the input stage should be biased with 10uA. It means that a 10uA
stable bias current source will be provided. Each group will have to decide suitable scaling factor
for the current mirror as per their circuit requirement to bias the input stage. Fig.1 clarifies the
difference between Ibias and Ivdd.
Ivdd: Limits the maximum bias current for the circuit. The biasing current (or the total current
consumption) of the circuit should be kept below 1mA.
Phase Margin (PM): Should be minimum 80 ° in the entire common mode range.
VCM: Common mode voltage range should be minimum 1.5V. This means that the difference be-tween
the maximum common-mode voltage and the minimum common-mode voltage, for proper
operation of the circuit should be minimum 1.5V. In other words, VCM_Max.- VCM_Min.=1.5V
Slew Rate (SR): Is given for a given capacitive load. It specifies the maximum rate of change of
the output voltage. Remember to verify the slew-rate performance each time you increase the
output capacitance.
Here is a suggestion for the design flow:
1. Calculations:
a) Refer the specifications for the parameters you need to achieve and the parameters
that are given to you, for example: DC gain, Unity Gain Frequency, available bias
current, load capacitance, etc.
b) Refer the MOS model summary document and process parameters document to
collect the device parameters, such as: Threshold voltage,µ , Cox, etc.
a) and b) constitute the set of ‘knowns’ for your calculations.
c) Collect the set of ‘unknowns’ needed to achieve the ‘knowns’ in a). Use the
‘knowns’ in a) and b) with suitable equations to calculate the ‘unknowns’. Exam-ple
of ‘unknowns’, for instance is: the transconductance of the input transistors,
the output impedance, the biasing current required, device dimensions etc.
2. An example:
i. Start with the slew-rate specification. Calculate the biasing current from this specification.
ii. Since the biasing current is now known, calculate the size of the biasing transistor – tail cur-rent
mirror (suitable mirroring ratio).
iii. Calculate the transconductance of the input transistors to achieve the required Unity Gain
Frequency with provided load capacitance.
iv. Calculate the required output impedance of the active-load current mirror to achieve the re-quired
DC gain. Calculate the sizes of the load transistors.
Usually one has to iterate a few times in steps i. through iv. along with the simulations to achieve
the required specifications.
This document is provided just as a reference or guidance. Students are not urged not to feel
bound to adopt the flow outlined above for this exercise.
Report structure
1. Title Page: With topic, course, group number, name of each candidate in the group.
2. Foreword: Providing the objective of the work and the outline of your report.
3. Contents: Listing individual sections in the report with page numbers.
4. Layout plots: with clearly marked device and port names. Explain the port names. Attach
the final schematic with clearly mentioned device dimensions.
5. Table with final circuit specific parameters – i.e., Data sheet of your design (Gain, UGF,
PM, Common-mode range, etc.) vs. the given specifications – Use Table 2.
6. Mention clearly if design changes were made after submission of the Mid-Term Report. If
yes, then enlist them, explaining the reason(s) to do so.
7. Simulations: Final plots for all the parameters after parasitics extraction – discuss them
individually as in Part I (6). Compare them both with suitable explanation.
8. Conclusion: What did you do? What was achieved? And a summary of your results.
9. Feedback: What did you learn? How was it? Your comments.