These are some of the commonly encountered questions regarding designing a Differential Amplifier –
Copyright – Gautam Vashisht
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Q-1) When the design technology node changes, say from 180nm to 90nm, what are process-dependent parameters whose values would be changed?
A- On the change of technology node, the process-dependent parameters- µnCox, µpCox, VDD, VTn, VTp would change their values accordingly. For example, for 180nm technology, the value of VDD = 1.8V but for 90nm, it is around 1.2V. Similarly, the rest of the parameters would change their values. For a successful design, care should be taken to find out the correct values of these parameters for a particular technology node.
Q-2) What is ICMR? What is its use in designing?
A- ICMR stands for Input Common Mode Reach. It is a range of common input voltage supplied to the Differential Amplifier circuit for which all the MOSFETs would be in saturation mode to operate as Diff Amp. Its value depends upon the technology as ICMR depends on VDD which itself depends on the technology. It is used to determine W/L ratios of the transistors 3,4,5,6.
Q-3) What is Slew Rate & how it is used in designing?
A- It is the rate of change of output voltage, used to determine the current ID.
Q-4) What is Leakage Power? How can it be determined for the circuit above?
A- Leakage power is primarily the result of unwanted sub-threshold current through the transistor when it is turned off. This sub-threshold-driven leakage power is strongly influenced by variations in the transistor threshold voltage VT. It can be determined by doing transient analysis of the circuit.
Q-5) How to determine the values of µnCox, µpCox, VTn, VTp for the technology used for designing?
A- The values of the above process-dependent parameters can be determined by keeping the nMOSFET & pMOSFET in saturation mode (each in separate circuit) & then looking into their model files for their exact values.
Gautam Vashisht
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