Rajesh Yadav12ECP01P

M-Tech (VLSI- Design)

1. Introduction2. Low-Frequency Small-Signal Equivalent Circuit Model

i. Regime of Operationii. Key pointsiii. Transconductanceiv. Output conductancev. Backgate Transounductancevi. Complete MOSFET Small-Signal Low-Frequency Model

3. High-Frequency Small-Signal Equivalent Circuit Modeli. Key Pointsii. Add in Capacitancesiii. Complete MOSFET Small-Signal High-Frequency Model

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Why do we need small-signal modeling?To linearize circuits. Linear circuits are much easier to work with: we

can use Thevenin/Norton equivalent circuits, superposition, etc.

How to obtain a linearized circuit?If we limit our signals to a relatively small

amplitude,the non-linear IV curves can be considered piece-wise linear =⇒ small signal model.

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Key pointsSmall signal is small response of nonlinear components

become linearSince response is linear, lots of linear circuit techniques

such as superposition can be used to determine the circuit response

Notation Total current = DC current + Small signal current

Mathematically

dDD iIi

),,(),,(),,,,,( bsdsgsdBSDSGSDbsdsgsBSDSGSD vvviVVVIvvvVVVi

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Complete MOSFET Small- Signal Low -Frequency Model

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What Happens at High Frequency?

There are intrinsic or parasitic capacitances related to the MOSFET structure, as we know Zc = 1/jwC . At low frequency, Zc is very large, can be approximated to open circuit, however at high frequency, Zc is small enough we have to consider.

We have 4 terminals. Considering the possible combinations between them.

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Add in Capacitances

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Complete MOSFET Small- Signal High Frequency Model

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