MOSFET Small signal model

Mosfet small signal model analysis

1. Rajesh Yadav
12ECP01P
M-Tech (VLSI- Design)

2. 1.Introduction
2.Low-Frequency Small-Signal Equivalent Circuit Model
i.Regime of Operation
ii.Key points
iii.Transconductance
iv.Output conductance
v.Backgate Transounductance
vi.Complete MOSFET Small-Signal Low-Frequency Model
3.High-Frequency Small-Signal Equivalent Circuit Model
i.Key Points
ii.Add in Capacitances
iii.Complete MOSFET Small-Signal High-Frequency Model
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3. 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 points
Small signal is small response of nonlinear components
become linear
Since 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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14. Complete MOSFET Small- Signal Low -Frequency Model
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15. 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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16. Add in Capacitances
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17. Complete MOSFET Small- Signal High Frequency Model
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