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Scientist: Examples: Electrical Engineering

Given an unknown MOS n-channel depletion-mode transistor wired to a variable positive voltage source, determine the transistor's transconductance parameter K and its pinch-off voltage Vp. If we load the circuit (V = +15V and RL = 220 Ohms) and measure the voltage at a test point Vo. We know that the transistor exhibits the transfer characteristic:

iD = 1/2*K*(Vgs - Vp)^2

as long as the transistor is in-saturation, that is as long as,

Vds > Vgs - Vp

The output voltage Vo is given by

Vo = 15 - RL*iD

The Scientist model for this system is shown below.

Transistor Circuit Model

By inspection of the data we see that 0.1 and -1 are reasonable starting values for K and Vp respectively.

Transistor Circuit Parameters

We perform a least squares fit calculation and find the following.

Transistor Circuit Least Square Fit