MOSFET Flashcards
What are the three parts of the MOSFET
Gate, Source, and Drain
What happens when there’s zero gate voltage?
Two back to back diodes prevent a current conduction from the drain to the source when a voltage V(DS) is applied
What happened when a voltage is applied to the gate and the source is grounded?
A voltage V(GS) is created allowing for a channel to form between the source and drain which creates a current when V(DS) is applied
Does the gate receive a current?
No because of the oxide insulator
What is threshold voltage V(TN)?
The value of V(GS) at which a sufficient number of mobile electrons accumulate in the channel region to form a conducting channel
What is enhancement mode?
When V(GS) is greater than V(TN)
What is the relationship between drain current I(D) and Drain-Source Voltage V(DS) in saturation mode?
I(D) is independent of V(DS)
What is the equation for I(D) in saturation mode?
I(D) = K * [V(GS)-V(TN)]^2
When is a MOSFET in saturation mode?
What is the saturation voltage V(DS)sat?
V(DS) >= V(GS) - V(TN)
V(DS)sat = V(GS) - V(TN)
What a MOSFET is not in Saturation mode what is the mode called?
What is the effect of each mode?
Linear mode the MOSFET is a voltage-controlled resistor.
Saturation mode the MOSFET is used as an amplifier.
What is the output resistance r(o) equation?
r(o) = V(A) / I(D)Q
V(A) = Early Voltage I(D)Q = Drain current at Q point
What is the first step for solving for Q point of a MOSFET (DC Analysis)?
- Use the Gate-Source loop to develop an equation as a function of V(GS) and I(D)
What is the second step for solving for the Q point of a MOSFET (DC Analysis)
- Assume the MOSFET is in saturation mode and use the I(D) equation
What is the third step for solving for the Q point of a MOSFET (DC Analysis)?
- Apply the equation in step 2 to find V(GS) and I(D)
What is the fourth step in finding the Q point of a MOSFET (DC Analysis)?
- Use Drain-Source loop to find the value of V(DS)
What is the fifth step for finding the Q point for a MOSFET (DC Analysis)?
- If V(DS) >= V(GS) - V(TN) then I(D) is valid. Otherwise the assumption of I(D) is invalid and use the Linear Mode equation to find for I(D) and V(DS)
What is the sixth step for finding the Q point for a MOSFET (DC Analysis)?
- Draw the DC load line by expressing I(D) as a function of V(DS) from the Drain-Source loop equation
What is the equation for I(D) in Linear mode?
I(D) = K * [2 * [V(GS)-V(TN)] * V(DS) - V(DS)^2]
What are the three types of MOSFET amplifiers?
Common-Source, Common-Drain, Common-Gate
What are the characteristics of a Common-Source amplifier?
Input to the Gate, output from the Drain, source is grounded in the AC equivalent circuit
What is the equation for transconductance g(m)?
g(m) = 2 * sqrt(K * I(D)Q)
What is the small signal equivalent circuit for a Common-Source amplifier?
———oG D———————
+ | |
V(GS) ♦️g(m)V(GS) } r(o)
- | |
————o—————————-
S
How do you find the small signal equivalent circuit for a Basic CS Amplifier (no R(s) Resistor)?
Short capacitors and short bias voltage sources and obtain r(g) = r1 || r2. Place R(D) drain resistance and R(L) load resistance in parallel after r(o)
How do you find the voltage gain Av in general and for a Basic Common-Source amplifier
General Av = Vo/V(GS) * V(GS)/Vsig
BCSA Av = -g(m) * [r(o) || R(D) || R(L)] *
[R(G) / [R(S)+R(G)]]
What is the input resistance R(i) in the small signal model?
Anything before the gate and after the signal resistance Rsig
What is the output resistance R(o) in the small signal model?
The resistance connected to the Drain in parallel with every resistance before the load resistance R(L)
How do you draw the AC load line for a basic CS amplifier?
Find R(AC) = r(o)||R(D)||R(L)
Use the Q point to find b in
i(D) = -V(DS)/R(AC) + b
Plot the points at V(DS) = 0 and at I(D) = 0 and at the Q point found in DC analysis.
How do you find the maximum allowable input signal such that the output is not distorted for a basic CE amplifier?
Look at the AC load line and do V(DS)Q - [V(GS)-V(TN)] and divide that by the voltage gain
