MOSFET

Question 1

What are the three parts of the MOSFET

Answer 1

Gate, Source, and Drain

Question 2

What happens when there’s zero gate voltage?

Answer 2

Two back to back diodes prevent a current conduction from the drain to the source when a voltage V(DS) is applied

Question 3

What happened when a voltage is applied to the gate and the source is grounded?

Answer 3

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

Question 4

Does the gate receive a current?

Answer 4

No because of the oxide insulator

Question 5

What is threshold voltage V(TN)?

Answer 5

The value of V(GS) at which a sufficient number of mobile electrons accumulate in the channel region to form a conducting channel

Question 6

What is enhancement mode?

Answer 6

When V(GS) is greater than V(TN)

Question 7

What is the relationship between drain current I(D) and Drain-Source Voltage V(DS) in saturation mode?

Answer 7

I(D) is independent of V(DS)

Question 8

What is the equation for I(D) in saturation mode?

Answer 8

I(D) = K * [V(GS)-V(TN)]^2

Question 9

When is a MOSFET in saturation mode?

What is the saturation voltage V(DS)sat?

Answer 9

V(DS) >= V(GS) - V(TN)

V(DS)sat = V(GS) - V(TN)

Question 10

What a MOSFET is not in Saturation mode what is the mode called?
What is the effect of each mode?

Answer 10

Linear mode the MOSFET is a voltage-controlled resistor.

Saturation mode the MOSFET is used as an amplifier.

Question 11

What is the output resistance r(o) equation?

Answer 11

r(o) = V(A) / I(D)Q

V(A) = Early Voltage
I(D)Q = Drain current at Q point

Question 12

What is the first step for solving for Q point of a MOSFET (DC Analysis)?

Answer 12

1. Use the Gate-Source loop to develop an equation as a function of V(GS) and I(D)

Question 13

What is the second step for solving for the Q point of a MOSFET (DC Analysis)

Answer 13

2. Assume the MOSFET is in saturation mode and use the I(D) equation

Question 14

What is the third step for solving for the Q point of a MOSFET (DC Analysis)?

Answer 14

3. Apply the equation in step 2 to find V(GS) and I(D)

Question 15

What is the fourth step in finding the Q point of a MOSFET (DC Analysis)?

Answer 15

4. Use Drain-Source loop to find the value of V(DS)

Question 16

What is the fifth step for finding the Q point for a MOSFET (DC Analysis)?

Answer 16

5. 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)

Question 17

What is the sixth step for finding the Q point for a MOSFET (DC Analysis)?

Answer 17

6. Draw the DC load line by expressing I(D) as a function of V(DS) from the Drain-Source loop equation

Question 18

What is the equation for I(D) in Linear mode?

Answer 18

I(D) = K * [2 * [V(GS)-V(TN)] * V(DS) - V(DS)^2]

Question 19

What are the three types of MOSFET amplifiers?

Answer 19

Common-Source, Common-Drain, Common-Gate

Question 20

What are the characteristics of a Common-Source amplifier?

Answer 20

Input to the Gate, output from the Drain, source is grounded in the AC equivalent circuit

Question 21

What is the equation for transconductance g(m)?

Answer 21

g(m) = 2 * sqrt(K * I(D)Q)

Question 22

What is the small signal equivalent circuit for a Common-Source amplifier?

Answer 22

———oG D———————
+ | |
V(GS) ♦️g(m)V(GS) } r(o)
- | |
————o—————————-
S

Question 23

How do you find the small signal equivalent circuit for a Basic CS Amplifier (no R(s) Resistor)?

Answer 23

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)

Question 24

How do you find the voltage gain Av in general and for a Basic Common-Source amplifier

Answer 24

General Av = Vo/V(GS) * V(GS)/Vsig
BCSA Av = -g(m) * [r(o) || R(D) || R(L)] *
[R(G) / [R(S)+R(G)]]

Question 25

What is the input resistance R(i) in the small signal model?

Answer 25

Anything before the gate and after the signal resistance Rsig

Question 26

What is the output resistance R(o) in the small signal model?

Answer 26

The resistance connected to the Drain in parallel with every resistance before the load resistance R(L)

Question 27

How do you draw the AC load line for a basic CS amplifier?

Answer 27

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.

Question 28

How do you find the maximum allowable input signal such that the output is not distorted for a basic CE amplifier?

Answer 28

Look at the AC load line and do V(DS)Q - [V(GS)-V(TN)] and divide that by the voltage gain