FET amplifiers

Question 1

What does FET stand for?

Answer 1

Field effect transisor

Question 2

What is a junction FET?

Answer 2

Vgs = 0 then current flows

Vgs << Negative then current will be stopped.

Question 3

When current is stopped in a junction FET what is this referred to?

Answer 3

The pinch off voltage

Question 4

What is a MOSFET?

Answer 4

Metal Oxide Field Effect Transistor

Question 5

How does an N-channel MOSFET work?

Answer 5

When the gate (Vgg) is given a Positive charge, electrons are attracted to it and this causes the area around the gate to become negatively charged allowing electrons to flow between the source and drain

Question 6

What is the equivalent circuit model for a FET?

Answer 6

A voltage controlled current source with a resistor r0 across it.

Question 7

What is the equation for the IV graph of a MOSFET in the saturation region?

Answer 7

Gradient of the slope = 1 / r0

Question 8

What is the equation for the Norton equivalent circuit of a MOSFET?

Answer 8

id = ( k(Vgs - Vth)^2 ) / 2

Question 9

What is a depletion-mode MOSFET?

Answer 9

Vgs = 0 then current flows

Vgs << -0 then current stops flowing

Question 10

What is an enhancement-mode MOSFET?

Answer 10

Vgs = 0 then no current flows

Vgs > 0 then current flows

Question 11

What is a BJT?

Answer 11

Bipolar Junction Transistor

Question 12

What are the terminals on a BJT?

Answer 12

Base = Gate

Collector = Source

Emitter = Drain

Question 13

What are the 2 equations for a BJT?

Answer 13

Ie = Ib + Ic

Ic = BIb

Question 14

What is a JFET?

Answer 14

Junction Field Effect Transistor

Question 15

What are the properties of a JFET?

Answer 15

Is ≈ Id

B >> 1

Question 16

What is the equation for the gain of an amplifier and what is the symbol for gain?

Answer 16

A = Vout / Vin

Question 17

For DC analysis of a normal transistor amplifier: What is the KVL equation for the output side of the circuit?

Answer 17

Vdd = Vds + IdRd

Vds = Vdd - IdRd

Question 18

How can we calculate the Drain-source current and voltage when we know: Vgs Rd Vdd

Answer 18

If we overlay the graph for the KVL equation for the output side with Vds as the subject (Vds = Vdd - IdRd) on top of the Id-Vds graph for the FET with different gate-source voltages. The point where the graphs intercept at the correct Vgs value is the drain current and drain-source voltage.

Question 19

What do we do to the circuit to simplify it during DC analysis of a FET amplifier?

Answer 19

> AC voltages sources short

> AC current sources open

> Capacitors open

> Inductors short

Question 20

What do we do to the circuit to simplify it during AC analysis of a FET amplifier?

Answer 20

> DC voltage sources short

> DC current sources open

> Large capacitors short

> Large inductors open

Question 21

For AC analysis of a transistor amplifier: What is the equation relating Drain current to Gate-source voltage

Answer 21

id = gmVgs

Question 22

What is gm?

Answer 22

The transconductance

Question 23

How is gm calculated graphically?

Answer 23

gm is equal to the gradient of the slope of the id-Vgs graph between the peak-peak voltages of the input.

Question 24

What is the gradient of the graph for Vds = Vdd - IdRd? why?

Answer 24

-1/Rd

Id is on the y-axis so we have to rearrange the equation:

Vdd - Vds = IdRd

-1/Rd(Vdd + Vds) = Id

Question 25

What can be said about the voltage source and Vdd during AC analysis?

Answer 25

Vdd = Gnd = 0V because during AC analysis DC voltage supplies short

Question 26

What is the equation to calculate gm?

Answer 26

gm = 2Id/(Vgs - Vth)

Question 27

What are the equations to calculate the output voltage during AC analysis?

Answer 27

Vout = idRd Vout = gmVinRd

Question 28

What is the equation to calculate amplification of a FET amplifier?

Answer 28

A = gmRd

Question 29

What is clipping?

Answer 29

If the gate-source voltage is too high, this can cause the range of voltages to not be in the linear region of a MOSFET and so will cause the MOSFET to switch off periodically and this will cause clipping to occur.

Question 30

What is a single stage common-source FET amplifier with Rf?

Answer 30

This is the same as normal FET amplifier except that there is a resistor between the transistor source and ground. The resistor is Rf

Question 31

What is the purpose of Rf?

Answer 31

It stabilises the Q-point against variation in FET parameters

Question 32

What is the Q point?

Answer 32

During DC analysis, it is the average point on the graph of Id-Vgs or Id-Vds

Question 33

For DC analysis of a FET amplifier with Rf: What is the KVL equation for the output side of the circuit?

Answer 33

Vds = Vdd - IdRd - IdRf

Question 34

What always accompanies the resistor Rf? Why and how does this affect equations?

Answer 34

A capacitor is in parallel with Rf. This allows the equations for AC analysis to be the same because the capacitor shorts during AC analysis.

Question 35

What would happen to the equations for AC analysis if there was no capacitor in parallel with Rf?

Answer 35

This would mean that Rf would be factored in series with Rd.

Question 36

What is the purpose of capacitors on the input out output of an amplifier?

Answer 36

Blocks out the DC bias

Question 37

What is the gradient of the graph for Vds = Vdd - IdRd -IdRf?

Answer 37

-1/(Rd+Rf)

Question 38

What happens to the input voltage if we have no capacitor in parallel with Rf?

Answer 38

Vin = IdRf + Vgs [Normally Vin=Vgs]

Question 39

What is the equation for amplification if we have no capacitor in parallel with Rf?

Answer 39

A ≈ Rd/(Rf + 1/gm)

Question 40

What is the equation for the output voltage if we have no capacitor in parallel with Rf?

Answer 40

Vout ≈ VinRd / (Rf + 1/gm)

Question 41

What is the equation for the drain current if we have no capacitor in parallel with Rf?

Answer 41

id = Vin / (Rf + 1/gm)