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ELEC1130 - Circuit Analysis and Design > FET amplifiers > Flashcards

Flashcards in FET amplifiers Deck (42)
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1

What does FET stand for?

Field effect transisor

2

What is a junction FET?

Vgs = 0 then current flows

Vgs << Negative then current will be stopped.

3

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

The pinch off voltage

4

What is a MOSFET?

Metal Oxide Field Effect Transistor

5

How does an N-channel MOSFET work?

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

6

What is the equivalent circuit model for a FET?

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

7

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

Gradient of the slope = 1 / r0

8

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

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

9

What is a depletion-mode MOSFET?

Vgs = 0 then current flows

Vgs << -0 then current stops flowing

10

What is an enhancement-mode MOSFET?

Vgs = 0 then no current flows

Vgs > 0 then current flows

11

What is a BJT?

Bipolar Junction Transistor

12

What are the terminals on a BJT?

Base = Gate

Collector = Source

Emitter = Drain

13

What are the 2 equations for a BJT?

Ie = Ib + Ic

Ic = BIb

14

What is a JFET?

Junction Field Effect Transistor

15

What are the properties of a JFET?

Is ≈ Id

B >> 1

16

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

A = Vout / Vin

17

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

Vdd = Vds + IdRd

Vds = Vdd - IdRd

18

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

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.

19

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

> AC voltages sources short

> AC current sources open

> Capacitors open

> Inductors short

20

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

> DC voltage sources short

> DC current sources open

> Large capacitors short

> Large inductors open

21

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

id = gmVgs

22

What is gm?

The transconductance

23

How is gm calculated graphically?

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

24

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

-1/Rd

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

Vdd - Vds = IdRd

-1/Rd(Vdd + Vds) = Id

25

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

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

26

What is the equation to calculate gm?

gm = 2Id/(Vgs - Vth)

27

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

Vout = idRd

Vout = gmVinRd

28

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

A = gmRd

29

What is clipping?

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.

30

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

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

31

What is the purpose of Rf?

It stabilises the Q-point against variation in FET parameters

32

What is the Q point?

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

33

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

Vds = Vdd - IdRd - IdRf

34

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

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.

35

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

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

36

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

Blocks out the DC bias

37

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

-1/(Rd+Rf)

38

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

Vin = IdRf + Vgs [Normally Vin=Vgs]

39

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

A ≈ Rd/(Rf + 1/gm)

40

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

Vout ≈ VinRd / (Rf + 1/gm)

41

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

id = Vin / (Rf + 1/gm)

42