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ELEC1130 - Circuit Analysis and Design > FET amplifiers > Flashcards

FET amplifiers Flashcards

1
Q

What does FET stand for?

A

Field effect transisor

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2
Q

What is a junction FET?

A

Vgs = 0 then current flows

Vgs << Negative then current will be stopped.

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3
Q

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

A

The pinch off voltage

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4
Q

What is a MOSFET?

A

Metal Oxide Field Effect Transistor

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5
Q

How does an N-channel MOSFET work?

A

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

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6
Q

What is the equivalent circuit model for a FET?

A

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

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7
Q

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

A

Gradient of the slope = 1 / r0

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8
Q

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

A

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

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9
Q

What is a depletion-mode MOSFET?

A

Vgs = 0 then current flows

Vgs << -0 then current stops flowing

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10
Q

What is an enhancement-mode MOSFET?

A

Vgs = 0 then no current flows

Vgs > 0 then current flows

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11
Q

What is a BJT?

A

Bipolar Junction Transistor

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12
Q

What are the terminals on a BJT?

A

Base = Gate

Collector = Source

Emitter = Drain

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13
Q

What are the 2 equations for a BJT?

A

Ie = Ib + Ic

Ic = BIb

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14
Q

What is a JFET?

A

Junction Field Effect Transistor

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15
Q

What are the properties of a JFET?

A

Is ≈ Id

B >> 1

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16
Q

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

A

A = Vout / Vin

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17
Q

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

A

Vdd = Vds + IdRd

Vds = Vdd - IdRd

18
Q

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

A

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
Q

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

A

> AC voltages sources short

> AC current sources open

> Capacitors open

> Inductors short

20
Q

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

A

> DC voltage sources short

> DC current sources open

> Large capacitors short

> Large inductors open

21
Q

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

A

id = gmVgs

22
Q

What is gm?

A

The transconductance

23
Q

How is gm calculated graphically?

A

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

24
Q

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

A

-1/Rd

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

Vdd - Vds = IdRd

-1/Rd(Vdd + Vds) = Id

25
Q

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

A

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

26
Q

What is the equation to calculate gm?

A

gm = 2Id/(Vgs - Vth)

27
Q

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

A

Vout = idRd

Vout = gmVinRd

28
Q

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

A

A = gmRd

29
Q

What is clipping?

A

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
Q

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

A

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
Q

What is the purpose of Rf?

A

It stabilises the Q-point against variation in FET parameters

32
Q

What is the Q point?

A

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

33
Q

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

A

Vds = Vdd - IdRd - IdRf

34
Q

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

A

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
Q

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

A

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

36
Q

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

A

Blocks out the DC bias

37
Q

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

A

-1/(Rd+Rf)

38
Q

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

A

Vin = IdRf + Vgs [Normally Vin=Vgs]

39
Q

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

A

A ≈ Rd/(Rf + 1/gm)

40
Q

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

A

Vout ≈ VinRd / (Rf + 1/gm)

41
Q

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

A

id = Vin / (Rf + 1/gm)

42
Q
A

ELEC1130 - Circuit Analysis and Design (4 decks)

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