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IA Engineering > 1P3 Analysis of Circuits > Flashcards

1P3 Analysis of Circuits Flashcards

1
Q

How do capacitors and inductors behave under DC conditions?

A

Capacitors act like an infinite resistance (open circuit)

All inductors act like zero resistance (short circuit)

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2
Q

How do we define RMS?

A

RMS value is equal to the DC value which would produce the same average power dissapation in a resistive load.

sqrt(1/T integral(f^2)dt)
where finding rms of f

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3
Q

What does the letter X represent?

A

Reactance! the modulus of the imaginary part of the impedance.
1/wC or wL

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4
Q

what does a capacitor do at high and low frequencies?

A

High frequencies it acts like a short circuit.
At low frequencies it acts like an open circuit.

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5
Q

How is the imperfection in an inductor or capactiro represented?

A

Inductor: series resistor representing winding losses.
Capacitor: parallel resistor representing the leakage resistance (M ohms)

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6
Q

What is the current in norton equivalent circuit?

A

Short circuit current of original network.

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7
Q

What is the voltage in a Thevenin equivalent circuit?

A

The open circuit voltage.

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8
Q

What is the Thevenin or Norton Resistance?

A

Voc/Isc

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9
Q

What is Kirchoff’s voltage law?

A

THe sum of voltages around a closed loop must be zero.

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10
Q

What is Kirchoff’s current law?

A

The sum of currents at every junction must be zero.

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11
Q

What approach is best for solving bridge circuits?

A

Loop (mesh) analysis, two top loops have equal and opposite direction currents when bridge is balanced so there is no current flow.

Or consider the bridge as two pairs of potential dividers with the same voltage.

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12
Q

Decibel in terms of both power and voltage.

A

G = 10 log10(P2/P1)
G = 20 log10(V2/V1)

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13
Q

Explain the 3dB point.

A

Where the magnitude of the voltage gain is reduced by a factor of root2, or approximately 0.7 .
Power is reduced by factor of a half.

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14
Q

What is a bode plot?

A

It indicates the frequency response of a particular component such as a low pass filter.

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15
Q

What is the attenuation slope of a first order filter?

A

20dB/decade.

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16
Q

How do you work out the resonant frequency of a resonance circuit?

A

Take a potential divider accross the capacitor, and find where the real part of the denominator goes to 0. Should end up with w = 1/sqrt(LC)

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17
Q

What is Q factor in terms of resonant circuits?

A

Q is the ratio of the reactance to the resistance.

Q = 1/(wCR) = wL/R

where w is the resonant frequency.

v2 = Qv1

More formally, Q is the ratio of energy stored in an oscillating resonantor to the energy dissipated per cycle due to damping.

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18
Q

Define bandwidth for resonant circuits.

A

dw = w0/Q

vmax/sqrt(2)

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19
Q

Define admittance.

A

The reciprocal of impedance. Complex form of conductance

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20
Q

How do you solve parallel resonant circuits?

A

Convert inductor in series with resistor into an inductor with a parallel resistor (with frequency dependent resistance).

Then use the total parallel resistance to find the overall Q factor for the circuit.

Convert the input into a current source, and use V = iR to find the voltage.

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21
Q

Max power for amplifier model.

A

When Rout = Rload

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22
Q

How do you solve for final voltage in the amplifier model?

A

By using repeated potential dividers.

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23
Q

How do you find the output impedance of an amplifier?

A

Short the input and apply a test voltage to the output, with a corresponding test current. Rout = V/I

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24
Q

How do you find the input impedance of an amplifier?

A

What current enters the amplifier for a corresponding voltage.

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25
Q

What is a decoupling capacitor?

A

A capacitor that blocks DC current, ie a high pass filter.

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26
Q

What does a capacitor in parallel with an amplifier usually represent?

A

Parasitic capitance, results in a low pass filter.

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27
Q

How does pure undoped silicon behave?

A

Like an insulator with 4 valence electrons, forming four covalent bonds, very strong.

At higher tmperatures even room temperature, a few electrons can break free leaving behind holes which move throughout the silicon based on the current. Holes move in the direction of the electric field, electrons move against it.

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28
Q

To add free electrons to silicon, what elements are added?

A

Pentavalent metals (Phosphorus, Antimony or Arsenic)

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29
Q

What element do we add to silicon to add extra holes?

A

Trivalent metal (boron, gallium or indium)

30
Q

What doping is used in n-type semi conductors?

A

Adding pentavalent elements, to add extra free electrons.

31
Q

What doping is used in p-type semi-conductors?

A

Adding trivalent elements, to add extra holes.

32
Q

What happens at the border between the n-type and p-type silicon?

A

Electrons and holes diffuse to the other side, producing a region with no free electrons or holes, the depletion region.
This creates an electric field which opposes diffusion.

33
Q

What happens when a positive voltage is applied to the n-type region?

A

More electrons move from the n-type to the p-type, increasing the size of the depletion region, creating a reverse saturation current, Io (very small, nA)

34
Q

How do photodiodes work?

A

A reverse biased p-n junction is used, such that light increaes the diffusion of electrons accross the junction, increase the reverse biased current.

35
Q

What happens when the p-type is connected to the positive voltage?

A

The holes are repelled b the voltage and move towards the depletion regions decreasing its size and creating a forward biased current on the order of mA.

36
Q

How does a diode work?

A

It is a p-n junction, with the p type being the anode (positive) and n type being the cathode (negative0

37
Q

What is the threshold voltage for silicon?

A

~0.6V

38
Q

What can the relationship be approximated to between current and voltage for a diode?

A

I is proportional to V^2.

Actuallly it is an exponential.

39
Q

How do LEDs work?

A

When an electron falls into a hole in the p-type layer, it involves a drop in its energy from the conduction band to a lower orbital, releasing a photon.

40
Q

What are the 4 types of MOSFETS and how can their circuit symbols be distinguished?

A

n channel enhancement/depletion
p-channel enhancement/depletion.

Enhancement the connections on the symbol is seperated. Depletion they are joined in one straight line.

p-type the arrow on the middle connection points away from the FET, n-type it points towards.

41
Q

What’s the difference between the enhancement mode and depletion mode MOSFET?

A

Enhancement - gate voltage can only be positive.
Depletion mode - can be negative or positive.

42
Q

Explain what happens when a Vgs = 0. (for an enhancement mode)

A

One of the p-n junctions in the transistor is reverse biased so current cannot flow.

43
Q

What happens when a positive voltage is applied to the gate? (for an enhancement mode)

A

Electric field accross the device, pulling electrons towards the gate. This creates an inverse layer just below the gate, connecting the two n-type terminals.

44
Q

What is the ohmic, and saturation region for transistors?

A

Ohmic, where Id is roughly proportional to Vds. Rapidly increasing.

Saturation, Id reaches an almost maximum.

45
Q

How are depletion mode MOSFETS different to enhancement mode?

A

There is a built in n channel inside the p-type substrate. This creates a layer which conducts when the gate voltage is zero.

When Vgs goes negative, it pushes the electrons from this substrate away, removing it causing it to be purely p-type substrate so it will no longer conduct.

46
Q

What is a common source amplifier?

A

A FET amplifier where the source is grounded.

47
Q

When biasing a FET amplifier, what is the optimum value of Vds.

A

Vds = Vdd/2 as this allows maximum voltage swing.

48
Q

What are the forbidden zones on a FET’s operating characteristics?

A

Non-linear, low-resistance region (below saturation region)

Maximum limit for Vgs.

Maximum power dissipation within the transistor (VdsId)max

Maximum value of Vds.

49
Q

How do you find the OP of a FET amplifier?

A

Find an equation relating Vds and Id, and plot a line on the operating characteristic. Find the intercept with the appropriate value of Vgs, this is your OP.

50
Q

How can you estimate the gain of an amplifier using the operating characteristic?

A

By finding what the change in Vgs on the op straight line, will result in what change in Vds.

Make sure to check whether the gain is negative.

51
Q

What value represents when Id changes a little based on a small change in Vds?

A

Drain resistance:
rd = d(Vds)/d(Id)
for a fixed Vgs

52
Q

What value represents a small change in Id for a change in Vgs?

A

Mutual Conductance:
gm = dId / dVgs
for a fixed Vds

53
Q

What is the equation for a FET with small signals, and how can this be expressed in a circuit?

A

id = vds/rd + gm vgs

With a current source and rd in parallel.

54
Q

How is a power supply modelled in small signal analysis of a FET?

A

As a short to ground. Essentially a very large capacitor.

55
Q

What are the advantages of having a source resistor for a FET amplifier?

A

More stable operating point, and doesn’t require impractical gatea voltage source.

56
Q

What type of transistor is required for a self-biased FET amplifier?

A

A depletion mode as Vgs operating point is negative.

57
Q

What is required for an enhancement mode MOSFET to be used in a FET amplifier without a DC gate voltage source?

A

A potential divider to set the Vgs operating point.

58
Q

For a self-biased amplifier, what assumption is often made to make the Small signal analysis easier?

A

That rd is very large and no current passes through it.

59
Q

For a self biasing amplifier, what assumption can be made about the output resistance?

A

That is approcimately rd, as it dominates.

60
Q

What is the purpose of a source follower?

A

It serves as a buffer, as the amplification is approximately one.
Can separate elements of a circuit which would interact with each other.

Doesn’t have a drain resistance.

61
Q

What is a bypass capacitor?

A

Reduces the effect of the source resistance at higher frequencies which would otherwise limit the gain of the amplifier.

62
Q

What is a decoupling capacitor?

A

It removes the DC component of the voltage from going between two amplifier circuits.

63
Q

How can decoupling capacitors be analysed?

A

By using the amplifier model (w/ input and ouput impedances)

64
Q

What parasitic capacitance is important for FET amplifier circuits?

A

Cgd, as it connects input and output (around 1pF). Limits the high frequency response of the amplifier.

65
Q

What assumptions are made for the ideal op amp?

A

Infinite open-loop gain.
Infinite input resistance.
Zero output resistance.

66
Q

Design of op-amp inverting amplifier.

A

v- connected to vout via V2,
input connected to v- via R1.
v+ grounded.

67
Q

Design of non-inverting amplifier.

A

input -> v+
ouput has potential divider to the ground, v- is connected to the middle of this.

68
Q

How is a buffer created with op amp?

A

input = v+
v- connected to the ouput

69
Q

How to work out 3dB point of an op-amp inverting amplifier w/ capacitors?

A

Find an expression for gain in terms of complex impedances. Don’t multiple them and should be two seperate terms which dominate at different frequencies.

70
Q

How does a voltage comparator work?

A

Potential divider connected to the v-. Vin connected to v+.
Will jump to positive supply rail when Vin>v-. and will jump to negative supply rail when Vin <v-

71
Q
A

IA Engineering (10 decks)

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