IB electrical ALL

Can a BJT have backwards current

No they have a diode in

What is base resistance

The resistance between the base and ground

What is miller capacitance and when is it relevant

Cbe, dominates at high frequencies

Why is miller capacitance bad

It slows switching

What is CMRR

Differential gain / common mode gain

Why do we want a big CMRR

Because difference not amplitude is relevant

How does a zener diode act in a transistor circuit

It behaves like a perfect voltage source

How do you treat a zener diode in the SSM

A short circuit

What are the things inside the triangle

Differential amplifier, high gain stage, current mirrors, capacitor

Why do opamps have a differential amplifier

For big CMRR and high input impedance

What do current mirrors do in an opamp

Replace zener diodes and provide constant voltage drop

Why is there a capacitor in an opamp

To reduce gain at high frequencies

Why is high input impedance good

To not draw too much current

Why do we want low output impedance

To minimise power loss

What output impedance gives optimum power transfer

Ro=Rload

How does negative feedback affect open loop gain

Falls by 1+AB

How does negative feedback affect 3dB freq

Rises by 1+AB

How does negative feedback affect the gain bandwidth product

It does not

How does negative feedback affect input impedance

It rises by 1+AB

How does negative feedback affect output impedance

It falls by 1+AB

What is A (feedback)

Open loop gain

What is B (feedback)

Potential divider ratio R2/R1+R2 where R2 connects in and out of opamp and R1 goes from R2 to earth

How do you do SSM when multiple transistors

Work out if only one can be on at a time, only draw one state (one model)

Are line or phase values larger

Line

How to find star line voltage

Phase x root 3

When are line and phase voltage the same

Delta

When are line and phase current the same

Star

How to find delta line current

Phase current x root 3

Do you want to find P Q or S from phase values

No, different eqns for star and delta

P from line values

P=root3 VI cos a

Q from line values

Q= root3 VI sina

S from line values

S= root3 VI

If voltage type is not specified what is it

Line voltage

What to do if multiple loads

Transform to star and branches are in parallel

What are the two main parts of a generator

Stator and rotor

How does a generator work

Primer-mover spins rotor which has a magnetic field, rotating magnetic field in air gap links to stator coils, emf in stator

What is faraday’s law

e=Nd(phi)/dt

Explain 2Hg=NI

Ampere’s law, g is air gap length, 2 because flux goes out once and in once

What is p

Number of coils (2 poles per coil)

What is the infinite bus

V with fixed magnitude and frequency

Explain wr=w/p

Rotor speed = electricity freq speed / number of coils

What is w/p called

The synchronous speed

What is excitation voltage proportional to

Rotor field current, or rotor excitation

What controls real output power of a generator

The primer mover

What values do you use for a phase diagram

Phase voltage and current

How does power in relate to torque

Power = torque x rotor speed

Physically, what is the load angle

The angle between the rotor-driven and total fields in the air gap

What can the load angle not exceed and why

It cannot be above 90 degrees or loss of synchronism occurs

At constant power P how does the phase diagram behave

Triangle height constant as Esind constant

What does over excited mean

E increases, current lags voltage, supply of VAR, inductive

What does under excited mean

E decreases, current leads voltage, VAR absorbed, capacitative

What are rated values

Values that cannot be exceeded due to heating (current) or flux (voltage)

How is real power controlled

By controlling the angle of Vs wrt Vr

What is the main limitation on power

Rated current, I2R losses heat cables and cause sagging

What happens to grid freq if demand exceeds generation

Primary response- f falls. Secondary response- reserve capacity increases Pgen, freq changes to compensate. Important that average freq remains 50

What is the eqn for total rotational kinetic energy

RKE= Kf² where L is the grid inertia constant

How does power change relate to frequency change

Change in P=dRKE/dt = 2Kf df/dt

What is the synchronous speed omega_s

The speed the motor field rotates at

Define slip and what is it

(Ws-Wr)/Ws. It is the relative soeed between the stator driven field Ws and the rotor speed

What is the same about the stator field and the rotor field

They have the same number of poles and rotate at the same speed

What happens if the rotor speed matches the synchronous speed

No torque

What is the rotor current frequency

Slip x w, where w is the speed of the field relative to the rotor

What is the equivalent circuit for a no load test (no torque)

Xm (magnetising reactance) in parallel with Ro (iron loss resistance)

What is the equivalent circuit for a locked rotor test (slip is 1)

(R1+R2’) in series with (X1+X2’) . Stator winding resistance plus referred rotor winding resistance in series with stator leakage reactance plus referred rotor leakage reactance

How to calculate gross torque

3(I2’)² R2’ / s(W_s)

How to find maximum torque

With maximum power transfer theorem-

Tmax = (3 x I2’ x R2’) / (w_s x s)

When does Tmax occur (torque)

At a very small value of slip

How does frequency affect the range of an EM wave

Range decreases with frequency as losses increase

If two waves superimpose what is their group velocity

w1-w2 / b1-b2 where b is the propagation constant 2pi/lambda

What is the phase velocity of a wave

w/b = 2 pi f / (2 pi / L) = fL

What is characteristic impedance

The ratio between voltage and current of a unidirectional wave at any point on a transmission line (root L/C)

Does characteristic impedance dissipate power

No

What are the telegraphers equations

dV/dx = -L dI/dt

dI/dx = -C dV/dt

L and C are per unit length

What is the transmission coefficient rhoT

Magnitude of transmitted voltage / forwards voltage in the first line

What is quarter wave matching (to stop reflections)

Connecting two lines with different impedances by a section of line with L= lambda/4 and impedance equal to the geometric mean of the two

What is Gauss law of electric fields (maxwell)

Del dot D is charge density. Charge produces an electric field

What is Gauss law of magnetic fields

Del dot B is zero. This says that there are no magnetic monopoles so lines of magnetic flux produce closed loops

What is the Faraday law of magnetic fields (maxwell)

Curl of E = - dB/dt. Changing flux through a coil produces an emf that acts to oppose the changing flux

What is the Ampere Maxwell law (maxwell)

Curl of H = current density + dD/dt

Both a moving charge and a changing electric flux density induce a magnetic field

How do the maxwell equations change in a dielectric

No free charges so rho and J are zero

How is the refractive index related to the permittivity

n= root(e)

What is the propagation constant

Beta = 2 pi / lambda

What is the power reflection coefficient

The voltage reflection coefficient squared

What is the simplest antenna

The half dipole antenna of length lambda/4

What direction are E H N on an antenna

E goes up and down/ longitudal loops, H goes round, N goes radially out

What is antenna gain

Maximum power density / isotropic (same in every direction) power density

What is radiation resistance

The resistor that would dissipate as much power as the antenna radiates

What is the effective area of an antenna

The absorbed power divided by the magnitude of the Poynting vector

What are the boundary conditions for EM waves at interfaces

Dn1 = Dn2

Et1 = Et2

Bn1 = Bn2

Ht1 = Ht2

n is normal component t is tangential component

What is the snell law of refraction

n1 sin theta1 = n2 sin theta2

What is the Brewster angle

The angle at which the component of E parallel to the plane of incidence is not reflected (wave polarised)

How thick should an anti reflection coating be

Quarter wavelength

What is gauss electric law in a conductor

Del dot D is zero- a metal has free charges but no net charge so charge density is zero

What is the skin effect

Resistance of a wire changes with frequency because at higher frequencies the depth of conduction decreases so the effective area decreases

What is the equation for skin depth

Root (2/permitivity x w x conductivity)