Semester 2 Final

Question 1

Different way to represent diodes, what rule do diodes not follow?
What’s a typical voltage for a silicon diode?

Answer 1

Diodes have a voltage drop Vd at a typical current Id
Voltage drop can be represented by a voltage source pointing the other way.
This source DOESN’t deliver any power-it consumes power.
Doesn’t follow Ohms law
0.7V

Question 2

Non-inverting Op Amp-what’s it look like, how to work Vout

Answer 2

Vout goes into V- through one resistor, with a second resistor in series going to ground.
Vout = Vin * (R1 + R2)/R2

Question 3

How to check for negative feedback in an amplifier?

What about if you get positive feedback?

Answer 3

1. Disconnect the circuit that feeds back from the output
2. Increase Vin (=V+) from 0V to a small positive voltage
3. Reconnect the feedback circuit
4. If Vout is reduced for the same Vin value, negative feedback
   If you get positive feedback, its not a fucking amplifier

Question 4

Inverting Op Amp-what’s it look like, how to find Vout

Answer 4

V+ goes to ground.
Vout = -(R2/R1)*Vin
R1 is first resistor from Vin, R2 is second

Question 5

What’s virtual ground and how does it work?

Answer 5

In an ideal Op Amp V+=V-
In an inverting Op Amp V+ is 0 as its connected to ground. This means that despite V- being connected to the power supply, its also 0

Question 6

Inverting Schmitt Trigger: what’s it look like, equations for trigger voltages, what’s the loop look like

Answer 6

Looks like a non-inverting Op Amp but with the potential dividor from Vout going into V+ (swap V+ and V- around)

Vtrigger = +/-Vsupply * R2/(R1 + R2)

Loop starts at top left, falls to bottom right

Question 7

Non-inverting Schmitt trigger: what’s it look like, equations for trigger voltages, what’s the loop look like

Answer 7

Looks like an inverting Op amp except V- goes to ground and V+ goes halfway between R1 and R2

Vtrigger = +/-Vsupply* R1/R2

Loops starts at bottom left, rises to top right

Question 8

What is a Schmitt trigger

Answer 8

Circuit where the output increases to a set maximum when the input rises above a certain threshold, and decreases to 0 when the input falls below another threshold
Gives positive feedback instead of negative

Question 9

4 types of filters and how they react to different frequencies

Answer 9

Low-pass filter: only lets low frequency signals through
High-pass filter: only lets high frequency signals through
Band-pass filter-only lets signals in a certain range of frequencies through
Band-stop/notch filter: stops frequencies in a certain range from going through

Question 10

JFET: what it stands for, how exactly the inputs affect each other

Answer 10

Junction Field Effect Transistor
Two inputs: drain and gate
One output: source
Id = Id (roughly)
Ig = 0 (roughly)
Vgs (voltage between gate and source) controls drain current

Question 11

What is the equivalent circuit model for a FET?

Answer 11

A voltage controlled current source with a resistor r0 across it

Question 12

What is a depletion-mode MOSFET?

What is an enhancement-mode MOSFET?

Answer 12

Vgs = 0, current flows
Vgs &laquo_space;0, current stops flowing
Vgs = 0, no current flows
Vgs > 0, current flows

Question 13

Given an equivalent circuit model for a FET, what equation would you use to work out Id involving a constant K

Answer 13

Id = k*((Vgs - Vth)^2)/2
Vgs = gate-source voltage
Vth = threshold voltage: the Vgs required for drain current to start to flow

Question 14

Equation for IV load line for an FET equivalent circuit

Equation to calculate gradient of the line

Answer 14

Vds = Vdd - Rd*Id
Vds = drain-source voltage
Vdd = power rail, voltage going into drain
Rd = resistor between power rail and circuit
Id = drain current

Rearrange for Id to be y in y=mx+c
Gradient = -1/Rd

Question 15

What is the Q point of a MOSFET IV graph?

What are the coordinates?

Answer 15

The intercept of load line and the MOSFET curve of gate-source voltage against drain current
Id, Vds

Question 16

What is gm
The equation linking drain current to gate-source voltage using gm
How to calculate it graphically when an AC voltage is applied to the MOSFET?

Answer 16

gm = transconductance
Id = gm*Vgs
Its equal to the gradient of the slope of the Id-Vgs graph between the peak-to-peak voltages of the input

Question 17

Calculating gm using current, gate-source voltage and threshold voltage
Calculating Vout using gm, drain resistor and input voltage

Answer 17

gm = (2*Id)/(Vgs - Vth)

Vout = -gm*Rd*Vin
Rd = drain resistor

Question 18

Average gain equation

Input resistance

Answer 18

Av = Vout/Vin = -gm*Rd

Rin = Vin/Iin = R1 || R2
Iin = current in

Question 19

What is clipping?

How do you determine maximum amplitude of a wave before clipping from a Vgs graph?

Answer 19

If the gate-source voltage is too high, the trange of voltages will not be in the linear region of the MOSFET (remember expontential graph). This will cause the MOSFET to switch off periodically, making the top of the wave flatten

Half of the voltage region it works in

Question 20

What is the purpose of Rf?

What always accompanies RF and why?

Answer 20

It stabilises the Q-point against variation in FET parameters
A capacitor is in parallel with Rf. Allows equations for AC analysis to be used as the capacitor shorts during AC analysis

Question 21

Equation for the load line of a DC FET amplifier with Rf-similar to equation without Rf

Answer 21

-(Rd + Rf)^-1

Question 22

How do these equations change when we have no capacitor in parallel with Rf:
Input voltage (Vin)
Average gain equation (Av)

Answer 22

Vin = IdRf + Vgs
Normally Vin = Vgs

Av = Rd/(Rf + 1/gm)
Normally Av = Vout/Vin = -gm*Rd

Question 23

How do these equations change when we have no capacitor in parallel with Rf:
Output voltage (Vout)
Drain current (Id)

Answer 23

Vout = (VinRd) / (Rf + 1/gm)
Normally Vout = -gmRd*Vin

Id = Vin / (Rf + 1/gm)
Normally id = gm*vgs

Question 24

How to use an FET as a switch

What kind of logic gate is it?

Answer 24

Take out R1.
When you connect input to power rail, it allows the power rail to go through the transistor straight to ground.
A = 8V, Q = 0V
When you connect input to ground rail, Vgs = 0V which means current will not flow through the transistor. This means it all exits via Q
A = 0V, Q = 8V

NOT gate

Question 25

How are AND and OR gates made from switchs in a circuit?

Answer 25

AND: Two switchs in a row
OR: Two switchs in parallel

Question 26

Making NOR gate out of two FET switchs?

What’s a NOR gate?

Answer 26

Two switchs attached in series to the same output line. Output is connected to one power rail. If either switch is closed the output goes to ground and becomes 0.
Only outputs 1 if both inputs are 0

Question 27

Making NAND gate out of two FET switchs?

Answer 27

Two FET switchs attached in parallel-the source of the top one is attached to the drain of the bottom one. The only way to get 0 output is to close both switchs
Only outputs 0 if both inputs are 1

Question 28

Making NOT gate out of two FET switchs

Answer 28

Two FET switchs in series. Q output from first goes into the gate on second

Question 29

Voltage gain in db
Current gain in db
Power gain in db

Answer 29

All logs are log10
Vdb = 20 * log(V/V0)
Idb = 20 * log(I/I0)
V/V0 / I/I0 = gain

Pdb = 10 * log(P/P0)

Question 30

How to work out roughly the E12 series?

Answer 30

Start at 1.0 and times by 10^(1/12)

Stops being accurate after 2.2