Semester 2 Flashcards
Superposition steps
- Remove all but one source
- Simplify the circuit using resistor laws
- Find unknowns
- Do the same for other sources
- Voltages and currents at a point are sum of all calculations
Current Dividor
I1 = Itotal * (R1 + R2)/R2
Thevenin equivalent circuit, how to find the different parts
Rthevenin = Vopencircuit/Ishortcircuit
Voc = voltage between A and B when no current flowing between them
Isc=current flowing between A and B when circuit is closed
Quick method to find resistance in thevenin circuits
Only works with no dependent sources
Turn down all sources to zero, reduce resistors down to one (find resistance across A and B)
Converting between Thevenin and Norton equivalent circuits
If you have a voltage source and resistor in series, you can change it to a current source and resistor in parallel.
Rth=Rn
Vth=Rn*In
Different way to represent diodes, what rule do diodes not follow?
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
Non-inverting Op Amp-what’s it look like, how to work Vout
Vout goes into V- through one resistor, with a second resistor in series going to ground.
Vout = Vin * (R1 + R2)/R2
How to check for negative feedback in an amplifier?
What about if you get positive feedback?
- Disconnect the circuit that feeds back from the output
- Increase Vin (=V+) from 0V to a small positive voltage
- Reconnect the feedback circuit
- If Vout is reduced for the same Vin value, negative feedback
If you get positive feedback, its not a fucking amplifier
For amplifiers written as voltage sources for no fucking reason how do you calculate Vout? Two potential dividor equations
Vin = Vs*(Rin/(Rs+Rin))
Rs is the first resistor, Rin is the second (inside the box)
Vout = AvVin(RL/(Rout+RL))
Rout is the first resistor, RL is the second (outside the box)
Inverting Op Amp-what’s it look like, how to find Vout
V+ goes to ground.
Vout = -(R2/R1)*Vin
R1 is first resistor from Vin, R2 is second
What’s virtual ground and how does it work?
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
Inverting Schmitt Trigger: what’s it look like, equations for trigger voltages, what’s the loop look like
Looks like a non-inverting Op Amp but with the potential dividor from Vout going into V+ (swap V+ and V- around)
Vtrigger = +/-Vsupply * R1/(R1 + R2)
Loop starts at top left, falls to bottom right
What is a Schmitt trigger
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
Non-inverting Schmitt trigger: what’s it look like, equations for trigger voltages, what’s the loop look like
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
4 types of filters and how they react to different frequencies
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
What does FET stand for
Field effect transistor
BJT: what it stands for, what are the relevent equations
Bipolar Junction Transistor Two inputs: Base current (side) and Collector current (top) One output: Emitter current Ie=Ib + Ic Ic=B*Ib where B>>1
JFET: what it stands for, how exactly the inputs affect each other
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
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 chaged allowing electrons to flow between the source and drain
What is the equivalent circuit model for a FET?
A voltage controlled current source with a resistor r0 across it
What is a depletion-mode MOSFET?
Vgs = 0, current flows
Vgs «_space;0, current stops flowing
What is an enhancement-mode MOSFET?
Vgs = 0, no current flows
Vgs > 0, current flows
Remind yourself what the inputs and outputs are on a JFET
JFET Gate-source voltage against Drain current graph. What does it look like, what’s on the axis?
Drain on top, Gate on side, Source on bottom
Drain current on Y, Gate-source voltage on X
Looks like an expontential graph starting at some value of Gate-source and 0 drain current
Remind yourself what the inputs and outputs are on a JFET
JFET Drain-source voltage against Drain current graph. What does it look like, what’s on the axis?
Drain on top, Gate on side, Source on bottom.
Multiple lines for different values of Vgs. Lines rise rapidly and then flatten out for different values of drain current depended on Vgs value.
Given an equivalent circuit model for a FET, what equation would you use to work out Id involving a constant K
Id = k*((Vgs - Vth)^2)/2 Vgs = gate-source voltage Vth = threshold voltage: the Vgs required for drain current to start to flow
What is the gradient for the graph of drain-source voltage against drain current in the saturation region?
1/r0
Calculating voltage gain for amplifiers at the systems level
Gain = Vout/Vs
Vs = Voltage source
Find potential dividor equations for Vs and Vout, put them together
Equation for IV load line for an FET equivalent circuit
Equation to calculate gradient of the line
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
What is the Q point of a MOSFET IV graph?
What are the coordinates?
The intercept of load line and the MOSFET curve of gate-source voltage against drain current
Id, Vds
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?
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
Two equations to calculate gm:one related to the MOSFET data and one related to the circuit equivalent data
MOSFET data: gm = (2Id)/(Vgs - Vth)
Vth = threshold voltage
CED: id = gmvgs
Calculating gm using current, gate-source voltage and threshold voltage
Calculating Vout using gm, drain resistor and input voltage
gm = (2*Id)/(Vgs - Vth)
Vout = -gm*Rd*Vin Rd = drain resistor
Average gain equation
Input resistance
Av = Vout/Vin = -gm*Rd
Rin = Vin/Iin = R1 || R2 Iin = current in
What is clipping?
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
Finding the overall gain of an amplifier at a systems level (amp represented by Av*Vin)
Vout/Vin
One potential dividor equation for each
Vin = Vs*(Rin/(Rs + Rin))
Rs is source resistor, Rin is amplifier resistor
Vout = Av*Vin*(Rl/(Rl + Rout)) Rl = Load resistor Rout = Amplifier out resistor
What is a single stage common-source FET amplifier with Rf?
Same as a normal FET amplifier except that there is a resistor between the transistor source and ground called Rf
What is the purpose of Rf?
What always accompanies RF and why?
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
For DC analysis of an FET amplifier with Rf: whats the equation for drain-source voltage (Vds)?
Vds = Vdd - IdRd - IdRf Vdd = Power rail Rd/Id = drain resistor/current Rf = source resistor
Equation for the load line of a DC FET amplifier with Rf-similar to equation without Rf
-(Rd + Rf)^-1
How do these equations change when we have no capacitor in parallel with Rf: Input voltage (Vin) Average gain equation (Av)
Vin = IdRf + Vgs
Normally Vin = Vgs
Av = Rd/(Rf + 1/gm)
Normally Av = Vout/Vin = -gm*Rd
How do these equations change when we have no capacitor in parallel with Rf: Output voltage (Vout) Drain current (Id)
Vout = (VinRd) / (Rf + 1/gm)
Normally Vout = -gmRd*Vin
Id = Vin / (Rf + 1/gm)
Normally id = gm*vgs
How to use an FET as a switch
What kind of logic gate is it?
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
How are AND and OR gates made from switchs in a circuit?
AND: Two switchs in a row
OR: Two switchs in parallel
Making NOR gate out of two FET switchs?
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.
Making NAND gate out of two FET switchs?
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
Making NOT gate out of two FET switchs
Two FET switchs in series. Q output from first goes into the gate on second
Voltage gain in db
Current gain in db
Power gain in db
All logs are log10
Vdb = 20 * log(V/V0)
Idb = 20 * log(I/I0)
V/V0 / I/I0 = gain
Pdb = 10 * log(P/P0)
