1. Remove all but one source 2. Simplify the circuit using resistor laws 3. Find unknowns 4. Do the same for other sources 5. Voltages and currents at a point are sum of all calculations

Semester 2 Flashcards by Matt Moran

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

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Current Dividor

I1 = Itotal * (R1 + R2)/R2

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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

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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)

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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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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What does FET stand for

Field effect transistor

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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

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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

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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 &laquo_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 = (2*Id)/(Vgs - Vth) Vth = threshold voltage CED: id = gm*vgs

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