FET amplifiers Flashcards
What does FET stand for?
Field effect transisor
What is a junction FET?
Vgs = 0 then current flows
Vgs << Negative then current will be stopped.
When current is stopped in a junction FET what is this referred to?
The pinch off voltage
What is a MOSFET?
Metal Oxide Field Effect Transistor
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 charged 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 the equation for the IV graph of a MOSFET in the saturation region?
Gradient of the slope = 1 / r0
What is the equation for the Norton equivalent circuit of a MOSFET?
id = ( k(Vgs - Vth)^2 ) / 2
What is a depletion-mode MOSFET?
Vgs = 0 then current flows
Vgs << -0 then current stops flowing
What is an enhancement-mode MOSFET?
Vgs = 0 then no current flows
Vgs > 0 then current flows
What is a BJT?
Bipolar Junction Transistor
What are the terminals on a BJT?
Base = Gate
Collector = Source
Emitter = Drain
What are the 2 equations for a BJT?
Ie = Ib + Ic
Ic = BIb
What is a JFET?
Junction Field Effect Transistor
What are the properties of a JFET?
Is ≈ Id
B >> 1
What is the equation for the gain of an amplifier and what is the symbol for gain?
A = Vout / Vin
For DC analysis of a normal transistor amplifier: What is the KVL equation for the output side of the circuit?
Vdd = Vds + IdRd
Vds = Vdd - IdRd
How can we calculate the Drain-source current and voltage when we know: Vgs Rd Vdd
If we overlay the graph for the KVL equation for the output side with Vds as the subject (Vds = Vdd - IdRd) on top of the Id-Vds graph for the FET with different gate-source voltages. The point where the graphs intercept at the correct Vgs value is the drain current and drain-source voltage.
What do we do to the circuit to simplify it during DC analysis of a FET amplifier?
> AC voltages sources short
> AC current sources open
> Capacitors open
> Inductors short
What do we do to the circuit to simplify it during AC analysis of a FET amplifier?
> DC voltage sources short
> DC current sources open
> Large capacitors short
> Large inductors open
For AC analysis of a transistor amplifier: What is the equation relating Drain current to Gate-source voltage
id = gmVgs
What is gm?
The transconductance
How is gm calculated graphically?
gm is equal to the gradient of the slope of the id-Vgs graph between the peak-peak voltages of the input.
What is the gradient of the graph for Vds = Vdd - IdRd? why?
-1/Rd
Id is on the y-axis so we have to rearrange the equation:
Vdd - Vds = IdRd
-1/Rd(Vdd + Vds) = Id
What can be said about the voltage source and Vdd during AC analysis?
Vdd = Gnd = 0V because during AC analysis DC voltage supplies short
What is the equation to calculate gm?
gm = 2Id/(Vgs - Vth)
What are the equations to calculate the output voltage during AC analysis?
Vout = idRd
Vout = gmVinRd
What is the equation to calculate amplification of a FET amplifier?
A = gmRd
What is clipping?
If the gate-source voltage is too high, this can cause the range of voltages to not be in the linear region of a MOSFET and so will cause the MOSFET to switch off periodically and this will cause clipping to occur.
What is a single stage common-source FET amplifier with Rf?
This is the same as normal FET amplifier except that there is a resistor between the transistor source and ground. The resistor is Rf
What is the purpose of Rf?
It stabilises the Q-point against variation in FET parameters
What is the Q point?
During DC analysis, it is the average point on the graph of Id-Vgs or Id-Vds
For DC analysis of a FET amplifier with Rf: What is the KVL equation for the output side of the circuit?
Vds = Vdd - IdRd - IdRf
What always accompanies the resistor Rf? Why and how does this affect equations?
A capacitor is in parallel with Rf. This allows the equations for AC analysis to be the same because the capacitor shorts during AC analysis.
What would happen to the equations for AC analysis if there was no capacitor in parallel with Rf?
This would mean that Rf would be factored in series with Rd.
What is the purpose of capacitors on the input out output of an amplifier?
Blocks out the DC bias
What is the gradient of the graph for Vds = Vdd - IdRd -IdRf?
-1/(Rd+Rf)
What happens to the input voltage if we have no capacitor in parallel with Rf?
Vin = IdRf + Vgs [Normally Vin=Vgs]
What is the equation for amplification if we have no capacitor in parallel with Rf?
A ≈ Rd/(Rf + 1/gm)
What is the equation for the output voltage if we have no capacitor in parallel with Rf?
Vout ≈ VinRd / (Rf + 1/gm)
What is the equation for the drain current if we have no capacitor in parallel with Rf?
id = Vin / (Rf + 1/gm)
