Field Effect Transisitors

Question 1

What is an important characteristic of an FET?

Answer 1

It has high input impedance

Question 2

What is the primary difference between a BJT and JFET?

Answer 2

A BJT is a current controlled device and a JFET is a voltage controlled device

Question 3

Define what an FET is

Answer 3

A transistor that uses an electric field to control current flow in a semiconductor
-Made up a gate, source and drain
-Voltage is applied to the gate terminal which controls the conductivity between the source and drain

Question 4

What is the direction of electron flow in an FET

Answer 4

Source to drain (tap analogy)

Question 5

For an n-channel JFET, what happens when a voltage (Vds) is applied?

Answer 5

-The depletion region of the p-type material increases, therefore the resistance increases
-As the n-channel narrows, the JFET obeys Ohm’s law until Vds = Vp where resistance is infinite
-At ‘pinch off’: Vgs = 0, Vds = Vp

Question 6

Define the I(dss) of a JFET and give the conditions

Answer 6

-Maximum drain saturation current
-Vgs = 0 and Vds >Vp

Question 7

For a fixed bias JFET, when Vgs < 0 , what bias components are connected to the drain and gate?

Answer 7

-Gate: a voltage source (Vgg), capacitor and a resistor (Rg)
-Drain: a resistor (Rd), Capacitor(vo)

Question 8

What is the major difference between the collector characteristic of a BJT and the drain characteristic of a JFET?

Answer 8

-BJT: the collector current is a linear relationship determined by beta and base current
-JFET: The non-linear relationship between Id and Vgs is defined by the Shockley equation

Question 9

In terms of KVL what is the drain-source voltage in a JFET?

Answer 9

Vds = Vdd - IdRd

Question 10

In self-bias for a JFET, what is added and why?

Answer 10

A resistor is connected to the source terminal
-eliminates the need for 2 voltage supplies
-Vgs can be determined by the voltage across Rs

Question 11

How do we find the Q point on a transfer characteristic for a JFET?

Answer 11

Draw a straight line y=x and the intercept is the q-point

Question 12

What are three important relationships of JFETs?

Answer 12

• Ig = 0 A
  -Id = Is
• Id = Shockley equation

Question 13

For self bias JFET what is Vgs equal to?

Answer 13

Vgs = -IdRs

Question 14

What can we assume about the input impedance of a JFET is?

Answer 14

Zi = infinity

Question 15

What is the ac drain resistance equal to?

Answer 15

rd = ∆Vds / ∆Id