Transistor Amplifiers

Question 1

What are the two primary transistor applications

Answer 1

Transistor as switch (digital) and transistor as controlled source (analog)

Question 2

How does analog like to use a transistor?

Answer 2

Analog prefers transistors in the active (or saturation) region. Then the transistor acts like a voltage controlled current source with only one independent variable (fet=vgs) following the square law

Question 3

In the first order model of FET, why doesn’t Ids depend on Vds

Answer 3

Because if we operate the FET in active (saturated) mode then the drain is pinched off isolating the drain from the channel.

Question 4

In the first order model of BJT, why doesn’t Ic depend on Vce

Answer 4

For the BJT in active mode the BC junction is reversed biased isolating the collector

Question 5

What are the FET operating conditions to ensure active mode

Answer 5

Vgs = Vt + Vov, Vds > Vov & Vgd < Vtn

Question 6

What are the BJT operating conditions to ensure active mode

Answer 6

Vbe = 0.7V, Vce > 0.3V therefore Vbc < 0.4V

Question 7

What is a bias point?

Answer 7

It is the DC operating point of the nodes and currents in the circuit. The DC operating point represents the average value.

Question 8

What is another word for bias point?

Answer 8

Quiescent point

Question 9

What is Vov

Answer 9

Vgs - Vt

Question 10

What is Id in terms of Vov

Answer 10

Id = (Kn*Vov^2)/2

Question 11

Why is CS amplifier less gain than CE amplifier

Answer 11

CS -> Av = -IdRd/(Vov/2)
CE-> Av = -IcRd/Vt
Usually Vov/2 > Vt

Question 12

What is a load line?

Answer 12

The effect of load resistance on the Vds vs Ids curve

Question 13

What is the closure resistance

Answer 13

Same as Rds(on)

Question 14

How should the small signal input amplitude compare to overdrive

Answer 14

vgs &laquo_space;2*Vov
this keeps the distortion term down in
id=K/2(VGS+vgs-Vt)^2 (expand the square)

Question 15

What is gm when small signal condition is true

Answer 15

Most useful-> gm=2*Id/Vov

gm=KnVov=kn’(W/L)Vov=Sqrt(2kn’)Sqrt(W/L)Sqrt(Id)

Question 16

What is small signal ro

Answer 16

ro=|Va|/Id

Question 17

What is Id (bias current) w/o channel length modulation when transistor is saturated

Answer 17

Id=(Kn/2)Vov^2=kn’(W/L)*Vov^2

Question 18

What are types of ideal buffers?

Answer 18

Voltage and current

Question 19

What is a voltage buffer?

Answer 19

Passes voltage unchanged, lowers resistance

Question 20

What is a current buffer?

Answer 20

Passes current unchanged, raises resistance. i.e. cascode FET in common gate arrangement

Question 21

What is a good way to find resistance into a node

Answer 21

Apply a test voltage Vx and determine Ix sourced into node: R = Vx/Ix

Question 22

What is input resistance of PCB CG amp

Answer 22

Rin=1/gm

Question 23

What is input resistance of VLSI CG amp

Answer 23

Rin=(1/gm)+(RL/(gmro))
RL translated to input attenuated by gmro
gm*ro will set current buffer effectiveness and therefore cascode fet effectiveness