Transistor

Question 1

What is a transistor?

Answer 1

a semiconductor device with three connections, capable of amplification in addition to rectification.

Question 2

Which way does current flow in a NPN transistor?

Answer 2

Collector, base, emitter

Question 3

Which way does current flow in a PNP transistor?

Answer 3

Emitter, base, Collector

Question 4

If the base current is 0, what is the collector current?

Answer 4

Always 0 as the collector current is controlled by the base current

Question 5

What does Vt (thermal voltage) equal?

Answer 5

kT/q

Question 6

What does the collector current equal?

Answer 6

Ic= Is*e^(Vbe/Vt)
Ic = beta Ib

Question 7

Which part of the graph shows the active region?

Answer 7

The parts with the steepest slope, between saturation voltages (graph of x=Vbe against y=Vce)

Question 8

How does a BJT amplifier work?

Answer 8

By using the base-emitter voltage to control the collector current.
Only possible in the active region otherwise it’s saturated

Question 9

What happens to Vce when there is a small change in Vbe?

Answer 9

Large change

Question 10

What does Vce equal?

Answer 10

Vce = Vs - IcR

Question 11

What is gm?

Answer 11

The mutual conductance

Question 12

What does gm equal?

Answer 12

gm = Ic/Vt

Question 13

What is the small signal ratio between Vc and Ic?

Answer 13

Vc = -Rc * ic

= -(Ic/Vt)*Rc

Question 14

What is the gain of a simple common-emitter amplifier?

Answer 14

A = - Rc * gm

Question 15

Why is the gain unpredictable?

Answer 15

related to beta which has a very wide tolerance (50-200)

Question 16

When does the transistor become saturated?

Answer 16

When Vce = 0 as it can’t go any lower and the collector current cannot go any higher

Question 17

What are the common assumptions for quiescent conditions?

Answer 17

• DC implies that capacitors look like open circuits
• Vbe = 0.5-0.7
• Transistor current gain is high so emitter and collector currents are equal, base is negligible

Question 18

What does input impedance equal?

Answer 18

rin = Rb // Beta/ gm

Question 19

What does Rc always equal?

Answer 19

r out

Question 20

What do coupling capacitors do?

Answer 20

limit the range of frequency for which it operates

Question 21

What is the cut off frequency?

Answer 21

• • 3dB point

- when the gain of the amplifier falls by a factor of root 2

Question 22

What does the cut off frequency equal?

Answer 22

fc = 1 / 2pirin*cin

Question 23

What should C out be greater than for the lower cut off frequency?

Answer 23

C out&raquo_space; 1/ 2pifc/10 *rout

Question 24

What does re equal?

Answer 24

re = Vt/Ic

Question 25

What should Ce be greater than?

Answer 25

Ce >= 1/ 2*pi*fc/10*re

Question 26

For large signal analysis what do Ic1 and Ic2 equal?

Answer 26

Ic1 = Is*e^(vbe1/vt)

Question 27

What is the input impedance for a differential amplifier?

Answer 27

rin = 2 beta/gm

Question 28

Why are input and output coupling capacitors used?

Answer 28

to remove dc bias voltages

Question 29

What are the non ideal dc effects?

Answer 29

-removes dc base base current and a small dc base bias current is required to get into the active region

Question 30

When are FET amplifiers used?

Answer 30

- required input impedance is very high | - load current requirements are high

Question 31

When are BJT amplifiers used?

Answer 31

- high voltage gain is needed | - input and output impedances are of the same order

Question 32

How do you calculate percentage distortion

Answer 32

A/4Vt

Question 33

What are the problems with the CE amplifier?

Answer 33

- fro inputs bigger then around 10 mV the output is heavily distorted - only works for ac signal - often a differential input is needed

Question 34

Compare CE with differential

Answer 34

``` CE -one transistor -single input, single output -high gain possibly with high input impedance DIF -need at least two transistors -differential input and out put -reduced gain possible with high input impedance ```

Question 35

What are the single stage amplifier limitations?

Answer 35

- high gain = high gm and high Rc - high run = low gm - low r out = low rc

Question 36

How do you increase the gain?

Answer 36

use two amplifiers instead of one

Question 37

Compare transistors and Op amps?

Answer 37

``` OP -simple to use -near ideal properties TRANS -better frequency response -lower noise ```

Question 38

What is the voltage swing of a differential amplifier?

Answer 38

Ic = gm Vt Vout = Vin - RcIc V swing = Vin - Vout

Question 39

What is the overall gain of a two stage amplifier?

Answer 39

ADiff * Ace* rce/(rdiff + rce)