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EE2B > Transistor > Flashcards

Transistor Flashcards

1
Q

What is a transistor?

A

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

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2
Q

Which way does current flow in a NPN transistor?

A

Collector, base, emitter

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3
Q

Which way does current flow in a PNP transistor?

A

Emitter, base, Collector

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4
Q

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

A

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

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5
Q

What does Vt (thermal voltage) equal?

A

kT/q

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6
Q

What does the collector current equal?

A 
Ic= Is*e^(Vbe/Vt)
Ic = beta Ib
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7
Q

Which part of the graph shows the active region?

A

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

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8
Q

How does a BJT amplifier work?

A

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

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9
Q

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

A

Large change

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10
Q

What does Vce equal?

A

Vce = Vs - IcR

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11
Q

What is gm?

A

The mutual conductance

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12
Q

What does gm equal?

A

gm = Ic/Vt

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13
Q

What is the small signal ratio between Vc and Ic?

A

Vc = -Rc * ic

= -(Ic/Vt)*Rc

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14
Q

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

A

A = - Rc * gm

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15
Q

Why is the gain unpredictable?

A

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

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16
Q

When does the transistor become saturated?

A

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

17
Q

What are the common assumptions for quiescent conditions?

A
  • 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
18
Q

What does input impedance equal?

A

rin = Rb // Beta/ gm

19
Q

What does Rc always equal?

A

r out

20
Q

What do coupling capacitors do?

A

limit the range of frequency for which it operates

21
Q

What is the cut off frequency?

A
    • 3dB point

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

22
Q

What does the cut off frequency equal?

A

fc = 1 / 2pirin*cin

23
Q

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

A

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

24
Q

What does re equal?

A

re = Vt/Ic

25
Q

What should Ce be greater than?

A

Ce >= 1/ 2pifc/10*re

26
Q

For large signal analysis what do Ic1 and Ic2 equal?

A

Ic1 = Is*e^(vbe1/vt)

27
Q

What is the input impedance for a differential amplifier?

A

rin = 2 beta/gm

28
Q

Why are input and output coupling capacitors used?

A

to remove dc bias voltages

29
Q

What are the non ideal dc effects?

A

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

30
Q

When are FET amplifiers used?

A
  • required input impedance is very high

- load current requirements are high

31
Q

When are BJT amplifiers used?

A
  • high voltage gain is needed

- input and output impedances are of the same order

32
Q

How do you calculate percentage distortion

A

A/4Vt

33
Q

What are the problems with the CE amplifier?

A
  • fro inputs bigger then around 10 mV the output is heavily distorted
  • only works for ac signal
  • often a differential input is needed
34
Q

Compare CE with differential

A 
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
35
Q

What are the single stage amplifier limitations?

A
  • high gain = high gm and high Rc
  • high run = low gm
  • low r out = low rc
36
Q

How do you increase the gain?

A

use two amplifiers instead of one

37
Q

Compare transistors and Op amps?

A 
OP
-simple to use
-near ideal properties
TRANS
-better frequency response 
-lower noise
38
Q

What is the voltage swing of a differential amplifier?

A

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

39
Q

What is the overall gain of a two stage amplifier?

A

ADiff * Ace* rce/(rdiff + rce)

EE2B (3 decks)

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