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B - Diodes And Semicon > 4.1 Semiconductions - Transistors Characteristics (2) > Flashcards

4.1 Semiconductions - Transistors Characteristics (2) Flashcards

1
Q

what are the different classes of amplifiers?

A

A
B
AB
C

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2
Q

what are the efficiency levels of a linear and non-linear amplifier?

A

linear = low efficiency
non-linear = high efficiency

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3
Q

how are the A - C amplifier classes defined?

A

by the length of their conduction state over some portion of the output waveform

fully on/off state

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4
Q

what class of amplifiers are the most common due to their simple design?

A

Class A

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5
Q

why are Class A amps designted “best in class”?

A

due to their low distortion levels and best sound

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6
Q

where does Class A amps operate?

A

in the linear portion of the characteristic curve

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7
Q

what is the main disadvantage to a Class A amp? why?

A

never fully turns off

because it is never driven to its cut off or saturation region due to a current always flowing through when it has no base signal

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8
Q

if the class A amp is constantly in its fully on state, what is happening?

A

continuous loss of power in the amp causing large amounts of heat and low efficiency as a result

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9
Q

what must be used in a class A amp due to its high idling current?

A

power supply must be appropriate size and well filtered to avoid hum and noise

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10
Q

what does a class B amp use for each of the wave forms? why?

A

2 complimentary transistors

so that each transistors amplifies only one half of the waveform.

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11
Q

what is the efficiency of a class B amp?

A

approx 50%

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12
Q

what is the problem with class B amp?

A

it can create distortion at the zero crossing point when the waveforms cross over

-0.7 to +0.7v required to turn a transistor to on state

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13
Q

what was the result of a class B to prevent cross distortion?

A

class AB amps developed

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14
Q

what makes the class AB amp work through the zero-crossing point?

A

both transistors are allowed to conduct at the same time, eliminating the crossover distortion

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15
Q

how does a Class AB amp prevent crossover distortion?

A

allowing both transistors to operate for just over a full half wave

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16
Q

what is the efficiency of a class AB amp?

A

50 to 60%

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17
Q

what is the efficiency of a class C amp? any why is this class no good audio amplifiers?

A

80%

due to the heavy distortion created

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18
Q

what is the conduction angle of a class C amp?

A

around 90 degrees

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19
Q

where might class C amps be used?

A

in high-frequency sine wave oscillators and a certain types of radio frequency amplifiers,

where the output can be converted to complete sine waves of a particular frequency

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20
Q

in a single-stage amplifier, why is a coupling capacitor placed after the supply voltage?

A

to prevent a shift in the operating point due to the external supply

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21
Q

in a single stage amplifier, why is there a coupling capacitor at the output?

A

to prevent a change in DC conditions at the transistor as a result of the connected load

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22
Q

in a single stage amp, why is there a resistor and capacitor placed after the transistors emitter terminal?

A

to help stabilise the temperature of the transistor

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23
Q

how are transistors connected if there are several of them?

A

connected in cascade

ie. output voltage of the first amp stage is the input to the next amp stage

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24
Q

what types of coupling is there in amplifiers?

A

AC coupling
DC coupling

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25
Q

what are the ways of accomplishing AC coupling? (2)

A

transformer coupling
resistance-capacitance coupling (RC coupling)

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26
Q

how is transformer coupling carried out?

A

the primary winding of the transformer creates the load resistance of the first stage
it passes it to the next stage through the secondary winding.
a capacitor prevents it from short circuiting

27
Q

what are the advantages and disadvantages of transformer coupling?

A

ad:
if the correct ratio of turns in the transformer is chosen, an optimum resistance match can be achieved.

dis:
expensive
additional distortions can be created
only transform specific frequency range

28
Q

how does RC coupling work?

A

the AC of the first stage is passed through a coupling capacitor onto the next stage

29
Q

what is the role of the coupling capacitor in a RC coupling circuit?

A

preventing the first stage from causing a shift in operation point in the second stage

30
Q

what must the capacitor be in a RC coupling circuit?

A

big enough so the capacitive reactance remains small enough in comparison to input resistance of the second stage

31
Q

how is a DC coupling circuit connected?

A

by simply connecting the output terminal to the input terminal of the next stage

provided that DC potential is the same in both stages

32
Q

how is DC coupling connected when the DC potentials of the stages are not the same?

A

using a voltage divider between the the output terminal and the input terminal

33
Q

what are the disadvantages of a DC coupling circuit?

how is this over come?

A

variations in the output voltage in the first stage minus only the voltage division ratio are directly passed on to the next stage

having a zener diode instead of a resistor as the voltage drop at the diode is nearly constant in the breakdown area

34
Q

what tends to happen with positive feedback?

A

it tends to vibrate

35
Q

how does positive feedback work?

A

supports the input increasing the amplification

36
Q

how does negative feedback work?

A

it reduces the amplication

37
Q

how is a series-shunt feedback system controlled?

A

voltage-voltage

38
Q

how does a series-shunt feedback system work?

A

error voltage fed back in series with input.

39
Q

in a series-shunt feedback system, how is the fed back voltage proportional?

A

directly proportional to output voltage as its parallel

40
Q

what is the ideal resistance of a series-shunt feedback system?

A

input resistance is very large
output resistance very small

41
Q

what is a series-shunt feedback system defined as?

A

output voltage to the input voltage

av=Vout/Vin

42
Q

how does a shunt-series feedback system get controlled?

A

current-current controlled

43
Q

how is the feedback signal of a shunt-series feedback system proportional?

A

directly proportional to the output current flowing in the load

44
Q

in a shunt-series feedback system, how is the feedback signal fed?

A

in parallel with the input

45
Q

how is a shunt-series feedback system defined?

A

as the output current to the input current
AI= Iout/Iin

46
Q

how does the shunt input and series output affect resistance in a shunt-series feedback system?

A

shunt = decreases resistance
series = increases resistance

47
Q

how is a series-series feedback system controlled?

A

voltage-current controlled

48
Q

what is fed back in a series-series feedback system?

A

a current signal which is converted to a voltage signal and subtracted from the input signal

49
Q

how is a series-series feedback system defined?

A

output current to the input voltage

Gm = Iout/Vin

50
Q

why is the impedance of the input and output systems increased in a series-series feedback system?

A

because the output current is connected in series and feedback as a voltage

51
Q

what is the ideal input and output resistances of a series-series feedback system?

A

both = very large resistances

52
Q

how is a shunt-shunt feedback system controlled?

A

current-voltage controlled

53
Q

how is a shunt-shunt feedback system defined?

A

output voltage to input current

Rm = Vout/Iin

54
Q

why does the impedance of both the input and output terminals reduce in a shunt-shunt feedback system?

A

because they’re connected in parallel

55
Q

what are the ideal resistances of the input and output terminals of a shunt-shunt feedback system?

A

both very small

56
Q

what is the main focus of a low frequency power amp?

A

to produce AC power necessary for a loudspeaker

57
Q

what issues could arise in power amplifiers? (3)

A

power output
efficiency
distortion

58
Q

where can a single ended amplifier’s output stage function?

A

class A operation

59
Q

if two transistors operating in B mode, what are these amplifiers called?

A

push-pull B power amplifiers

60
Q

in a push-pull B power amplifier, how is distortion made so small?

A

due to that non-linear distortions cancel themselves out in a push-pull circuit

can be removed by moving operating point towards class AB operating point

61
Q

what affect does having a lower distortion factor mean in push-pull B power amplifier?

A

the efficiency of the system is less

62
Q

what option could be used to avoid matching the loads with an output transformer?

A

setting the output stage as a collector circuit due to lower output resistance

63
Q

what are two transistors that have a PNP and NPN config called?

A

complementary

B - Diodes And Semicon (14 decks)

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