4.1 Semiconductions - Transistors Characteristics (2)

Question 1

what are the different classes of amplifiers?

Answer 1

A
B
AB
C

Question 2

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

Answer 2

linear = low efficiency
non-linear = high efficiency

Question 3

how are the A - C amplifier classes defined?

Answer 3

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

fully on/off state

Question 4

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

Answer 4

Class A

Question 5

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

Answer 5

due to their low distortion levels and best sound

Question 6

where does Class A amps operate?

Answer 6

in the linear portion of the characteristic curve

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

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

Answer 9

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

Question 10

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

Answer 10

2 complimentary transistors

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

Question 11

what is the efficiency of a class B amp?

Answer 11

approx 50%

Question 12

what is the problem with class B amp?

Answer 12

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

Question 13

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

Answer 13

class AB amps developed

Question 14

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

Answer 14

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

Question 15

how does a Class AB amp prevent crossover distortion?

Answer 15

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

Question 16

what is the efficiency of a class AB amp?

Answer 16

50 to 60%

Question 17

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

Answer 17

80%

due to the heavy distortion created

Question 18

what is the conduction angle of a class C amp?

Answer 18

around 90 degrees

Question 19

where might class C amps be used?

Answer 19

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

Question 20

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

Answer 20

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

to help stabilise the temperature of the transistor

Question 23

how are transistors connected if there are several of them?

Answer 23

connected in cascade

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

Question 24

what types of coupling is there in amplifiers?

Answer 24

AC coupling
DC coupling

Question 25

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

Answer 25

transformer coupling resistance-capacitance coupling (RC coupling)

Question 26

how is transformer coupling carried out?

Answer 26

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

Question 27

what are the advantages and disadvantages of transformer coupling?

Answer 27

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

Question 28

how does RC coupling work?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

what must the capacitor be in a RC coupling circuit?

Answer 30

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

Question 31

how is a DC coupling circuit connected?

Answer 31

by simply connecting the output terminal to the input terminal of the next stage provided that DC potential is the same in both stages

Question 32

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

Answer 32

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

Question 33

what are the disadvantages of a DC coupling circuit? how is this over come?

Answer 33

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

Question 34

what tends to happen with positive feedback?

Answer 34

it tends to vibrate

Question 35

how does positive feedback work?

Answer 35

supports the input increasing the amplification

Question 36

how does negative feedback work?

Answer 36

it reduces the amplication

Question 37

how is a series-shunt feedback system controlled?

Answer 37

voltage-voltage

Question 38

how does a series-shunt feedback system work?

Answer 38

error voltage fed back in series with input.

Question 39

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

Answer 39

directly proportional to output voltage as its parallel

Question 40

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

Answer 40

input resistance is very large output resistance very small

Question 41

what is a series-shunt feedback system defined as?

Answer 41

output voltage to the input voltage av=Vout/Vin

Question 42

how does a shunt-series feedback system get controlled?

Answer 42

current-current controlled

Question 43

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

Answer 43

directly proportional to the output current flowing in the load

Question 44

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

Answer 44

in parallel with the input

Question 45

how is a shunt-series feedback system defined?

Answer 45

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

Question 46

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

Answer 46

shunt = decreases resistance series = increases resistance

Question 47

how is a series-series feedback system controlled?

Answer 47

voltage-current controlled

Question 48

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

Answer 48

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

Question 49

how is a series-series feedback system defined?

Answer 49

output current to the input voltage Gm = Iout/Vin

Question 50

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

Answer 50

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

Question 51

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

Answer 51

both = very large resistances

Question 52

how is a shunt-shunt feedback system controlled?

Answer 52

current-voltage controlled

Question 53

how is a shunt-shunt feedback system defined?

Answer 53

output voltage to input current Rm = Vout/Iin

Question 54

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

Answer 54

because they're connected in parallel

Question 55

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

Answer 55

both very small

Question 56

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

Answer 56

to produce AC power necessary for a loudspeaker

Question 57

what issues could arise in power amplifiers? (3)

Answer 57

power output efficiency distortion

Question 58

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

Answer 58

class A operation

Question 59

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

Answer 59

push-pull B power amplifiers

Question 60

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

Answer 60

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

Question 61

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

Answer 61

the efficiency of the system is less

Question 62

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

Answer 62

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

Question 63

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

Answer 63

complementary