4.1 Semiconductions - Transistors Characteristics (3)

Question 1

in the switching stages, how are transistors operated?

Answer 1

high resistance = switch open
low resistance = switch closed

Question 2

which circuits are often used with transistors in their switching function?

Answer 2

emitter circuits

Question 3

how does an emitter circuit switching transistor work?

Answer 3

no voltage as base = blocks current flow
high resistance = full voltage at output terminal
when voltage increases above 0.7v = transistor switches = no voltage found at output terminal

photosensitive transistors work in a similar fashion when light placed on them

Question 4

what are the operating conditions of a transistor in the cut off region?

Answer 4

zero input base current
zero output collector current
maximum collector voltage

large depletion layer
fully switched off

Question 5

what are the operating conditions of a transistor in the saturation region?

Answer 5

biased so that
maximum base current = maximum collector current = minimum collector voltage

depletion layer small = maximum current flowing through transistor
fully on state

Question 6

what should an ideal switching transistor be? (4)

Answer 6

1. no resistance when closed
2. infinite resistance when open
3. switching response which triggers immediately and without delay
4. require as little power as possible

Question 7

what are the two possible condition of a switching transistor?

Answer 7

block (1)
conducts (0)

Question 8

in a switching transistor, what should the resistance values be for a forward and blocking states?

Answer 8

forward = 30 ohms
blocking = over 100 mega ohm

Question 9

in a switching transistor, what is delay time?

Answer 9

the time between switching and actual increase in collector current to 10% of its max value

Question 10

in a switching transistor, what voltage is required before current can flow due to delay time?

Answer 10

0.6v

Question 11

in a switching transistor, what is rise time?

Answer 11

the time that a collector current needs to rise from 10% max to 90% max current

Question 12

in a switching transistor, what is storage time?

Answer 12

the time, after cutting off control signal which the collector sinks to 90% max current

Question 13

in a switching transistor, what is fall time?

Answer 13

the time it takes for current to fall from 90% to 10% max current

Question 14

in a switching transistor, what can influence the fall time?

Answer 14

external resistances and capacitors
small=less time
higher =more time

Question 15

in a switching transistor, what is turn on time?

Answer 15

delay time + rise time

Question 16

in a switching transistor, what is turn off time?

Answer 16

storage time+ fall time

Question 17

in a switch for active loads, how does collector voltage and current change with on/off states?

Answer 17

if collector current drop = voltage increase

vice-versa

Question 18

in a switch for active loads, what is the power dissipation difference between off and on states?

Answer 18

same amount when turned on or off

Question 19

in a switch for capacitive loads, what is the power dissipation for on and off states?

Answer 19

on = large power loss
off = lower power loss

Question 20

in a switch for capacitive loads, what are the current peaks when switching on and off?

Answer 20

on = high current peak
off = no current peaks

Question 21

in a switch for capacitive loads, what is the length of switch time for on and off states?

Answer 21

on = long
off = short

Question 22

in a switch for an inductive load, what power loss occurs during on and off switching?

Answer 22

on = minimal
off = high

Question 23

what are generators called that generate more than one wave?

Answer 23

waveform generators

Question 24

what is the basic layout of an oscillator?

Answer 24

same as an amplifier with a feedback signal

Question 25

which 2 conditions must be fulfilled for the oscillation of an amp?

Answer 25

1. displacement of phase must be 0 or 360 degrees
2. product of amplification and feedback must be kv=1

Question 26

is oscillation possible if the feedback signal is negative?

Answer 26

no

Question 27

what will occur if kv doesn’t equal 1?

Answer 27

above 1 = amplifier overdrive, operates as a rectangular wave generator
below 1 = can not oscilate

Question 28

what is used to prevent overdrive?

Answer 28

amplitude limiter

Question 29

what are the 3 basic circuits used for sine wave generators with electronic components?

Answer 29

1. meissner-oscillator
2. hartley-oscillator
3. collpitts-oscillator

Question 30

why are signal generators with electronic components used almost only for high frequency?

Answer 30

at lower frequencies, the inductances have large values

Question 31

how is feedback generated in Meissner-oscillator?

Answer 31

via a transformer

Question 32

how is feedback generated in hartley-oscillator?

Answer 32

via a tapped transformer

Question 33

how is feedback generated in collpitts-oscillator?

Answer 33

capacitive voltage divider circuit

Question 34

what are used as phase shifters in sinewave generators with Rc components?

Answer 34

high-pass and low-pass filters