4.1 Semiconductions - Transistors Characteristics (3) Flashcards
in the switching stages, how are transistors operated?
high resistance = switch open
low resistance = switch closed
which circuits are often used with transistors in their switching function?
emitter circuits
how does an emitter circuit switching transistor work?
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
what are the operating conditions of a transistor in the cut off region?
zero input base current
zero output collector current
maximum collector voltage
large depletion layer
fully switched off
what are the operating conditions of a transistor in the saturation region?
biased so that
maximum base current = maximum collector current = minimum collector voltage
depletion layer small = maximum current flowing through transistor
fully on state
what should an ideal switching transistor be? (4)
- no resistance when closed
- infinite resistance when open
- switching response which triggers immediately and without delay
- require as little power as possible
what are the two possible condition of a switching transistor?
block (1)
conducts (0)
in a switching transistor, what should the resistance values be for a forward and blocking states?
forward = 30 ohms
blocking = over 100 mega ohm
in a switching transistor, what is delay time?
the time between switching and actual increase in collector current to 10% of its max value
in a switching transistor, what voltage is required before current can flow due to delay time?
0.6v
in a switching transistor, what is rise time?
the time that a collector current needs to rise from 10% max to 90% max current
in a switching transistor, what is storage time?
the time, after cutting off control signal which the collector sinks to 90% max current
in a switching transistor, what is fall time?
the time it takes for current to fall from 90% to 10% max current
in a switching transistor, what can influence the fall time?
external resistances and capacitors
small=less time
higher =more time
in a switching transistor, what is turn on time?
delay time + rise time
in a switching transistor, what is turn off time?
storage time+ fall time
in a switch for active loads, how does collector voltage and current change with on/off states?
if collector current drop = voltage increase
vice-versa
in a switch for active loads, what is the power dissipation difference between off and on states?
same amount when turned on or off
in a switch for capacitive loads, what is the power dissipation for on and off states?
on = large power loss
off = lower power loss
in a switch for capacitive loads, what are the current peaks when switching on and off?
on = high current peak
off = no current peaks
in a switch for capacitive loads, what is the length of switch time for on and off states?
on = long
off = short
in a switch for an inductive load, what power loss occurs during on and off switching?
on = minimal
off = high
what are generators called that generate more than one wave?
waveform generators
what is the basic layout of an oscillator?
same as an amplifier with a feedback signal
which 2 conditions must be fulfilled for the oscillation of an amp?
- displacement of phase must be 0 or 360 degrees
- product of amplification and feedback must be kv=1
is oscillation possible if the feedback signal is negative?
no
what will occur if kv doesn’t equal 1?
above 1 = amplifier overdrive, operates as a rectangular wave generator
below 1 = can not oscilate
what is used to prevent overdrive?
amplitude limiter
what are the 3 basic circuits used for sine wave generators with electronic components?
- meissner-oscillator
- hartley-oscillator
- collpitts-oscillator
why are signal generators with electronic components used almost only for high frequency?
at lower frequencies, the inductances have large values
how is feedback generated in Meissner-oscillator?
via a transformer
how is feedback generated in hartley-oscillator?
via a tapped transformer
how is feedback generated in collpitts-oscillator?
capacitive voltage divider circuit
what are used as phase shifters in sinewave generators with Rc components?
high-pass and low-pass filters
