4.1 Semiconductions - Transistors Characteristics (2) Flashcards
what are the different classes of amplifiers?
A
B
AB
C
what are the efficiency levels of a linear and non-linear amplifier?
linear = low efficiency
non-linear = high efficiency
how are the A - C amplifier classes defined?
by the length of their conduction state over some portion of the output waveform
fully on/off state
what class of amplifiers are the most common due to their simple design?
Class A
why are Class A amps designted “best in class”?
due to their low distortion levels and best sound
where does Class A amps operate?
in the linear portion of the characteristic curve
what is the main disadvantage to a Class A amp? why?
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
if the class A amp is constantly in its fully on state, what is happening?
continuous loss of power in the amp causing large amounts of heat and low efficiency as a result
what must be used in a class A amp due to its high idling current?
power supply must be appropriate size and well filtered to avoid hum and noise
what does a class B amp use for each of the wave forms? why?
2 complimentary transistors
so that each transistors amplifies only one half of the waveform.
what is the efficiency of a class B amp?
approx 50%
what is the problem with class B amp?
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
what was the result of a class B to prevent cross distortion?
class AB amps developed
what makes the class AB amp work through the zero-crossing point?
both transistors are allowed to conduct at the same time, eliminating the crossover distortion
how does a Class AB amp prevent crossover distortion?
allowing both transistors to operate for just over a full half wave
what is the efficiency of a class AB amp?
50 to 60%
what is the efficiency of a class C amp? any why is this class no good audio amplifiers?
80%
due to the heavy distortion created
what is the conduction angle of a class C amp?
around 90 degrees
where might class C amps be used?
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
in a single-stage amplifier, why is a coupling capacitor placed after the supply voltage?
to prevent a shift in the operating point due to the external supply
in a single stage amplifier, why is there a coupling capacitor at the output?
to prevent a change in DC conditions at the transistor as a result of the connected load
in a single stage amp, why is there a resistor and capacitor placed after the transistors emitter terminal?
to help stabilise the temperature of the transistor
how are transistors connected if there are several of them?
connected in cascade
ie. output voltage of the first amp stage is the input to the next amp stage
what types of coupling is there in amplifiers?
AC coupling
DC coupling
what are the ways of accomplishing AC coupling? (2)
transformer coupling
resistance-capacitance coupling (RC coupling)
how is transformer coupling carried out?
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
what are the advantages and disadvantages of transformer coupling?
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
how does RC coupling work?
the AC of the first stage is passed through a coupling capacitor onto the next stage
what is the role of the coupling capacitor in a RC coupling circuit?
preventing the first stage from causing a shift in operation point in the second stage
what must the capacitor be in a RC coupling circuit?
big enough so the capacitive reactance remains small enough in comparison to input resistance of the second stage
how is a DC coupling circuit connected?
by simply connecting the output terminal to the input terminal of the next stage
provided that DC potential is the same in both stages
how is DC coupling connected when the DC potentials of the stages are not the same?
using a voltage divider between the the output terminal and the input terminal
what are the disadvantages of a DC coupling circuit?
how is this over come?
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
what tends to happen with positive feedback?
it tends to vibrate
how does positive feedback work?
supports the input increasing the amplification
how does negative feedback work?
it reduces the amplication
how is a series-shunt feedback system controlled?
voltage-voltage
how does a series-shunt feedback system work?
error voltage fed back in series with input.
in a series-shunt feedback system, how is the fed back voltage proportional?
directly proportional to output voltage as its parallel
what is the ideal resistance of a series-shunt feedback system?
input resistance is very large
output resistance very small
what is a series-shunt feedback system defined as?
output voltage to the input voltage
av=Vout/Vin
how does a shunt-series feedback system get controlled?
current-current controlled
how is the feedback signal of a shunt-series feedback system proportional?
directly proportional to the output current flowing in the load
in a shunt-series feedback system, how is the feedback signal fed?
in parallel with the input
how is a shunt-series feedback system defined?
as the output current to the input current
AI= Iout/Iin
how does the shunt input and series output affect resistance in a shunt-series feedback system?
shunt = decreases resistance
series = increases resistance
how is a series-series feedback system controlled?
voltage-current controlled
what is fed back in a series-series feedback system?
a current signal which is converted to a voltage signal and subtracted from the input signal
how is a series-series feedback system defined?
output current to the input voltage
Gm = Iout/Vin
why is the impedance of the input and output systems increased in a series-series feedback system?
because the output current is connected in series and feedback as a voltage
what is the ideal input and output resistances of a series-series feedback system?
both = very large resistances
how is a shunt-shunt feedback system controlled?
current-voltage controlled
how is a shunt-shunt feedback system defined?
output voltage to input current
Rm = Vout/Iin
why does the impedance of both the input and output terminals reduce in a shunt-shunt feedback system?
because they’re connected in parallel
what are the ideal resistances of the input and output terminals of a shunt-shunt feedback system?
both very small
what is the main focus of a low frequency power amp?
to produce AC power necessary for a loudspeaker
what issues could arise in power amplifiers? (3)
power output
efficiency
distortion
where can a single ended amplifier’s output stage function?
class A operation
if two transistors operating in B mode, what are these amplifiers called?
push-pull B power amplifiers
in a push-pull B power amplifier, how is distortion made so small?
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
what affect does having a lower distortion factor mean in push-pull B power amplifier?
the efficiency of the system is less
what option could be used to avoid matching the loads with an output transformer?
setting the output stage as a collector circuit due to lower output resistance
what are two transistors that have a PNP and NPN config called?
complementary
