Download - Transistor as a voltage regulation
Transistor
as A VOLTAGE REGULATOR
1Electronic Devices and
Circuits (2131006)
Index
1.Voltage Regulation
2.Zener follower
3.Two transistor regulator
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What is ”Voltage Regulation” ?
Besides being use in buffer circuits and
impedance , the emitter follower is widely
use in voltage regulators.
In conjunction with a zener diode ,the
emitter follower can produce regulated
output voltages with much larger output
currents.
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Zener Follower
In fig. shows a zener
follower, a circuit that
combines a zener
regulator and emitter
follower.
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How its works?
The zener voltage is the input to the base of the emitter follower.
The d.c output voltage of the emitter follower is
VOUT = VZ – VBE
This output voltage is fixed so that it is equal to the zener voltage minus the VBE drop of the transistor .
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If the supply voltage changes , the zener voltage remains approximately constant , and so
does the output voltage.
In other word the circuit acts like a voltage regulator because the output voltage is always
one VBE drop less than the zener voltage.
The zener follower has two advantages over an ordinary zener regulator: first zener diode
of in fig. has to produce a load current of only
IB=IOUT / βdc
Since this base current is much smaller than the output current , we can use a much
smaller zener diode .
For instance , if you are trying to supply several amperes to a load resistor , and ordinary
zener regulator requires a zener diode capable of handling several amperes .
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The second advantage of a zener follower is it low output impedance. In an ordinary zener regulator , the load resistor sees and output impedance approximately Rz, The
zener impedance.
But in the zener follower the output impedance is
Zout = r’e + Rz /βdc
In the fig(b) shows the equivalent output circuit. Because zout is usually very small
compared to RL , an emitter follower can hold the D.C. output voltage almost constant
because the source looks stiff .
In summary , the zener follower provides the regulation of a zener diode with the
increased current handaling capability of an emitter follower.
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Two-Transistor Regulator
In fig. shows two transistor
regulator
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The d.c. input voltage vin comes from a
unregulated power supply such as a bridge
rectifier with a capacitor input filter.
Vin has a peak-to-peak ripple of about 10
percent of the d.c. voltage.
The final output voltage Vout has almost no
ripple and is almost constant in value ,
even though the input voltage and load
current may vary over a large range
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How its works?
Any attempted changes output voltage produce an amplified feedback voltage that opposes the original change .
For instants , suppose the output voltage is increases. then , the voltage appearing at
the based of Q1 will increase .
Since Q1 and R2 form a CE amplifier , the collector voltage of Q1 will decrease
because of the voltage gain.
Since the collector voltage of Q1 has decreased the voltage of Q2 decreases .
Because Q2 is an emitter follower the output voltage will decrease.
In the other words we have negative feedback .
The original increase in output voltage produces an opposing decrease in output voltage.
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The overall effect is that the output voltage will increase only slightly , much less then it would without the negative feedback .
Conversely , if the output voltage tries to decrease less voltage appears at the Q1
base more voltage appears at the Q1 collector , and more voltage appears at the
Q2 emitter .
We have a returning voltage that opposes the original changes in the output voltage.
Therefore , the output voltage decrease only a little far less than without the negative feedback.
Because of the zener diode , the Q1 emitter voltage equals Vz. The Q1 base
voltage is one VBE drop higher.
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Therefore, The voltage across R4 is :
V4=VZ+VBE
With ohm’s law , the current through R4 is :
I4=(Vz+VBE)/R4 Since , This current flow through R3 in series with R4, The output voltage is:
Vout = I4(R3+R4)
After expanding;
Vout=((R3+R4)(VZ+VBE))/R4
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THANK YOU
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