This document discusses different transistor voltage regulator circuits. It begins by explaining voltage regulation and how a zener diode and emitter follower can be used together in a zener follower circuit to produce a regulated output voltage. Next, it describes a two-transistor voltage regulator circuit that uses negative feedback to keep the output voltage nearly constant even as the input voltage and load current vary. It explains how changes in the output voltage are opposed by the feedback to maintain regulation. In both circuits, the transistor is used to increase the current handling capability compared to using just the zener diode alone.

1. 1 Electronic Devices and
Circuits (2131006)
Transistor
as
A VOLTAGE REGULATOR

2. Index
1.Voltage Regulation
2.Zener follower
3.Two transistor regulator
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3. 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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4. Zener Follower
In fig. shows a zener
follower, a circuit that
combines a zener
regulator and emitter
follower.
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5. 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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6.  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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7.  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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8. Two-Transistor Regulator
In fig. shows two transistor
regulator
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9. 9
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

10. 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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11.  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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12.  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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13. THANK YOU
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