Bipolar Transistors - Voltage Amplifiers

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					Power Supply Design
                   Power Supplies

For most electronic devices it is necessary to provide a
stable source of DC power. Batteries often serve this
function but they are limited in the amount of energy
available (see Chapter 14 - a 9 v alkaline will last a day at
100 mA, a car battery 2 hours at 1 A).
One of the simplest and most useful devices you can design
is a linear voltage source. We already have many of the
components: in lab 3: Diodes we saw the full wave bridge,
transformer and filter capacitor. We now need a regulator
circuit.
There are two common ways of doing this (if the circuit does
not involve an operational amplifier (next week’s topic)
• Pass transistor with voltage reference
• integrated circuit with internal pass transistor (“three
terminal regulator”)
        Unregulated DC supply
                 We already know how to make an
             I   unregulated DC supply. The first
                 component is a full wave bridge, which
                 can be purchased as a package.
                 (http:// www.irf.com/product-
                 info/bridges/articles.html)



                 There are two equations to
                 remember:
T   1
                     DC level V0  Vpeak  2Vdiode
  
2 2f
        V                  I 1        I       V0
                 Ripple V          2 fC  2 fCR
                                     
                            C 2f
                                                  load

V0               Then the choice of capacitors depends
                 on the level of ripple tolerated and the
                 current drawn.
                 Unregulated DC (2)
One reason that we still need a regulator is the variations in the
input AC. This can be in the form or surges or sags. A two
year study by Bell Laboratories showed that most locations will
experience approximately 25 power line disturbances/year, many
sagging below 96 volts. This is a result of power loading, and
you probably have noticed light bulbs flicker or dim. A two
year study by IEEE members Martzloff and Hahn completed in
1970 shows that surges and impulse voltage spikes can occur as
frequently as twice per hour in a typical residence with peak
values of 1500V to 2500V. All these variations in voltage
(they are all somewhat smoothed by the capacitor) result in
changes in the output voltage.

Another application of a regulator is for a battery-powered
circuit, which varies with load and with time.
                  Better Zener diode voltage regulator
                                    The voltage output Vout is VZ - VBE ~ VZ - 0.7.
                                    Remember that VZ depends on the current
                                    through the zener. However since the base
                                    current is small compared to the current
Unregulated Vin




                          RC
                    R               through the zener, which depends only on Vin
                                    the output is well controlled.
                                    The choice of resistor R is dependent an the
                                    current we want in the zener and the current in
                                    the supply. If the supply needs 1 A of
                                    collector (and emitter) current through the pass
                        Regulated   transistor, this means the base current is about
                        Vout        10 mA. Then the zener current should be
                                    greater than this so that changes in the output
                                    current don’t change the regulated voltage
                                    much. Also for a 1 W zener the voltage is
                                    normally specified at ~20-50 mA (for a 0.5 W
                                    zener it is 10’s of mAs)
     Better Zener diode voltage regulator (2)

                                  The resistor RC is for current limiting in
                                  the event that the output is shorted (you
                                  have probably done this in lab at least
                                  once already). If this happens then it is
Unregulated Vin



                        RC        not regulated anymore and the full Vin
                  R               falls across both R and RC (since the
                                  emitter and base are nearly grounded).
                                  A guideline for selection of this resistor:
                                  make it so the voltage drop across RC is
                                  less than the voltage drop across R when
                      Regulated   the maximum current is flowing. So if
                      Vout        the maximum output current is 1 A and
                                  the unregulated voltage is at most 5 V
                                  greater than the regulated output RC ~
                                  4.3 V/1 A = 4W. This keeps the BJT
                                  from saturation and if there is a short on
                                  the output current is less than Vin/RC.
           Three-terminal regulators
The previous design shows that a regulator is just a power
dissipating element, the pass transistor, that is adjusted by an
input voltage so that the voltage drop across it is controlled to
fix the output voltage.
This is a fairly simple design, but it is sensitive to changes in
temperature. In addition you are limited to the values and
precision of zener diodes available.
                            An alternative is a three-terminal
                            regulator. An example are the 7800
                            series fixed positive voltage regulators,
                            which come as a three pin package and can
                            be connected with a few optional
                            capacitors to make a regulated supply.
         Three-terminal regulators (2)
How do they work? For a standard three-terminal there are 20-30
transistors internally as well as a voltage reference, often a zener
diode! The output stage is a Darlington pair. The output voltage is
sampled and compared to the reference. If there is an error a
correction is applied to the base of the Darlington to make the
output correct.
An important consideration in
the use of this is that there
is a minimum voltage
difference between Vin and
Vout called the dropout
voltage. This is typically ~2 V.
The reason for this is that
the Darlington is kept far
from saturation which slows
its response to changes. From
the diagram you can see that
it is 2 VBE + VCE(PNP).
       Adjustable 3-terminal regulators
There are only a handful of voltage that
fixed regulators are made for. In
addition if you want to change the
voltage you need to rebuild the supply.
An alternative is an adjustable supply
where you change a potentiometer
(variable resistor) to get a voltage out.
 The value of VREF for a LM117-series
 is 1.25 V. The current through the
 adjustment connection IADJ is less
 than 100 mA. A typical value for R1 is
 240 W. The actual value of the
 output voltage is normally tweaked
 with the adjustable resistor since
 there are variations from chip-to-chip
 of the actual values of VREF and IADJ.
  Adjustable 3-terminal regulators (2)
The rest of the parameters that
you need for design are listed in
the datasheets for the regulator.
Typically a designer uses a
reference such as Horowitz and
Hill to help with selection once the
performance criteria are chosen.
More recently manufacturers
provide web info that helps with
design selection in their product
line (see www.national.com).
                                       317 pinout

Ultimately in either case the
actual circuit is based on material
in the datasheets. If you have
not done so already, download and
read the LM317 data sheet.

				
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posted:11/4/2011
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