Voltage follower
PARTS AND MATERIALS
-
One NPN transistor -- models 2N2222 or
2N3403 recommended (Radio Shack catalog # 276-1617 is a
package of fifteen NPN transistors ideal for this and
other experiments)
-
Two 6-volt batteries
-
Two 1 kΩ resistors
-
One 10 kΩ potentiometer, single-turn,
linear taper (Radio Shack catalog # 271-1715)
Beware that not all transistors share the
same terminal designations, or pinouts, even if they
share the same physical appearance. This will dictate how
you connect the transistors together and to other
components, so be sure to check the manufacturer's
specifications (component datasheet), easily obtained from
the manufacturer's website. Beware that it is possible for
the transistor's package and even the manufacturer's
datasheet to show incorrect terminal identification
diagrams! Double-checking pin identities with your
multimeter's "diode check" function is highly recommended.
For details on how to identify bipolar transistor terminals
using a multimeter, consult chapter 4 of the Semiconductor
volume (volume III) of this book series.
CROSS-REFERENCES
Lessons In Electric Circuits, Volume
3, chapter 4: "Bipolar Junction Transistors"
LEARNING OBJECTIVES
-
Purpose of circuit "ground" when there is
no actual connection to earth ground
-
Using a shunt resistor to measure current
with a voltmeter
-
Measure amplifier voltage gain
-
Measure amplifier current gain
-
Amplifier impedance transformation
SCHEMATIC DIAGRAM
ILLUSTRATION
INSTRUCTIONS
Again, beware that the transistor you select
for this experiment may not have the same terminal
designations shown here, and so the breadboard layout shown
in the illustration may not be correct for you. In my
illustrations, I show all TO-92 package transistors with
terminals labeled "CBE": Collector, Base, and Emitter, from
left to right. This is correct for the model 2N2222
transistor and some others, but not for all; not even
for all NPN-type transistors! As usual, check with the
manufacturer for details on the particular component(s) you
choose for a project. With bipolar junction transistors, it
is easy enough to verify terminal assignments with a
multimeter.
The voltage follower is the safest
and easiest transistor amplifier circuit to build. Its
purpose is to provide approximately the same voltage to a
load as what is input to the amplifier, but at a much
greater current. In other words, it has no voltage gain, but
it does have current gain.
Note that the negative (-) side of the power
supply is shown in the schematic diagram to be connected to
ground, as indicated by the symbol in the lower-left
corner of the diagram. This does not necessarily represent a
connection to the actual earth. What it means is that this
point in the circuit -- and all points electrically common
to it -- constitute the default reference point for all
voltage measurements in the circuit. Since voltage is by
necessity a quantity relative between two points, a "common"
point of reference designated in a circuit gives us the
ability to speak meaningfully of voltage at particular,
single points in that circuit.
For example, if I were to speak of voltage
at the base of the transistor (VB), I
would mean the voltage measured between the transistor's
base terminal and the negative side of the power supply
(ground), with the red probe touching the base terminal and
the black probe touching ground. Normally, it is nonsense to
speak of voltage at a single point, but having an
implicit reference point for voltage measurements makes such
statements meaningful:
Build this circuit, and measure output
voltage versus input voltage for several different
potentiometer settings. Input voltage is the voltage at the
potentiometer's wiper (voltage between the wiper and circuit
ground), while output voltage is the load resistor voltage
(voltage across the load resistor, or emitter voltage:
between emitter and circuit ground). You should see a close
correlation between these two voltages: one is just a little
bit greater than the other (about 0.6 volts or so?), but a
change in the input voltage gives almost equal change in the
output voltage. Because the relationship between input
change and output change is almost 1:1, we say
that the AC voltage gain of this amplifier is nearly 1.
Not very impressive, is it? Now measure
current through the base of the transistor (input current)
versus current through the load resistor (output current).
Before you break the circuit and insert your ammeter to take
these measurements, consider an alternative method: measure
voltage across the base and load resistors, whose
resistance values are known. Using Ohm's Law, current
through each resistor may be easily calculated: divide the
measured voltage by the known resistance (I=E/R). This
calculation is particularly easy with resistors of 1 kΩ
value: there will be 1 milliamp of current for every volt of
drop across them. For best precision, you may measure the
resistance of each resistor rather than assume an exact
value of 1 kΩ, but it really doesn't matter much for the
purposes of this experiment. When resistors are used to take
current measurements by "translating" a current into a
corresponding voltage, they are often referred to as
shunt resistors.
You should expect to find huge differences
between input and output currents for this amplifier
circuit. In fact, it is not uncommon to experience current
gains well in excess of 200 for a small-signal transistor
operating at low current levels. This is the primary purpose
of a voltage follower circuit: to boost the current capacity
of a "weak" signal without altering its voltage.
Another way of thinking of this circuit's
function is in terms of impedance. The input side of
this amplifier accepts a voltage signal without drawing much
current. The output side of this amplifier delivers the same
voltage, but at a current limited only by load resistance
and the current-handling ability of the transistor. Cast in
terms of impedance, we could say that this amplifier has a
high input impedance (voltage dropped with very little
current drawn) and a low output impedance (voltage dropped
with almost unlimited current-sourcing capacity).
COMPUTER SIMULATION
Schematic with SPICE node numbers:
Netlist (make a text file containing the
following text, verbatim):
Voltage follower
v1 1 0
rpot1 1 2 5k
rpot2 2 0 5k
rbase 2 3 1k
rload 4 0 1k
q1 1 3 4 mod1
.model mod1 npn bf=200
.dc v1 12 12 1
.print dc v(2,0) v(4,0) v(2,3)
.end
When this simulation is run through the
SPICE program, it shows an input voltage of 5.937 volts and
an output voltage of 5.095 volts, with an input current of
25.35 �A (2.535E-02 volts dropped across the 1 kΩ Rbase
resistor). Output current is, of course, 5.095 mA, inferred
from the output voltage of 5.095 volts dropped across a load
resistance of exactly 1 kΩ. You may change the
"potentiometer" setting in this circuit by adjusting the
values of Rpot1 and Rpot2, always
keeping their sum at 10 kΩ.
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