Experiment 3
The Transistor
Objectives
1.
2.
Measure the characteristics of an emitter follower circuit.
Construct and analyze a voltage regulator circuit for a solar panel.
Equipment
1.
3.
4.
5.
6.
7.
8.
9.
10.
1 2N2222 NPN bipolar transistor
Multimeter
Topward power supply
2 - 2.2 k and 1 - 1 k resistors
Breadboard
12 volt - 5-Watt zener diode
1 npn high power transistor (2145 Motorola or 2N3055)
Solar panel
Various high power resistors
Introduction
Transistors are the most important example of an active component. They are 3 terminal
devices, which can amplify signals producing an output signal with more power than the input signal.
They have distinct advantages over their predecessor, the vacuum tube, namely smaller size, no heat
producing filament and a much longer life. In combination with resistors and capacitors they make up
the components in an integrated circuit.
Bipolar Transistor
The bipolar transistor was invented in 1948 at AT & T Laboratories. The two types, the NPN
and PNP, are illustrated in figure 1.
collector
collector
base
base
emitter
emitter
NPN
PNP
Figure 1. The two types of bipolar transistors.
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The three terminals are designated as the collector (of electrons like the plate in a vacuum tube), the
base and the emitter (of electrons like the cathode in a vacuum tube). The rules for an NPN transistor
are as follows (for a PNP you simply reverse the polarities):
1.
2.
3.
4.
The collector must be more positive than the emitter.
The base-emitter behaves like a forward biased diode.
There are maximum values for IC, IB, VCE etc. that cannot be exceeded.
If rules 1-3 are obeyed then Ic = hFEIB = βIB and β ~ 100 typically.
Emitter follower
One simple application for the transistor is as an emitter follower, shown in figure 2. Vout is
equal to Vin minus the 0.6-volt diode drop.
+10 V
IC
IB
Vin
IE
Vout
RL
Figure 2. Emitter follower bipolar transistor circuit.
Thus Vout = Vin - 0.6 volts. As long as Vin doesn't swing below +0.6 volts then Vout ~ Vin. Note also
that Vout can't go above 10 volts since from rule 1, Vcollector > Vemitter.
The usefulness of such a circuit can be seen if one considers the input impedance:
Rin = ∆Vin/∆Iin =∆VB/∆IB
but ∆Vin = ∆Vout = ∆VB and ∆Vout = RL∆IE
thus Rin = RL∆IE/∆IB
Now IE = IC + IB and β = IC/IB (rule #4)
so that IE = (1 + β)IB and ∆IE/∆IB = 1 + β
Therefore Rin = RL(1+β)
and since β = 100, Rin = 101RL.
Thus the load resistance has increased by a factor of 101 by using the transistor circuit.
Questions
1.
For the voltage divider circuit from experiment 1, shown in figure 3, determine Vout with RL =
1k and with RL = 101k. Which value of RL gives Vout closer to 5 volts?
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2.2k
10 V
2.2k
Vout
Figure 3. Voltage divider circuit from experiment 1.
2.
What is the power dissipated in the load resistor for each of the above two cases?
The above questions illustrate the advantage of increasing the load impedance through an emitter
follower circuit. First, the voltage supply is more constant (voltage source loves a high impedance
load). Second, the power drain from the source is much smaller.
Emitter Follower Application Circuit
A simple voltage regulator circuit can be constructed using an emitter follower and a zener diode, as
shown in figure 4. For this specific application, we would like to power a 12 Volt DC water pump
from a solar panel. This type of application is quite common when the location of the water source is
significantly far from power lines. The voltage from the solar panel varies from 15 to 25 volts, but the
pump will be destroyed if its voltage exceeds 14 volts. To keep a constant ~ 11.4 volts across the
pump, the circuit uses a 12-volt zener diode.
R
Pump (RL)
Figure 4. Constant voltage circuit for a solar panel.
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Questions
3.
Why not use a 12-volt zener circuit without the emitter follower? For the circuit in figure 5,
determine the range of power dissipated in the 12-volt zener diode as the input voltage changes
from 15 to 25 volts. Assume a 10 Ω load resistance. Would a 5-W zener diode be able to
handle this?
R = 2 ohms
15 - 25 volts
Figure 5. Zener diode circuit without the emitter follower.
4.
For the emitter follower circuit from figure 4, with R = 100 ohms, determine the range of
power dissipated in the 12-volt zener diode as the input voltage changes from 15 to 25 volts. Again,
assume a 10 Ω load resistance. Would a 5-W zener diode be able to handle this?
Procedure
Construct the voltage divider circuit from figure 3. Attach a 1-k resistor across the output.
Draw the circuit. Measure and record Vout. What is the percent difference between Vout with and
without the 1k load resistor?
Construct the emitter follower circuit from the introduction section. Use the left channel of
Topward supply adjusted to 10 volts as the collector voltage. Use the 1-k resistor as the load resistor.
Attach the output of the voltage divider to the base of the emitter follower circuit. Use the pin out
diagram in figure 6 to determine the connections for the transistor. Draw the combined circuit.
Measure and record Vout of the voltage divider (which is Vin for the emitter follower). Now remove
the connection from the ouput of the voltage divider to the input of the emitter follower and measure
Vout of the voltage divider again. What is the percent difference between Vout (of the voltage divider)
with and without the emitter follower attached to its output? What is the input impedance of the
emitter follower? Determine β for the transistor.
E B C
Figure 6. Bottom view of the 2N2222 NPN transistor.
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For the emitter follower application circuit from figure 4, there will be two groups that will
determine the properties of the solar panel and the zener diode. The field group will measure the
voltage output of the solar panel in terms of the load resistance. The indoor group shall measure the IV curve for the zener diode. Using these two sets of data, the emitter follower circuit sould be
optimized to deliver between 11 and 13 volts across a 10 Ω load, for an input voltage that varies from
15 to 25 volts. Construct your circuit using the high power transistor and a high power 10 Ω resistor
for the load (does the other resistor in the circuit need to be high power?). Draw the circuit with your
component values labeled. Measure the voltage across the load as you adjust the input voltage from 15
to 25 volts. Graph your data and comment on whether your circuit exhibited the desired results.
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