Transistor Basics I
PHYS 309
Name:
A. Introduction
Transistors are the heart of all of our modern electronics. Think of them as “current valves,” giving you
control over the amount of current flowing through them. This “valve” behavior happens by combining ptype and n-type semiconductors in groups of three. Recall that diodes combine a single p-type with a
single n-type semiconductor, and that there is that ≈ 0.6𝑉𝑉 quantum mechanical “cost” they impose.
Transistors that sandwich two p-type semiconductor layers around one layer of n-type are called “pnp
transistors.” Conversely, two n-type semiconductors sandwiched around one p-type yields an “npn
transistor.” These two are given schematically below next to their circuit symbols. Note the direction of
the arrow—like the diodes, this shows the direction of the conventional current flow.
p
n
n
p
p
n
There are three wires that go to these transistors; there are others with 4 wires but we’re not dealing
with those for now. The base, collector and emitter all have specific functions related to the transistor’s
ability to regulate the current that flow through it from the collector to the emitter. Note how there is an
arrow in the emitter-base junction that points in the direction of pn so you can immediately tell the
type of the transistor.
A typical transistor circuit is shown to the right. Recall the “junction
potential” between two types of semiconductors that must be
overcome in order for the current to start flowing, 𝜙𝜙 ≈ 0.4𝑣𝑣 − 0.8𝑉𝑉
depending on the materials used in each semiconductor.
With this circuit, if the voltage difference between the emitter and
the base, 𝑉𝑉𝑏𝑏𝑏𝑏 is high enough then the two semiconductors involved in
the base-emitter junction will conduct. Conversely, if the voltage
difference between the collector and the base, 𝑉𝑉𝑐𝑐𝑐𝑐 is high enough
then the two semiconductors involved in the collector-base junction
will conduct. This is the “forward bias” situation.
A simpler way to think about this is “If the base voltage is high enough, the transistor turns on and lets the
current flow.” In this lab you will investigate what “high enough” means.
Think of the base as the valve that turns the transistor on/off (more/less), depending on how strong the
forward biasing is with the junctions. In this lab you will investigate one of the two most common
transistor configurations and how it works in relation to this biasing.
Transistors I-1
Transistor Basics I
PHYS 309
Name:
B. Basic transistor circuit 1—emitter follower
Build this circuit. The 10𝑘𝑘 fixed and 100𝑘𝑘 variable resistors are a “voltage
divider” that will “bias” the base voltage. Use color-coded wire for your
hookups to make trouble-shooting—and visualization of how this circuit
works—easier. Use red for +12𝑉𝑉, green for signal, and black for 0𝑉𝑉
(ground). Have this checked before turning on any power! Do the following:
1. Record 𝑉𝑉𝑖𝑖𝑖𝑖 and 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜 for many dc inputs starting with 0𝑉𝑉.
a. Be sure the first few voltages are 0𝑉𝑉, 0.1𝑉𝑉, 0.2𝑉𝑉, etc.
b. Take the input voltage as close to 12𝑉𝑉 as you can.
2. Now set the voltage at the base of this emitter follower circuit at 5𝑉𝑉 and
send in a 1𝑘𝑘𝑘𝑘𝑘𝑘 ac signal with amplitude of 2.0𝑉𝑉 (use a capacitor to feed
this into the voltage divider 5𝑉𝑉).
a. Record what happens with the output voltage, both the dc and the ac part of it.
Write-up (due 2.0 weeks from today):
• Draw the two types of transistors on the first page of this lab (use 5Spice Analysis).
o State how you know the direction of the conventional current flow in each.
• Draw the two transistor circuits (one above, plus one with capacitor added) for your write-up.
o Write down the actual values for your resistors and capacitor so your diagrams are accurate.
• For the first circuit using the various 𝑑𝑑𝑑𝑑 inputs:
o Record your numbers in a spreadsheet for the emitter follower circuit and graph them.
 Be sure you have enough numbers so your graph will have smooth lines. However, be
sure to have your individual points shown in addition to the overall line.
 Include your table of numbers in your write-up (multiple columns—save space).
o Graph the following:
 𝑉𝑉𝑜𝑜𝑜𝑜𝑜𝑜 and 𝑉𝑉𝑖𝑖𝑖𝑖 will be on the vertical axis, 𝑉𝑉𝑖𝑖𝑖𝑖 will be on the horizontal axis. Note that
there will be two lines, with one a simple 𝑦𝑦 = 𝑥𝑥 straight line.
 You must label your axes and the units for each.
o Answer the following questions in your write-up:
 What happened with the output for the first several tenths of a volt for the input?
 The voltage divider is a necessary part of a transistor circuit. Why is this dc “bias”
voltage at the base of the transistor necessary for proper operation?
 How does the current going through the left-hand voltage divider circuit relate to the
current running through the output resistor. Hint: Calculate these two currents and
include those two values in your write-up.
 Where does the semiconductor “junction potential” appear in this circuit?
o Of what use might this circuit be? Hint: You can amplify either current (i.e. power) or voltage
with electronics. Which does this circuit do (explain how you can tell)?
• For the second circuit with the 5𝑉𝑉 dc plus the ±2𝑉𝑉 ac input, explain
o What happened to the power of the 𝑎𝑎𝑎𝑎 signal? Explain how you can tell (think “current”).
o Explain why I had you to set the base “bias voltage” at 5𝑉𝑉 initially. Why would e.g. 1.5𝑉𝑉 not be
helpful?
o Explain why I had you to use the capacitor to feed the 𝑎𝑎𝑎𝑎 signal into the voltage divider in this
circuit. Hint: Recall what types of signals capacitors pass, and what they don’t.
Transistors I-2