Transistor Basics II
PHYS 309 Name:
A.
Introduction
The two basic types of transistors—“pnp” and “npn”—are given schematically below next to their circuit symbols. p n n p p n
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. The arrow in the emitter-base junction 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. This circuit differs from the previous lab’s circuit through the addition of the collector resistor 𝑅𝑅 𝑐𝑐
. The emitter resistor 𝑅𝑅 𝑒𝑒
remains as before.
Recall the “junction potential” between two types of semiconductors that must be overcome for the current to flow:
Φ ≅ 0.4 − 0.8𝑉𝑉 depending on the materials used in each semiconductor.
If the potential difference between the emitter and the base, 𝑉𝑉 𝑏𝑏𝑒𝑒 overcomes this junction potential then the two semiconductors involved in the base-emitter junction will conduct. Conversely, if the potential difference between the collector and the base, 𝑉𝑉 𝑐𝑐𝑏𝑏 overcomes this junction potential then the two semiconductors involved in the collector-base junction will conduct. This is
“forward bias.”
Think of the base as the valve that turns the transistor off and on, depending on how strong the forward biasing is with the junctions. In this lab you will investigate the second of the two most common transistor configurations and how it works in relation to this biasing. You will also discover how the two resistors 𝑅𝑅 𝑐𝑐
and 𝑅𝑅 𝑒𝑒
serve to amplify signals.
Transistor Basics II-1

Transistor Basics II
PHYS 309 Name:
B.
Basic transistor circuit 2—common collector
Set up the transistor circuit at the right. Note that there are two differences between this circuit and the one from last week: The collector resistor in the circuit 𝑅𝑅 𝑐𝑐 output voltage.
, and where you measure the
For this circuit you will have an 𝑎𝑎𝑎𝑎 input with 𝑓𝑓 ≅ 1𝑘𝑘𝑘𝑘𝑘𝑘 and an amplitude of 𝑉𝑉 ≅ ±400𝑚𝑚𝑉𝑉 . Use a capacitor to ensure that you only feed an ac signal into your properly biased base. Note that you will need to do some adjusting of the bias to make this circuit work optimally. And by “optimally” I mean so that you can see the entire output signal with no “clipping.”
One-week check : Use the following approximate values for 𝑅𝑅 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑒𝑒𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
:
500Ω, 1𝑘𝑘, 2𝑘𝑘, 5𝑘𝑘, 10𝑘𝑘, 15𝑘𝑘, 20𝑘𝑘, 30𝑘𝑘, 50𝑘𝑘
Observe and record two things about the input and output voltages for each of the 𝑅𝑅 𝑐𝑐 above:
• The phase of 𝑉𝑉
values listed
, 𝑅𝑅 𝑒𝑒𝑒𝑒𝑖𝑖𝑐𝑐𝑐𝑐𝑒𝑒𝑐𝑐 𝑐𝑐𝑜𝑜𝑐𝑐
as compared to 𝑉𝑉 𝑖𝑖𝑖𝑖
. Are they the same, 90° or 180° different? Or what?
• For simplicity, make a data table with the following headings: 𝑅𝑅
, 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑒𝑒𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
, 𝑉𝑉 𝑐𝑐𝑜𝑜𝑐𝑐
, 𝑉𝑉 𝑖𝑖𝑖𝑖
, 𝑟𝑟𝑟𝑟𝑟𝑟𝑎𝑎𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 𝑝𝑝ℎ𝑎𝑎𝑎𝑎𝑟𝑟 , 𝑅𝑅 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
/𝑅𝑅 𝑒𝑒𝑒𝑒
, 𝑉𝑉 𝑐𝑐𝑜𝑜𝑐𝑐
Writeup: Due 2.0 weeks from today
/𝑉𝑉 𝑖𝑖𝑖𝑖
• Put the transistor circuit in this lab into an MSWord document.
• For this circuit there is a relationship between the input and output voltages, and the resistors on the right-hand side of this circuit. o
Include your data table o
Explain what the capacitor 𝐶𝐶 o
Address how the ratio 𝑉𝑉 𝑐𝑐𝑜𝑜𝑐𝑐
1
/𝑉𝑉
was doing in this circuit. 𝑖𝑖𝑖𝑖
related to the resistors 𝑅𝑅 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
and 𝑅𝑅 𝑒𝑒𝑒𝑒
. o
Explain how you had to adjust the bias to make this circuit work optimally.
 Explain what is meant by “clipping” of your output signal.
 Explain why clipping became inevitable (i.e. always present) for one or more values for 𝑅𝑅 𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑒𝑒𝑐𝑐𝑐𝑐𝑐𝑐𝑐𝑐
. o
What is going on with the phase of the output vs. the input voltages?
 Explain conceptually how this common collector circuit makes this happen. o
Power calcualtions:
 Calculate the power drawn from the power supply 𝑉𝑉 on the input side of the circuit
 Calculate the power drawn from the power supply 𝑉𝑉 on the output side of the circuit for each of your collector resistors. Show your work only for the 500Ω case. o
What every day uses might you see for this transistor circuit? Again, address this clearly by giving an example.
Transistor Basics II-2