Lab 6 - Transistor Switching

The transistor is like a relay in that it can switch a flow of electricity.  The components contains a piece of silicon divided in to three paths known as; collector, base emitter.  The collector receives current from power source.  The base controls the output of the collector.  The emitter sends out the current or signal.  The schematic symbol for a transistor looks like a combination of symbols that we have previously learned.  Three lines/leads connect to a circle and inside the circle looks like a relay switch in the shape of a "T" and a diode coming from the relay switch.  The transistor is always labeled as "Q" not as "T" as one would expect, because the letter "T" is already used for transformer.  How I think of it is the base receives a voltage signal of HIGH or LOW that the transistor mechanically interprets as ON or OFF.  The collector is like a tank of water, when the transistor is in OFF mode or LOW state, that collects the current and voltage before it is sent to the emitter.  The emitter is essentially a diode and amplifies the current called "gain".  Also diodes behave like a one way valve so that current will not travel in the reverse direction.  There are pros and cons for transistors just like the relay switch.  The transistor has a small footprint no moving parts.  Usually, they can only hand small amounts of current and voltage.  Pretty neat stuff in this lab so far :)

We are shown two types of transistors NPN and PNP, which are bipolar semiconductors.  There three layers of silicon that are sandwiched together in a transistor.  The P-type silicon has a positive charge and the N-type silicon has a negative charge.  The middle silicon type is the base, which allows a positive or negative charge to activate the transistor.  In a transistors passive state the NPN and PNP block electricity between the collector and the emitter just like a relay.  They also allow a tiny bit of current called leakage.

Transistor Basics:

• Never apply a power supply directly across a transistor.  You WILL BURN IT OUT with too much current.  Always read the manufacturers specifications.
• Protect a transistor with a resistor in series with either the emitter or collector, in the same way you would protect an LED. 
• Avoid reversing the connection of a transistor between positive and negative voltages.
• Sometimes a NPN transistor is more convenient in a circuit; sometimes a PNP happens to fit more easily.
• They both function as switches and amplifiers, the only difference being that you apply a relatively positive voltage to the base of a NPN; and a relatively negative voltage to a PNP.
• Remember that bipolar transistors only amplify current, and not voltage.  A small fluctuation of current through the base enables a large change in current between the emitter and collector.
• Schematics show transistors with a circle around them, and sometimes do not.
• Transistors com in various different sizes and configurations.  In many of them, there is no way to tell which wires connect to the emitter, collector, or base and some transistors have no parts number on them.  Before you through away the packaging/paperwork/labels that came with the transistor , check to see whether it identifies the terminals
• If you forget which wire is which, some multi-meters have a function that will identify the emitter, collector, and base for you.

Demonstrate the Finger Switch Transistor
Materials:

• 2 VOM's for measuring current
• AC adapter, breadboard, wires/leads, and a DMM
• 1 LED
• Various resistors
• 1 Pushbutton, SPST
• 1 NPN Transistor, 2N2222 or 2N3904

Summary:

When the circuit is hooked up as shown on the diagram, and the button is in the OFF position, the LED lights up, but very dimly.  The LED is receiving a trick of electricity from the collector.  Even though the base is not activated the transistor sends a tiny amount of current through the emitter.  When the button is pushed the solid state switch opens and allows current to flow through the third pin, emitter, and the LED shines more brightly.  Wow!!!

To show that a finger can activate the transistor just as well as a resistor, we yank out R2 and hold two wires to our fingertip.  The LED lights up, but not as bright; unless you have sweaty hands or lick your finger tip.     

The Gain of the Transmitter

Materials:

• R1: 180ᘯ
• R2: 1Kᘯ
• R3: 180ᘯ
• R4: 1Kᘯ
• P1: 100Kᘯ linear potentiometer
• Q1: 2N3904 transistor

Summary:

The circuit diagram shows the potentiometer and transistor in parallel and have the same resistance in both loops.  The VOMs are connected in series between the potentiometer and base, and between the emitter and R3 to ground.  By measuring the current before the base and after the emitter we can determine the amount of gain Q1 is rated for.  First, we adjust the pot so that the input current to the base is 10µA and measure the out put current from the emitter.  We divide the output over the iput current to know our "Gain".  As we approach 100µA into the base the output current levels off at 14mA and the gain drops from about 200 to 175 and so on.