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Talking of Transistors
The word transistor doesn’t come from some obscure Latin noun; actually the man who built the first one, Walter Brattain, figured that, just as the vacuum tube had the property of transconductance, this new thingie had the electrical property of transresistance. He also knew that a number of electronic devices had come out recently with names, such as varistor and thermistor. Transistor seemed to fit the bill, which is all well and good, but what exactly is the thing? Simply put, a transistor controls the flow of electric current by opening and closing a kind of valve within it.
You can use transistors as either a switch or an amplifier. In the following sections, we describe both applications.
Using a transistor as a switch
A switch simply opens or closes a path through which current flows. You can use a transistor as an electrically operated switch. You can see the circuit for a transistor used as a switch in Figure 7-9.
You can use a transistor to switch on current.
Take a closer look at what makes up a transistor. A transistor has three leads: Base, emitter, and collector (which we discuss in Chapter 4). When you use a transistor as a switch, the base lead of the transistor works like the toggle on a mechanical switch. term LinG - live, informative, Non-cost and Genuine !
152 Part III: Putting It On Paper
When you’re not applying current to the base (that is, there’s no input current), the transistor is off, which is equivalent to an open switch. Even with a voltage difference between the other two leads of the transistor, no current flows through the transistor.
When you supply current to the base of the transistor, it turns the transistor on, which is equivalent to a closed switch. With the transistor on, a voltage difference between the other two leads of the transistor causes a current to flow through the transistor and out to whatever doohickey you want to turn on.
How does this on-off thing work in practice? Say that you use an electronic gadget to automatically scatter chicken feed at dawn. The gadget is controlled by a photodiode (similar to a solar cell) in your henhouse, which supplies the input to the transistor. At night, the photodiode doesn’t generate any current, and the transistor is off. When the sun rises, the photodiode generates current, and the transistor turns on. When the transistor turns on, current goes to the gizmo that you built to scatter chicken feed so that you can sleep late and the chickens stay happy.
Wait a minute you ask, why not just supply the current from the photodiode to the gizmo? Your gizmo might need a larger current than can be supplied by the photodiode. For example, it might need the current you get from a battery. By using the transistor as a switch you can control the current from the battery with the much smaller current supplied by the photodiode.
In ICs (integrated circuits) that contain logic gates like the ICs used by calculators and computers, transistors wired as switches are an integral part.
When is a transistor an amplifier?
We all need a helping hand from time to time. Why should electronic signals be any different? You often need to amplify signals to get things done. For example, you may have to amplify a signal from a microphone to drive a speaker. Figure 7-10 shows the circuit of a basic one-transistor amplifier.
An amplifier must have a transistor partially turned on. To turn the transistor partially on, you apply a small voltage to the base of the transistor. This procedure is called biasing the transistor. In the example in Figure 7-10, in order to bias the transistor, resistors R1 and R2 are connected to the base of the transistor and configured as a voltage divider (see the section “Exploring a Voltage Divider Circuit,” earlier in this chapter). The output of this voltage divider supplies enough voltage to the base of the transistor to turn the transistor on and allow current to flow through it.
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Boosting things with a basic amplifier.
Filtering out DC offsets maintains the bias.
Chapter 7: Understanding the Basics of Electronics Circuits 153
R2 > R4
R1 > R3
When the amplifier receives an AC input signal, such as from a microphone, the signal must be centered around 0 (zero) volts to maintain the bias. The capacitor at the input filters out any offsets from 0 (zero) volts DC (called a DC offset) in the input signal. You can see this effect in Figure 7-11.
This biased state is the major difference between using a transistor as an amplifier and using a transistor as a switch. When you use a transistor as a switch, you have the transistor either off or on. When using a transistor as an amplifier, you apply a voltage, or bias, to the base to keep the transistor partially turned on. Think of it like keeping a car running at idle.