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SIGNAL WITH DC OFFSET
Thereĺs an advantage in leaving a transistor biased on because it responds to any change in the input signal. A transistor requires about 0.6 volts applied to the base (from the base to the emitter) to turn on. If you donĺt have the transistor turned on, any input signal below 0.6 volts doesnĺt produce an output signal. With the transistor biased on, it amplifies the entire input signal. Figure 7-12 shows the effect of biasing a transistor on the output signal. Note that in the output signal without bias, only a portion is amplified; the rest is lost. In the output signal with bias, the entire signal is amplified.
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154 Part III: Putting It On Paper
The middle output signal has been amplified all the way.
OUTPUT SIGNAL WITH BIAS
OUTPUT SI GNAL WI THOUT B AS
The other two resistors that you can see in the circuit in Figure 7-10, R4 between the emitter and ground and R3 between the collector and +V, control the gain. The gain is simply how much the signal is amplified. For example, with a gain of 10, a 1-volt input signal becomes a 10-volt output signal.
What else can you do with transistors?
The circuit that we discuss in this section is a common emitter circuit. You can also do the following things with circuits that include transistors:
Wire them in common base circuits, which you use in radio frequency applications or voltage regulators.
Use PNP transistors rather than the more commonly used NPN transistors.
Wire them with more than one transistor, producing multiple stages of amplification.
Let's have more transistor amplifiers
The section titled "Talking of Transistors" gives you a taste of transistor amplifiers, but where's the rest of the meal, you ask? In this chapter, we explain the basics of electronics circuits; we don't really have room to give transistor amplifiers a thorough going-over. If you now have a basic understanding of how transistor amplifiers work and can look at the schematic for a
project and understand how the transistors are being used, we're happy.
For you budding electronics Einsteins who just have to know more, try getting your hands on a good electronics design book, such as The Art of Electronics by Thomas C. Hayes and Paul Horowitz (Cambridge University Press). It's not cheap, but it's a classic.
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Chapter 7: Understanding the Basics of Electronics Circuits
An Operational Amplifier
If one transistor is good, more transistors are better, right? An operational amplifier is an IC containing several transistors, as well as other components. An operational amplifier, usually called simply an op amp, performs much better than an amplifier made from a single transistor. For example, an op amp can provide uniform amplification over a much wider range of frequencies than can a single-transistor amplifier.
Check out a basic circuit that uses an op amp in Figure 7-13.
Providing better amplification with an op amp circuit.
Just put a signal (for example, from a microphone) to the input; the signal, amplified several times, then appears at the output, where it can drive a component, such as a speaker. The values of the resistors adjust the gain of the amplifier (remember, gain simply means how much the signal is amplified). You calculate the gain by dividing R2 by R1:
Gain = R2
If R2 is 10 times R1, the gain is 10. This gain results in a 1-volt input signal producing a 10-volt output signal.
An op amp requires both negative and positive supply voltages. A positive supply voltage in the range of 8 to 12 volts and a negative supply voltage in the range of -8 to -12 works.
The circuit in Figure 7-13 uses the op amp in an inverting mode, which means that the input signal is flipped to produce the output signal. You generally should use the inverting mode because of signal noise problems that you can encounter with the non-inverting mode.
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156 Part III: Putting It On Paper
Simplifying a Project with an Integrated Circuit
Whoever said that less is more must have been a fan of integrated circuits (ICs). Using an IC in a project allows you to substitute one component for several because many components are built into an IC. In this section, you discover how to connect an IC into a circuit by connecting inputs, outputs, ground, power, and some resistors and capacitors to the correct pins of the IC, as you can see in Figure 7-14.
A 555 timer IC wired into a circuit makes more of less.
+V ╬ V
Which pin number you use to connect different parts of the circuit depends on the design of the IC. You can identify these pins for each IC on the manufacturerĺs data sheet or the schematic for a particular project.
In Figure 7-14:
+V connects to pin 8, which is power, and pin 4, which is reset.