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Understanding Electronic Components |
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8. Microphones, speakers and headphones
Microphones, speakers and headphones are components commonly used as the electro-acoustic transformers. Theory of operation of microphones is that they convert sound waves into electric signals, which are, at some later point, transformed back into sound through speakers or headphones. Schematic symbols for these components are on 8.1. 8.1 Microphones There are several different types of microphones in existence today: carbon, dynamic, crystal, capacitative (electret microphones), etc. Dynamic microphones are in most widespreaded usage, it is nearly impossible to even consider not having them when recording music or speech, and other in some other occasions when high fidelity is demanded. Basically dynamic microphone is exactly the same as dynamic speaker, with only difference being it's size. (In reality, when sound quality is not the first priority, like small portable transceivers (walkie-talkies), interphones and such, it isn't rare that instead of a microphone you find a miniature dynamic speaker.) Internal structure and the cross-section of a dynamic microphone are represented on 8.2. Paper cylinder, to Because of their low resistance, dynamic microphones usually need separate transformer so they could be connected to some kind of amplifier. This transformer is usually built-in the microphone's case. If this transformer is absent from the circuit, it is necessary to connect a preamplifier with low input resistance instead. Carbon microphones were mainly used in telephony applications, but they are being pushed back from there by some, more advanced, modern microphones. In past couple of years, electret microphones gained in the number of applications in which they are being used, mostly in portable and mass production devices. Electret microphone is a modern variant of an old capacitative microphone. Some of it's main advantages are it's small form factor, resistance to physical impacts, quality and it's low price. Shape, size and other characteristics of an electret microphone are displayed on 8.3. Pay attention to it's miniature size, equal to a thicker shirt button, and it's wide frequency range. Metal case which houses this microphone has a single FET amplifier, which means that this microphone needs DC voltage for supply. Bottom side has two pins marked with 1 and 2. According to 8.3d, pin 2 is connected to the common ground of the device (amplifier, etc.). Pin 1 is so called a “live pin”. FET's resistor is connected between it and the positive battery end. FET is used to amplify the low-frequency signal from the microphone. This, amplified signal, is further connected over the capacitor CS. 8.2 Speakers Speakers, as well as microphones and headphones are various in their shape and function. There are dynamic, crystal, capacitative, but, as with microphones, most frequently used ones are the dynamic speakers. Two most important characteristics of a speaker are it's resistance and it's power. Resistances that are most commonly found in appliances are 4, 8 and 16 Ohm, but there are also ones with 3,25,40 and 80 Ohm. Speaker powers range from only a fraction of Watt to several tens of Watts, even hundreds of Watts. Both this characteristics should be taken into consideration while using or repairing audio equipment. If schematic states that 8 Ohm speaker should be used, that is the resistance that should be used, because when using for example lower value, there is a possibility that the amplifier or the speaker may become permanently damaged (which one of the two would be rendered useless depends on which of the two has lower power). Speaker and amplifier would remain functioning if this happened to be a higher resistance speaker than supposed, but the output power would be considerably lower. Situation is a bit different when considering speaker power. If maximum power is, for example, 1W, then speaker's power shouldn't be lower than that (if this is for some reason an unavoidable situation, level regulator should not be turned to the max, because output power then exceeds 1W, which could prove devastating for the speaker). Any speaker above 1W will work with our example. This makes distortions lower and sound reproduction gains in quality. It is advisable to choose larger speakers and avoid miniature ones whenever possible. This is so because larger speakers tend to be more efficient (they are significantly louder) and sound reproduction is of much higher quality, especially within the lower frequencies range. Speaker should be housed in a large, massive case because the speaker box functions as a resonance box, and this greatly adds to the overall quality of the sound reproduction. As you can see, when dealing with speakers, size does make a difference. For smaller appliances (alarm clocks and other alarm and indication circuits) it is good enough to use miniature speakers in plastic cases of the same shape as the housing of the electret microphone on the 8.3a. Similar speaker is used in the alarm device on 7.8 “Buzzers” are miniature ceramic speakers, which could also be used in these devices. They have very high resistance which makes them easily compatible with digital ICs, since they could be connected directly to them. For example, on 7.8 buzzer would have been connected between pin 11 and ground. 8.3 Headphones
Same goes for headphones as with microphones and speakers. There are several types: dynamic, crystal and electromagnetic, but dynamic ones are most commonly found nowadays. They are functioning on the same principles, with same internal parts as dynamic speakers, with obvious difference in construction, since they are intended for much lower powers than speakers. Their main characteristic is their resistance, which comes in range from several Ohms (dynamic), to several hundreds of kiloohms (crystal headphones). Among amateurs, electromagnetic speakers are a bit more popular, since they come in range from several hundreds of ohms to several kiloohms, which enables them to be directly plugged into various simple transistor amplifiers. 8.4 Examples
In this equation CCB is representing a capacitance of the collector circuit, or simpler, capacitance between the collector and the base. Value of this capacitance depends on the voltage over the base: higher the voltage, lower the capacitance and vice versa - lower voltage, higher capacitance. Voltage over base is constant while there is no sound, which means that the frequency of the oscillator is constant as well. When sound appears, point 1 receives amplified LF voltage from the microphone. It is, over C1, forwarded to the base of the transistor, and because of that overall voltage over base changes (it actually oscillates around the middle value determined by R2 and R3). With same frequency variates the capacitance of the CCB, and thus the overall frequency of whole oscillator variates as well. In this way is facilitated the frequency modulation (FM).
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