How to design A class amplifier
Posted on Mar 14, 2013 5109
Under: Audio Amplifier Circuits
Feedback - This is where part of the output signal is fedback to the input BUT 180o out-of-phase (i.e. partially cancels the input). If it were in-phase feedback then we would have an oscillator - which in this case we definitely do not want. Fidelity - This means many things to many people but to us it means the output must be an exact replica of the input but only magnified or amplified. Efficiency - The theoretical limit to this amplifier's efficiency is 50%, meaning for every watt of output we will use up at least 2 watts of d.c. power input in to the amplifier. Depending upon the application this may or may not be significant. If we have a large power transformer available to us and power, literally to burn, who cares?.
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On the other hand if miniaturisation were the keyword and precious battery supply is the only power source available thennnnnn! that's another matter. For this I have selected a very popular, cheap and extremely versatile transistor and reproduced it's pertinent specifications below. This is the 2N2222A. There are many other similar types available. 1. Id - It is a silicon NPN general purpose type. 2. Pd - 0.5W Meaning it is capable of dissipating 500mW. 3. Vce - 40V Meaning do not use it above 20V (preferably less) d.c. supply. 4. Ic - 0.8A Meaning the maximum collector current allowed is 800 mA. 5. Hfe - 100 @ Ic of 150 mA This means it has an amplification factor of 100. 6. Ft - 300 Mhz this means that by the time the frequency reaches 300 Mhz the amplification factor has dropped off to 1. 7. Case - TO-18 type package (not always). Rule 1: Do not always believe all of those specs. Most specs are the conservative limits but important items such as the gain Hfe, varies widely. On a bunch of these transistors I have measured gain figures ranging from 60 to 170. Always use your transistor well under the published specifications. O.K. we have our little 2N2222A sitting on the bench in front of us. What good is that? Alright let's look at it (or even a similar type). It has three legs or leads coming out of it. Look at figure 1. One is called BASE (B), another is called EMITTER (E) and the other is...