In this Introduction to the Amplifier tutorial, we have seen that there are different types of amplifier circuit each with its own advantages and disadvantages. In the next tutorial about amplifiers, we will look at the most commonly connected type of transistor amplifier circuit, the common emitter amplifier. Most transistor amplifiers are of the Common Emitter or CE type circuit due to their large gains in voltage, current and power as well as their excellent input/output characteristics.
First and probably the most important thing I learned is all grounds should not go directly to the chassis. This seems contrary to logic. A little side trip into electricity and conductors is appropriate now. Any conductor has some finite resistance and anytime some current is passed through it a voltage will be correspondingly created. Actually the voltage is first, but in this case we want to concentrate on the current. In a typical piece of audio equipment (most everything else as well) there are usually three distinct ground circuits. The first is obvious, the signal ground, the second is the power supply ground and third and often ignored is the case or chassis ground. Each has a particular function and all interact. The signal ground usual and primary function is to provide a return path for the audio. Likewise the power supply ground is the return path for the power used by the circuitry. The case ground I will cover a bit later. Remembering that anytime current flows through a conductor it will cause a voltage, let us see what happens if you mix the signal and power grounds. Each will generate a proportional corresponding voltage. So for an example: the conductor is a piece of wire that has a 1 ohm resistance, the signal voltage (and thus its return) has one millivolt (mV)of amplitude and the power supply is causing a flow of 100 milliamperes (mA) through the wire. The contribution to the voltages on the conductor is then, 1 mV for the signal and 0.1 Ampere times 1 ohm equals 100 mV for the power - see Ohm Law for equations. Since it is really unlikely that the power is perfectly clean DC it will contaminate the signal with hum and noise. Even a 1% noise level in the DC will result in a noise voltage equal to the signal voltage. I grant that this is a gross simplification, but it does illustrate the situation. So my rule number one is to absolutely avoid having any ground conductor handling both signal and power. So how do you do this? I find that a sort of modular arrangement is best. This does not mean that the physical components ca not be on the same board though. What it does mean is that the power supply is wired independently from the active signal portion. Eventually these grounds need to connect, but I will get to that later.
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