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I may fudge a little technically, but this is an intuitive description.
In a push pull amp, the upper and lower outputs take turns conducting. Ideally in a sine wave, we would have the upper start immediately as the waveform rises from zero. As it rises and falls back to zero, we would want the upper side to shut off right at zero and the lower start conducting right at zero as the waveform continues to the negative. And so on.
because a transistor base has to be at a slight potential higher than the emitter to start conducting, let us say half a volt, then in reality as we apply the signal to the base, as it starts to rise from zero, it isn't until it gets to that half volt level that the transistor actually starts conducting. That means that the first tiny bit of the waveform is not there on the output. SO looking at the output we see what looks like a sine wave, but at each zero crossing we see a brief little moment where the waveform is stuck at zero. This is called crossover distortion, for distortion caused by crossing zero unsmoothly.
By applying a little voltage to the base to start with - the bias - we can have the base of the transistor already right at the brink of conduction so when the signal starts up, so can the transistor. If it needs to sit at half a volt, if we bias it at a quarter of a volt, it will eliminate part of that crossover "notch" in the waveform, but not all. The higher we raise the base of that transistor, the less crossover notch we have, until the bias reaches that half a volt level. The setting that still have crossover distortion like that are called cold settings.
Now what if we turn that bias up more than the half a volt. Now the transistor is not only ready to conduct, it actually now IS conducting. And the more we apply, the more it conducts. The more it conducts, the more current flows, and that current flowing through the emitter, and so the emitter resistor will be measured as a voltage drop across the resistor.
Both the upper and lower sides are biased. In yours separately, but in most, there is one circuit biasing both. If they are biased cold, underbiased, then we have the crossover notch. As we bias warmer than our half a volt each, then we have the steady flow of current. When both sides are conducting, we have current flowing between positive and negative power supplies. The more current flows, the warmer the transistors get.
We bias these things so there are a few milliamps flowing just to eliminate the crossover distortion, but we do not want to bias them so so much current flows as to make them run hot.
When you start from cold, you increase the bias until the crossover notch disappears, From that point on, the hotter you bias the thing, the more extra current flows from positive to negative supply, making heat. So what do you gain with a higher bias current? More heat.
In a push pull amp, the upper and lower outputs take turns conducting. Ideally in a sine wave, we would have the upper start immediately as the waveform rises from zero. As it rises and falls back to zero, we would want the upper side to shut off right at zero and the lower start conducting right at zero as the waveform continues to the negative. And so on.
because a transistor base has to be at a slight potential higher than the emitter to start conducting, let us say half a volt, then in reality as we apply the signal to the base, as it starts to rise from zero, it isn't until it gets to that half volt level that the transistor actually starts conducting. That means that the first tiny bit of the waveform is not there on the output. SO looking at the output we see what looks like a sine wave, but at each zero crossing we see a brief little moment where the waveform is stuck at zero. This is called crossover distortion, for distortion caused by crossing zero unsmoothly.
By applying a little voltage to the base to start with - the bias - we can have the base of the transistor already right at the brink of conduction so when the signal starts up, so can the transistor. If it needs to sit at half a volt, if we bias it at a quarter of a volt, it will eliminate part of that crossover "notch" in the waveform, but not all. The higher we raise the base of that transistor, the less crossover notch we have, until the bias reaches that half a volt level. The setting that still have crossover distortion like that are called cold settings.
Now what if we turn that bias up more than the half a volt. Now the transistor is not only ready to conduct, it actually now IS conducting. And the more we apply, the more it conducts. The more it conducts, the more current flows, and that current flowing through the emitter, and so the emitter resistor will be measured as a voltage drop across the resistor.
Both the upper and lower sides are biased. In yours separately, but in most, there is one circuit biasing both. If they are biased cold, underbiased, then we have the crossover notch. As we bias warmer than our half a volt each, then we have the steady flow of current. When both sides are conducting, we have current flowing between positive and negative power supplies. The more current flows, the warmer the transistors get.
We bias these things so there are a few milliamps flowing just to eliminate the crossover distortion, but we do not want to bias them so so much current flows as to make them run hot.
When you start from cold, you increase the bias until the crossover notch disappears, From that point on, the hotter you bias the thing, the more extra current flows from positive to negative supply, making heat. So what do you gain with a higher bias current? More heat.
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