DC is a steady voltage, no appreciable fluctuations.
AC is a varying voltage.
The AC may be riding on (or offset) by a DC voltage but it's still AC, even if it doesn't actually reverse its polarity with respect to ground.
Thus an AC signal can go through an amplifier and, at various points in the amplifier, may experience different DC bias levels, but it's still AC.
At the amplifier output the signal typically is coupled through a DC block capacitor (high-pass filter) so that only the AC signal (going positive and negative around the 0V ground level) remains.
That help clarify things?
understanding the difference or the ‘actual distinction’ between fluctuating dc current and ac current...
AC is any periodic component; DC is any constant component. Most signals (includes power supply voltages) has a DC component plus an AC component.
Non-periodic signals (say an audio signal) can be decomposed into several AC components. There are several limitations in this discussions but we need not be concerned ...
Basically that is the story...
I understand that ac has a positive and negative componant (polarity -??)...Is it possible for dc to have a ‘negative’ voltage??
I am having some difficulty understanding the difference or the ‘actual distinction’ between fluctuating dc current and ac current, say with a microphone input into a small audio amplifier...Both are sinusoidal, but ac appears to regularly reverse it’s direction or polarity (+ to -) while dc appears to merely vary its voltage potential from a position of more positive to one of less positive...In many sources I’ve searched I find that the term ‘alternating current’ is routinely interchanged for both an ac source and a dc source, but without any added apparent clarity...
Thank You
Bunkysdad
For either voltage or current, I look upon AC as meaning "alternating cycle" and DC as "defined constant". You can mix and match the two waves, but usually the result is then considered an arbitrary waveform
Although for practical purposes both physical AC and DC (voltage and current) units can be seen as behaving in different ways, they could mathematically be modeled in the same way, such as the electrical grid voltage as a cosine of frequency 50/60Hz and peak voltage of Sqrt(2)*220V, whereas the voltage of an automotive battery as a cosine of 0Hz and peak voltage of 12V. This would help a beginner understand why a series capacitor (whose impedance is inversely proportional to the dominant frequency) is used for decoupling different DC bias and to couple AC signals.
Be careful. A lot of engineers and mathematicians do not like to think of steady voltage/current as zero frequency sinusoidal. Impedance has no meaning at steady voltage/current. You cannot discern a component's value at steady zero frequency voltage/current. They are really two different waveforms. For w=0, sinusoidal Cos(wt) equals a steady one for all t, but Sin(wt) equals a steady zero for all t. Yet, Sin and Cos are different only in phase and give the same impedance value for w > 0. Capacitors do not block DC like a switch does, they develop a reverse voltage that neutralizes the DC component of the signal.
Ratch
So a soundwave is sinusoidal as it enters a microphone...
So do you mean that this added bias occurs via say the voltage (or potential) divider, prior to the signal input to the transistor base??...
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