AC versus DC question

[Bunkysdad]

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

 

[crutschow]

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?

 

[Bunkysdad]

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?

Can you clarify a microphone’s output (actually the input into a small audio amplifier) that is driven by one’s voice vibrations...

The question is whether this output is defined by various levels (potentials) of positive voltage, meaning sinusoidal varying levels (fluctuating) positive, or of positive then falling to negative (relative to positive), thus would these fluctuations be considered both positive and negative values??...

It somehow seems to me that dc voltage could also have varying voltage levels, but then I’m not clear if this would be called dc or ac?

Thank You

 

[KlausST]

Hi,

What you talk about is AC combined with DC.
This is true for an electret microphone. (I'm no microphone specialist) But an electret microphone is just a (variabke) capacitor, it can't produce AC voltage by itself. It needs a DC bias voltage...to be able to generate AC voltage.

A dynamic microphone can produce AC. No DC bias needed. All the electrical power comes from the acoustical power.

******
Consider the signal of an electret microphone like the waves on the sea.
It needs some depth of water (DC) for the wave (AC) to travel on it.
(It's not exactly like this, but maybe thus picture helps...)

Klaus

 

[c_mitra]

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...

 

[Bunkysdad]

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...

Got it, but for a follow-up question, I understand that ac has a positive and negative componant (polarity -??)...Is it possible for dc to have a ‘negative’ voltage??.....Or maybe this is really a physics question??

Thank You

Bunkysdad

 

[c_mitra]

I understand that ac has a positive and negative componant (polarity -??)...Is it possible for dc to have a ‘negative’ voltage??

Of course yes. AC voltage, if averaged over a cycle, gives ZERO. Sine waveforms are popular because they form a basis function (mathematically speaking). sin(x), cos(x) and sin(x).cos(x) when integrated over a full cycle gives ZERO. Therefore sin(x) and cos(x) are orthogonal functions.

You can have either zero, positive and negative offset (DC voltage) added to the sine wave- the sum over a full cycle will now give you the DC offset voltage.

In electronics, we are concerned with potential differences- hence the need for a ground potential (a reference point)- we can have a ground point in an aircraft or spaceship too- that is a mere convention.

In a power supply, for example, you can have two grounds- one for the input and one for the output. These two grounds can be completely independent. But that is beside the original question.

Hence we have AC voltages, DC voltages and any arbitrary combination of two in real life.

 

[BradtheRad]

When hooking up jumper cables we might say 'red goes to positive, black goes to negative'. Or in a different circumstance we say 'black goes to ground'. It's up to us to be aware of the context of our circuit. There was a time automobiles had positive chassis ground. I suppose a mechanic would read positive volts on his meter, then invert them to a negative value. Yet our nomenclature does not make a real difference as to the direction of electron flow.

A bipolar supply (or any bipolar circuit action) means we need to be aware of positive and negative, and 0V ground. Where to use N-devices or P-devices, what color code we should have for supply wires. Etc.

 

[Ratch]

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.

Ratch

 

[andre_luis]

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.

 

[Ratch]

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

 

[Bunkysdad]

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

This is all great info for me to ponder, but brings to mind a bit simpler question that I canÂ’t resolve - Like the sound wave into a microphone...Hopefully I can try to coalesce my nagging question properly...

So the sound wave into a voice coil type microphone is sinusoidal, but should this be considered an alternating wave which actually changes (periodically reverses) its direction (say, + -) along with itÂ’s amplitude & modulation, or is this a wave that varies itÂ’s amplitude & modulation only??.....

IF the latter is true (no periodic + to - reversal), is this STILL considered to be an alternating wave current?....

And, IF the latter is true, then once this wave enters the input section of a pre-amp (say, if we were to plot it as volts vs time) does it’s volts potential ‘range’ vary say from zero volts positive (with respect to ground circuit - but considering the ac function only) rising to some even more positive sinusoidal voltage, or does it ever plot say to the negative dc side??

I hope this makes sense!

Bunkysdad

 

[Bunkysdad]

Thanks for the info - more for me to ponder over!!....But now a follow-up question about soundwaves entering a voice coil microphone...

So a soundwave is sinusoidal as it enters a microphone, but does this mean that it is sinusoidal in it’s amplitude and modulation OR does its direction periodically reverse too as what is commonly referred to as alternating current?....(i.e. when does this wave become electrically alternating or ac?)...

Then the soundwave enters a voice coil microphone to the input side of a basic pre-amp...My best guess (and from what I’ve absorbed...) is that it is at this point where this sinusoidal soundwave becomes an alternating electrical signal - but at this juncture, does this signal periodically reverse itself in the fashion of household ac (say, actual + and -), OR, if plotted (say, a volts vs time plot) does this varying signal voltage range from a point of zero then alternating to a slightly higher voltage and back to zero again - but (considering ac function only) not actually going negatively below zero volts??

Sorry for my dogged confusion!!

Bunkysdad

 

[c_mitra]

So a soundwave is sinusoidal as it enters a microphone...

Sound waves, common spoken sounds, are composed of many different sine waves- it is not correct to call that a sinusoidal waveform.

A dynamic microphone will produce a voltage that will have both positive and negative sides. When the coil is pushed inside the magnetic field, you will get a positive voltage and a negative voltage when the coil moves away from the magnet.

The voltage is zero when the coil does not move. Sound waves are pressure waves- they are longitudinal waves (in air) and consist of compression and rarefaction phases. If you ignore the phase, one produces positive and the other produces negative voltage.

Usually the output from the microphone is AC coupled (connected via a capacitor) and that removes the DC part completely. The preamplifier adds some bias so that the negative part is shifted up (all the voltages are now positive)- this is just adding a DC voltage.

Usually we do this so that the amplifier operates in the linear region so that distortion will be minimum.

 

[Bunkysdad]

Awesome Sir, thank you very much for your very concise description!!...This is a problem that up to now I have been unable to resolve in my own mind - again, my sincere appreciaton to all !!

-----------------

IÂ’m still digesting this - Yet another follow-up:
“ The preamplifier adds some bias so that the negative part is shifted up (all the voltages are now positive)- this is just adding a DC voltage.”

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??

Bunkysdad

 

[c_mitra]

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??...

Right. You can figure out the linear region of the transistor from the curves given in the datasheet- if the base voltage is close to zero, there will be considerable distortion.