AC audio signal superimposed on to DC question

[mckenzie]

Sorry, this is very probably a very stupid question & I am sure I'm missing some fundamental concept but here goes.

In say an audio amplifier an AC signal can be superimposed on a DC offset/bias, or some explanations say the AC is riding on top of the DC. I get that now the signal is moved away from 0v & is offset, useful for amplifying if say your only using a single supply. I understand this bit. However I'm having trouble understanding how this fixed DC component is still intact/present as DC after the AC signal is applied. On a scope if you apply a signal to a fixed DC bias voltage the DC appears to change into the AC signal & you can now see & measure the peak to peak value (instead of the flat fixed DC voltage line that was there before the signal was applied). How is there still a fixed DC voltage present when the scope now shows just AC ? How does the AC signal interact with the DC exactly & how can they both be present ?

 

[danadak]

A sine with DC is a simple sum of the DC component and the AC.

So you take the value of the sine at a specific phase and add the
DC to it, you get a scope pic that shows the Sine shifted by the
DC. If you then AC coupled that sum, out of the summer, you would
recover the sine, symmetrical about ground, but lose the DC.

The DC after the summer is, on a scope, "hidden" in the modified
sine sample poiint being displayed. So you no longer see a DC line, just
each sample offset by the DC.

If you want to see two lines, the sine + DC and DC, use a two channel
scope, DC into one channel, DC + AC into the other.

You can effect a summer by feeding into a scope the DC value in one channel,
the AC w/o DC into another, and use the math f() in scope to sum them and
show the result, which is a sine offset with respect to 0.


Regards, Dana.

 

[hevans1944]

In a word: superposition.

In a linear circuit, voltages from different sources, AC or DC or both, can be algebraically "added" or superimposed. Later, circuits can be used to "restore" the DC levels "lost" as a result of AC coupling. In the case of a biased transistor, this means that the DC "offset" needed to bias the transistor into conduction at or "above" the "knee" of the transfer curve is independent of, and superimposed on, the AC "signal" that is to be amplified. In an oscilloscope measurement, a "DC blocking" capacitor is often used to AC-couple the o'scope input, thereby hiding and omitting the DC component in the waveform trace, allowing for greater channel gain without deflecting the trace (uselessly) off-screen.

The same logic, AC coupling, is/was used in analog multi-meters to measure audio signal levels while blocking the hundreds of volts present in vacuum tube amplifiers. This was usually mysteriously named the "output" mode, although the multi-meter terminal in question was actually an AC-coupled input.

 

[crutschow]

How does the AC signal interact with the DC exactly & how can they both be present ?

When you have an AC signal riding on a DC level, the value of the DC is the average value of the total.
Thus if you have a 1Vac signal riding on a 2Vdc level, the value of the signal averaged over one cycle of the AC will be 2V.

So if you measure the voltage through a high-pass filter (large series capacitor followed by a resistor to ground) you will see only the AC signal.
Similarly if you measure the voltage through a low-pass filter (resistor in series followed by a large capacitor to ground) you will see only the DC value.

Make sense?

 

[hevans1944]

Make sense?

Yes, but the OP may not understand the significance of the superposition principle. It did not seem intuitively obvious to me, when I first encountered it, slightly more than midway through the previous century. It later turned out to be of inestimable value when performing circuit analysis and certain types of mathematical operations. It simplifies many computational problems.

It is absolutely imperative that the newbie understand the principle of linear superposition if there is to be any hope of understanding electricity and electronics. I shit you not: it is that important! More important than knowing Ohm's Law. More important than remembering your own birthday. Read the link posted in the previous paragraph and also this link until you fully understand the superposition principle.

Note, however, that linear superposition applies only to those systems exhibiting a linear response, which is mostly what we consider here as a practical point of view. Even highly non-linear systems can be analyzed as "piece-wise linear" for purposes of applying the superposition principle, but this does require more thought, and perhaps inspiration.

 

[mckenzie]

Thanks very much for the explanations, it's much clearer now. I was thinking too much in terms of visual it seems.

When you have an AC signal riding on a DC level, the value of the DC is the average value of the total.

I had a feeling it was something to do with the average so thanks for making it clear.

Yes, but the OP may not understand the significance of the superposition principle.

No I had no idea thanks for bringing it up. It's really interesting reading about it & I'm starting to understand why it's important.

Hopefully in time it will become clearer to me what is actually physically happening but I think, for now, what I'll take away from this is to treat the AC & DC as two separate sources, to a degree independent of each other, then rely on (or trust) coupling/decoupling/maths to bring them together/separate them.

 

[crutschow]

I was thinking too much in terms of visual it seems.

I also like the visual to help me understand a function.
Below is a display showing a 0.4Vpp, 10Hz sinewave added to a DC level (square-wave) that changes value every half second between 2V and 3V.
Does that help?

 

[mckenzie]

Does that help?

Yes it definitely helps looking at it visually, thanks for this.