what is signal source offset? - Page 1
Forum section: electronics / beginners

author: khatus
author_rank: Regular Contributor
timestamp: October 24, 2020, 06:41:16 am
content: What does it mean by signal source offset ?

author: ataradov
author_rank: Super Contributor
timestamp: October 24, 2020, 06:48:10 am
content: A literal DC offset of the signal. So neutral signal level is not at 0 V anymore. That DC offset would be amplified and would propagate itself though the stages. So you use blocking capacitors to remove that offset.

author: TimFox
author_rank: Super Contributor
timestamp: October 24, 2020, 02:02:45 pm
content: In the textbook example highlighted, the offset would be relatively large (assuming a transistor with grounded emitter and collector pulled up to Vcc through a resistor). The term "offset" is perhaps more commonly applied to relatively small voltages, such as the voltage that appears at the input of an operational amplifier due to mismatch of the input devices.

author: Bud
author_rank: Super Contributor
timestamp: October 24, 2020, 03:17:43 pm
content: You should not make handwritten notes on books, it is a bad habit, unless it was a copy.

author: tooki
author_rank: Super Contributor
timestamp: October 24, 2020, 04:03:47 pm
content: Quote from: Bud on October 24, 2020, 03:17:43 pmLOL says who? If it helps the owner understand it better, I say go for it!

author: tooki
author_rank: Super Contributor
timestamp: October 24, 2020, 04:06:41 pm
content: Quote from: khatus on October 24, 2020, 06:41:16 amThey mean a DC offset, like in this image I found somewhere (note where 0V is)

author: TimFox
author_rank: Super Contributor
timestamp: October 24, 2020, 05:10:10 pm
content: That's another meaning of offset, but not what is meant by the text highlighted in red by the OP.Your source is referring to an output offset voltage, produced by the extra resistor at the input.His source refers to removing an input offset voltage.

author: JacobPilsen
author_rank: Regular Contributor
timestamp: October 24, 2020, 07:51:03 pm
content: Quote from: Bud on October 24, 2020, 03:17:43 pmThis is obviously a copy.Can't you see the signs of (bad) scanning?

author: tooki
author_rank: Super Contributor
timestamp: October 25, 2020, 02:52:48 am
content: Quote from: TimFox on October 24, 2020, 05:10:10 pmThose aren't different meanings at all. The definition of "offset" doesn't include where it came from.

author: pwlps
author_rank: Frequent Contributor
timestamp: October 25, 2020, 10:03:47 am
content: If it were for DC they wouldn't call it "offset" probably, it would just be a non-zero input voltage. It seems they use this term thinking of an AC application where "offset" means DC component (that you usually want to get rid of). I agree it may be confusing when the DC vs AC context is not specified.

author: TimFox
author_rank: Super Contributor
timestamp: October 25, 2020, 02:10:41 pm
content: Quote from: tooki on October 25, 2020, 02:52:48 amThe original question is "what is signal source offset?" The high-lighted part of the text corresponds to that question, where the point of the capacitor is to eliminate that offset.

author: tooki
author_rank: Super Contributor
timestamp: October 25, 2020, 11:46:40 pm
content: Quote from: pwlps on October 25, 2020, 10:03:47 amDC offset, DC component, and DC bias are all synonyms that meanthe exact same thing.

author: pwlps
author_rank: Frequent Contributor
timestamp: October 26, 2020, 12:06:48 am
content: Quote from: tooki on October 25, 2020, 11:46:40 pmYes, but only in the AC context, as stated in this wikipedia article :"When describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean amplitude of the waveform. If the mean amplitude is zero, there is no DC bias.""periodic function" means AC (if you don't consider the pathological case of zero frequency). This is what I wanted to say, if the circuit is analyzed in DC only then you don't use the word "offset" to say there is a non-zero voltage somewhere. Using this term the authors imply the input voltage is AC but maybe they didn't say it explicitly this is why I said it may be confusing for someone focused on DC analysis.

author: tooki
author_rank: Super Contributor
timestamp: October 26, 2020, 12:50:04 am
content: Quote from: pwlps on October 26, 2020, 12:06:48 amOf course you wouldn’t call a DC-only signal an offset, because that’s not what “offset” means! But clearly the OP’s book is talking about an AC signal, and thus offset is one of a few correct words. Plus, the authordidsay it explicitly: “...it is usually a good idea to use blocking capacitors if you’re only interested in ac signals anyway.”It seems to me you’re arguing that the word “offset” is unclear without specifying that it’s an AC environment, butby definition“offset” refers to something that doesn’t exist in a DC-only context.

author: pwlps
author_rank: Frequent Contributor
timestamp: October 26, 2020, 01:43:35 am
content: Quote from: tooki on October 26, 2020, 12:50:04 amOk but we are in the "beginners" section here

author: Cerebus
author_rank: Super Contributor
timestamp: October 26, 2020, 01:49:10 am
content: I'm sorry @khatus, but this seems to have dissolved into a squabble over English comprehension rather than electronics that's going to take a junior school English teacher to sort out, probably by handing out a lot C-'s and scribbling "see me" on people's home work.Let me try and answer your question.I'm assuming you are trying to understand the phrase "[have a] signal source offset [from ground]" in the context given in that text book.The original sentence might be a bit clearer with a comma or a "that is": "So if you have a signal sourcethat isoffset from ground, ...". The phrase "signal source offset" isn't a commonly used phrase or term of art in itself. The 'source' there is just saying that they are talking about a signal that is a 'source' from the point of view of the circuit in question - a "signal source", something that is the source of a signal.What Horowitz and Hill are describing there is an AC signal that is riding on top of a constant DC signal, on top of anoffset. Let's say it's a sine wave and it is 2 volts measured from peak to peak (the lowest to the highest points in the waveform). Looking at only the AC part in isolation, assuming it has ground as a reference point, then it's sweeping from -1 volt to + 1 volt. If that had a 2V DC offset, then the signal would be sweeping between 1V and 3Vrelative to ground, the reference point for the AC part of the signal has beenoffsetfrom the natural centre point of an AC signal at ground (aka 0V), just as in the illustration Tooki posted:On the left it is showing two signals, one AC centred around 0V, and one DC signal of VDCvolts relative to 0V. This particular op amp circuit is shown adding the signals algebraically to produce the signal shown on the right. On the right the new AC signal, complete with DC offset, is shown in green. The red on the right isnota signal, simply an indication of where thevalueof VDC(as input) is relative to the green output waveform. The new green output signal on the right now varies around an offset from ground, the green signal on the left has no offset from ground. As Tooki said before, note well where the 0V indications are (two on the left, one on the right).