DC offset

[whiz115]

hi! i need to know what is DC offset and why are we trying to reduce it
in audio output?

has to do anything with the centering of the waveform?
http://imageshack.gr/viewer.php?id=624938diagram3.jpg

thank you!

 

[Sceadwian]

DC offset is just that, the positive or negative DC voltage that's still being outputted when there is no signal applied. Too much DC offset will cause both distortion and heating in the speakers voice coil. Capacitive coupling is the most common way to getting rid of DC bias, however it also kills bass responce.

 

[dknguyen]

Audio is heard by your ears, and your ears only detects vibrations which are CHANGES in air pressure. DC offset is not changing and not vibrating. So aside from biasing audio components too much and driving them to saturation and distortion, it also uses extra power that isn't contributing to sound production.

 

[Analog]

Audio is heard by your ears, and your ears only detects vibrations which are CHANGES in air pressure. DC offset is not changing and not vibrating. So aside from biasing audio components too much and driving them to saturation and distortion, it also uses extra power that isn't contributing to sound production.

And isn't good for speakers.

 

[whiz115]

guys i have trouble to understand it

the photo i posted is it useful to help me understand how is a signal
with DC offset and without?

P.S some of the things you said are already known to me but i don't understand
them too...like if we have DC offset we loose output power or that if a device (let's say an op amp) does not have output capacitor it is possible to damage the next device connected with our op amp which also does not have an input capacitor.

 

[Nigel Goodwin]

You need to ask more specific questions - DC offset can make no difference, it can be harmful, it can be beneficial - it all depends where it is, and what the circuit needs to be do.

 

[dknguyen]

The output capacitor removes most of the DC offset (if it's big enough). WIthout it, you can damage the device it is connected to because that device is not designed to accept a signal with DC offset. Here's why it might be damaged or might not operate properly...

Transistors and speakers have upper and lower limits. A transistor will only do what it's supposed to do within a certain voltage range. A DC offset (a bias voltage) is applied to the transistor to make sure it is in the center of this operating range. An AC signal is then applied to the transistor. The AC signal mixes with the DC offset and if the AC signal is not too big, the voltage will still be within the range where the transistor operates properly and you get a good output signal with a DC offset. THis DC offset is filtered with a capacitor before it leaves the circuit since the next device is expecting zero DC offset (it makes things easier than having to guess what the DC offset from the input device is and uses less energy too).

If you apply a AC+DC signal to the transistor, the DC offset of the transistor adds into the DC offset that is being used to bias the transistor. This increased DC offset pushes the voltage range outside of the proper operation of the transistor, especially when you add the AC signal in. If the two DC offsets are large enough they push the transistor out of it's operating range even when the AC = 0.

Speakers are the same way. A speaker makes sound by pushing in and out. When it is off the speaker membrane sits in the center. An AC signal makes the speaker push in and out to make sound about the center point so the speaker can push in and out by an equal amount so the sound is good (and the range is symmetric so there is no distortion). But when a DC offset is applied, the speaker no longer sits in the middle. It sits a too far in or too far out when no sound is being produced. Now the distance that the speaker can push in and out is not the same. And when an AC signal is now applied with the DC offset the speaker might try to push in or out too far (too much voltage is being applied because the voltage is the AC+DC). The speaker saturates when it pushes too far in or out and causes distortion and might be damaged because it's trying to move too much.

I hope that helps.

 

[whiz115]

ok...let's start from how the waveform looks like if there is DC offset and if there isn't!

 

[eng1]

the photo i posted is it useful to help me understand how is a signal with DC offset and without?

The plots show the same signal with different offset. An ideal high-pass filter would remove the offset and the signal would be centered around 0 V.
The simulator might help in this case.

 

[Sceadwian]

It looks exactly like the image your provided, though typically DC offsets are so dramatic as that. You're using an electrete microphone which explains the DC offset, it's part of how they operate.

A simple example is a 1 volt p-p sine wave with a 0 volt dc offset will have a positive peek of +0.5 volts and a negative peek of -0.5 volts
A 1 volt p-p sine wave with a +1 volt dc offset will have a positive peek of +1.5 volt and a negative peek of +0.5 volts.

 

[Torben]

ok...let's start from how the waveform looks like if there is DC offset and if there isn't!

My understanding of it goes like so:

Assuming the scope works, then the DC offset is how far the imaginary centre line of your wiggly signal line is from the 0V line.

So in your top image you have +1V DC offset, and in the bottom one you have +6V DC offset.

With no DC offset, your signal line is centred on 0V.


Torben

 

[whiz115]

thank you guyz!!! it wasn't so difficult after all!

i have LTspice can someone help me to create a circuit that produces DC offset and after the capacitor i get zero offset? i want to experiment with it so i can understand better some of the additional things we talk about on this thread.

dknguyen your post is very helpful!

 

[dknguyen]

You connect a bunch of AC sources in series (if you change the frequency, magnitude, and delay of each one and have enough it can approximate other non-sinusoid signals using Fourier). It just superimposes each AC signal onto another. Anways, then you connect a DC source in series with the AC source. The DC source is your offset.

Put a large capacitor in series with the source, and you will find that the AC+DC sources are superimposed on one side of the capacitor, but only the AC appears on the other side of the capacitor (if the capacitor is large or if the AC source frequencies being used are high enough).

I forget the code to do it, but that's how you do it (maybe it's GUI now).

 

[whiz115]

SPICE programs are great...i've done a simulation and helped me much!

so...a capacitor centers the waveform to zero and the output signal is the
difference between the p-p voltage and DC offset voltage? (i know it's not the correct way to describe it but i can't do it better)

i've noticed that the outcome in volts after a capacitor is always less than before the capacitor...i understand that in most cases DC offset is not useful signal so it doesn't bother us if we get less output, but what's going on with op amps?

check this schematic...the power supply is 6V and this op amp has half the supply voltage mixed with the signal of an electret... this probably means a DC offset of 3,1V! at it's input pin what do we expect as an output after the coupling capacitor? let's say the gain is 20...do i expect 20 times the electret voltage? if yes...probably there is a DC offset in that voltage that's way there is an output capacitor, what's the outcome after the capacitor?

https://www.electro-tech-online.com/attachments/preamp-for-electret-mic-png.11263/

 

[Sceadwian]

There's an option in AC voltage for a DC offset... If you use ideal capacitors the DC bias is gone on the other side after a stabilization period (few ms's tops)

A capacitor only transfers AC voltages. Real world capacitors introduce a lot of losses you may not be compensating for in your simulations.

 

[whiz115]

A capacitor only transfers AC voltages. Real world capacitors introduce a lot of losses you may not be compensating for in your simulations.

ok probably my simulations don't reflect the real world but i think i got most of the idea...right?? (if you want to..you can help me make more realistic simulalations...every part has a bunch of parameters i can change!)

what about the schematic any comments?

 

[audioguru]

The opamp has a DC offset of 3.0V as explained here:

 

[whiz115]

on more question on this topic...it's relevant so i believe it's better i ask here than opening a new topic.

when do we use bipolar caps? (i know mostly on speakers) but what is
so special in such situations and it needs NP capacitors?

 

[audioguru]

A speaker uses an AC signal that keeps changing its polarity. If a polarized capacitor is used in a speaker's crossover network then the reversed polarity of the signal for half of each cycle would ruin it. So NP (non-polarized) capacitors are used.

 

[Dean Huster]

As you work your way through most discrete Class A amplifier circuitry, you'll find DC offset everywhere. The offset is used to bias the amplifier stage to the center of its operating range to minimize clipping and maximize undistorted gain. Coupling capacitors (sometimes called blocking capacitors) are used to isolate the dc voltage between stages. Only at the output is a large capacitor inserted to block the dc and couple the ac to the speaker.

Dean