AC signals and DC level

Discussion in 'General Electronics Chat' started by welkin87, Dec 2, 2010.

Nov 18, 2010
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I'm a little confused about something. I just recently finished a differential amplifier project and was looking back over the simulation when I noticed that the output voltage swing was centered around 5 volts. Is that what DC offset is?

I was just reading in the online book, "The amplifier is still being supplied with 30 volts overall, but with the split voltage DC power supply, the output voltage across the load resistor can now swing from a theoretical maximum of +15 volts to -15 volts, instead of +30 volts to 0 volts. This is an easy way to get true alternating current (AC)..." (Ch.8, Sect. 2; OpAmps)

In my diff amp, I was using +10 and -10 volt supplies. How could I center my output around 0 so that I could get a bigger voltage swing? Or is that even possible in a simple 2 BJT diff amp design?

I was just curious about that, it's not crucial for me to know.

2. R!f@@ AAC Fanatic!

Apr 2, 2009
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If the Voltage swing is centered in any voltage other than zero, then this value is referred as the DC offset voltage.

It is sometimes not necessary for the offset to be zero, depending on application.

But for Audio amps to drive speakers the offset shud be as close as possible to zero.
This way the voltage can swing to both rails.

In modern op-amps the offset is internally set to zero, but old once has connection to set the offset to the desired value. But in Some modern op-amps variable offset is also achievable.

Inverting, summing or differential amp has nothing to do with offset typically, as offset always depends on what level the out put is biased at. This is at least what I know.

3. Kermit2 AAC Fanatic!

Feb 5, 2010
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It may be as simple as using a capacitor on your input line to block any DC components from being amplified. That should give you a waveform roughly centered on zero. Without seeing the circuit you are refering to this is just basic info. Your situation may require something else. Depends on the circuit.