# İnput signal of op-amp

Where I am confused is that the signal coming to the opamp should be superimposed on dc, or could it only consist of ac? Because when we want to make a filter consisting of a few transistors, we need to superimpose the input signal to dc. Is there the same thing here?I am confused here because while analyzing, for example, we are analyzing differential amplifer as large signal and small signal, but here I do not fully understand what the large and small signal is based on.So how should the input signal be, what is the analysis based on, you can explain it, I would appreciate it.

• Generally speaking, if the opamp is powered by both a positive supply AND a negative supply, you can directly feed AC signal. If only positive supply is given to opamp, then the input signal should be superimposed on a sufficiently large DC value so that the total input signal doesn't go below zero volt.
– AJN
May 16, 2021 at 13:21

simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) Dual supply. (b) Single-ended supply.

• In Figure 1a the op-amp is supplied by a dual supply. The input can accept signals of alternating polarity. The output can drive positive and negative.
• In Figure 1b the op-amp is supplied by a single ended supply. The op-amp can only accept and output voltages which are between ground and V+ (but not all can operate rail-to-rail so this is an important parameter when choosing an op-amp).

To work with alternating signals such as audio when restricted to a single-ended supply the configuration of Figure 1b is common:

• C1 decouples the DC component of the input (if there is any) from the DC bias provided by R1 and R2 - usually equal values so that the input operates around V+/2.
• The output now has a DC bias which is removed or blocked by C2.
• Thank you so much, now everything is in my head. Thank you very much. May 16, 2021 at 14:32
• Good. Wait a while to see if any other answers give you any other ways of looking at the problem. Then accept one of the answers to tell the system that your question has been answered. May 16, 2021 at 14:36
• @KeremSaruhan I think what's missing in your head may still be there. (But I may be projecting my own struggles based on what I read from you, too.) I think it would be better for you to look at the input stage of a bipolar opamp. That's what clarified things completely for me. The opamp does require a DC bias. It just isn't shown because the (+) and (-) rails hide how that's done. Also, the difference between the two (+) and (-) input terminals must normally be small -- on the order of millivolts or tens of them. Do you understand well what I just wrote?
– jonk
May 16, 2021 at 16:46
• I'm actually working with differential amplifiers and I couldn't quite understand whether the input signal is only dc and the combination of dc and ac,So how is it used as an amplifier? What happens to the dc signal? but I understood a little better when I worked. Thank you. May 20, 2021 at 8:10