Question: I'm designing a DC Lab Power Supply; the voltage regulation circuit looks like this:

[![enter image description here](https://i.stack.imgur.com/pywTK.png)](https://i.stack.imgur.com/pywTK.png)

I'm developing the BOM, component by component, and next in line is specifying the capacitor in the feedback loop for the op amp, C1 in this schematic.

As I'm going through the selection of specific components, I've developed a new appreciation for the diversity of available capacitors and the relative complexity of what I originally thought was a pretty straightforward component type. So my dwelling on this particular component is as much for the learning opportunity it presents as the desire to pick the right item for this specific case.

My PCB is SMD wherever possible, and I'm inclined to believe a garden-variety 0805 X7R would do the job just fine. However, I've learned they can have surprising behaviors depending on, for example, the voltage applied, so wanted to get the perspective of more experienced designers.

The design of the feedback loop itself was by far the biggest time investment in the circuit overall. I had to refresh my foggy recollection of Body plots, transfer functions, op amp particulars, etc. And it took me quite a few tries to get it this right (and understand why it worked then :). So I'm inclined to think if there's anywhere in the overall where the capacitance in critical that this would be it.

So my question is: ***Is a regular surface-mount ceramic cap the right choice for using in this feedback loop? Or should I be thinking something a little fancier, like perhaps a film capacitor of some type?***

Answer: 
DC bias effects have already been noted (there is an [excellent application note](http://www.murata.com.sg/Portal/ASEANWeb.nsf/dx/faq04.pdf/$file/faq04.pdf) from Murata on this). This link appears to be broken; [this FAQ page](http://www.murata.com/en-eu/support/faqs/products/capacitor/mlcc/char/0005) may be of use.

C0G, although marginally more expensive, bring other things to the table, and in a feedback loop such as yours (I am designing some right now in an interesting application switching a few hundred volts that requires a linear ramp), I want to have a part that will remain at its rated capacitance across bias, time and temperature.

C0G: No DC bias effect from most manufacturers (this has to do with the material used). This is definitely true of AVX, Murata and Johnson.

No [capacitor ageing](http://www.johansondielectrics.com/ceramic-capacitor-aging-made-simple.html)

Tempco <= 30ppm: This will be important if the power supply box heats up significantly.

Compare that to X7R and you will find that in a control loop, C0G is the best choice in a ceramic. I would not normally need to use a better part than that.

I am actually using a 1nF C0G, 50V, 5%, 0603 part from AVX (but all the usual suspects have them).
The type isn't critical, as long as it meets your tolerance needs.

Be aware that ceramics (not sure they are available so small) lose >> 20 % (to 50 % or more ) of their capacitance as the applied voltage reaches their rating. To avoid this, you can use a cap that is rated at a higher voltage than you need.

On your circuit -- if your + and - 15 V supplies don't come up properly, is there any possibility that your output will become unregulated ? The 1k will turn on Q1, but requires the opamp to turn it off...