The short answer is that the input of the second stage has input impedance which acts to load the output of the first stage which has output impedance, and anything in between the two (like a capacitor) will act as part of the first stage output impedance and so the signal getting to the second stage will be lowered because of that too.Hi, I have a problem I need to understand. Probably the question will be simple If explained in a simple way, and for this reason I chose this section. Very often it happens that the output voltage of the signal of one amplification stage is lowered once hooked to the next amp stage, even with a coupling capacitor. It often happens to me with discrete components like JFETs or transistors, and a little less with opamps. It is probably a physiological behavior of the signal chain, but I would like to understand why, otherwise I can not carry out some of my projects. Thank you for your contribution!
So you have three main things to consider:
1. The output impedance (or just output resistance) of the first stage.
2. The input impedance (or just input resistance) of the second stage.
3. The intercircuit coupling component(s) like a capacitor or resistor.
Those are the three main things that affect the signal attenuation between stages in amplifiers.
The higher any or all of these component values are, the amount of the loss of signal will be greater and so less signal amplitude will be seen at the very output of the second stage.