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C1 and C2 represent stray capacitances.

I was told that C2 leads to negative feedback and C1 leads to positive feedback (pg 371 of Learning the Art of Electronics) and therefore C2 is good while C1 is bad, if your goal is to minimize unwanted oscillations.

I understand that due to the Miller effect, the effective input impedance is much greater due to the amplification effect between the input and output terminals. I wasn't able to find an intuitive explanation of what C1 and C2 are doing in terms of feedback.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I can't tell without reading what you are reading, but I suspect there are some needed parasitics (not shown in your diagram) that will cause a 180 degree phase shift, which results in the positive feedback that the author(s) were talking about. An inductance (such as lead or wiring inductance) is probably needed. It's also not clear where the output is being observed. Is this an emitter follower? \$\endgroup\$ – jonk Nov 30 '18 at 2:16
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    \$\begingroup\$ Please note that the MILLER effect drastically REDUCES the input impedance. However, this applies only if the collector is not at signal ground (as in your - uncomplete? - drawing). \$\endgroup\$ – LvW Nov 30 '18 at 6:47

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