Hello, I have a question about how to find the input resistance of an transistor amplifier of the first circuit below. It says to make an equivalent circuit shown in the second circuit so that the input resistance is R1 in parallel with R2. So, the book I have tells me that to an ac signal at the input, the power supply acts like a short to ground. Why is this so? I understand the role of R2 but why would R1 be a path to ground? Wouldn't a signal appearing at the input travel to the ground through R2 and ignore R1 completely so that the input resistance is just R2 only? This is confusing me, appreciate it if anyone can help. Thanks