# Why do we use open loop voltage gain in Miller's theorem? Shouldn't we use the closed loop voltage gain?

Let us take the case of the circuit in the image, while splitting the capacitor connected from base to collector into separate input and output capacitors as shown in the second figure. We use Miller's theorem.

C1= C ( 1- v2/v1) and C2= C ( 1- v1/v2)

Here for calculating the value v2/v1 we must use the closed loop values or the circuit where the BJT has a feedback path. Most of the text books recommend using the v2/v1 value when there is no feedback (capacitor 'c') present or simply to use the open loop voltage gain. My questions are:

1. Wouldn't it be wrong to use the open loop voltage gain as it might give wrong input and output capacitor values?

2. Moreover, we use Miller's theorem to simplify any complex circuits that has parallel connections, but to use Miller's theorem it seems we must first find the voltages across the particular impedance in parrallel connection. To find that voltage drop we must first solve the circuit containing that particular impedance. Wouldn't it make the theorem useless?