Miller effect and gain band width

The voltage multiplier will be used to illustrate the circuit reaction to component changes.

I added a 1p capacitor to see the frequency response. You can see it falls off on the high end.

I changed it to 10p and you can see it falls off faster.

The 20p falls off even sooner. From this we can see the capacitance is our enemy when trying to get high frequency response. Our circuit will have base collector capacitance and it will limit out high frequency. What can we do about it?

A circuit such as this can be resolved to a single vector at a specific angle. In simple terms Xc and Xl cancel each other. One will dominate. So What if we put a coil in our circuit?

 Look at the effect the coil had.

As you can see the coil helped. What else can we do?

I changed the frequency to show it is a frequency response in play.

I reduced the input Z to show the effect.

I varied the gain to show the effect is at a certain frequency. At low frequencies the gain effected the output but a higher frequencies the gain fell.
If you followed this you should see the gain effects the roll off frequency and the input Z effects the roll off  frequency. To get higher gain at high frequencies we need less capacitance. Placing a coil in the circuit can help with this. Using a lower Z will also improve the high frequency response.

Remember the capacitance will be there. You may find a better transistor for high frequencies. Look at the Ft.

A lower Z circuit will have better frequency response. Some RF designers use 50 Ohms for their I/O for this reason. The low Z circuit will have a higher current draw. For this reason a 5 volt amp may have lower power consumption than a 12 volt amp and produce the same gain. You can use this calculator to see the cut off frequency for different R C values.

1/(2*pi*r*c) calculator