This is a very basic question. I want to have a very simple BJT Signal Amplifier with variable voltage gain. Let's think about a common emitter with an emitter resistor like this:
The input signal is a sine wave and the signal amplitude is small enough to be considered as small signal (around 500mV pk-pk ) and RB,RE and RC are selected properly to work in good bias condition . According to the equation we expect the voltage gain( AV) to be approximated as AV=-RC/RE:
So I expect by turning the potentiometer, I should see different gains. This was on the paper but in reality it does not happen exactly: the gain changes well but some wave form distortion is seen in some frequencies for example in my case these distortions look like skewness to left in 2.5MHz, 7MHz ,.... What am I missing ?