Would someone be able to explain why this is happening? Thanks.
For a nominal hFE of 200 (+400% /-50%);
Rb= hFE Re = 200 * 47 = 8 k. This is in parallel with R4 = 100k so the base bias voltage is reduced by this input load.
You are only looking at the initial capacitor charging voltage step (inverted).
The problem is the base voltage bias is regulated by hFE and the low Re from “Load Regulation” error of the high bias R Values. The equivalent Rb ought to be <= 100 x Re and lower is better regulated which affects low frequency response of input RC time constant.
a far better solution is to use negative feedback to help regulate the collector voltage and current for wide tolerances on hFE yet only need to reduce the base Req to about 8k or 200x Re so you can get away with slightly lower bias as it self-regulates a bit.
Now looking like an inverting Op Amp gain defined by Rfb/Rin ratios with low open loop gain, you can use Cin as your Rin if you want but the current gain increases with collector current but the. The voltage gain Reduces with collector current using Rfb due to negative feedback, so there is a sweet spot in the 100 to 200 Re range. The concept is far better than the classic H bridge CE amplifier taught in school because this reduces the variation in Vbe , which means the the nonlinear quadratic THD distortion on an asymmetric sine wave is significantly improved according to how much excess gain you feedback.
You can short out the 40 ohms but then you need a small pull-down R on the base to reduce the feedback bias current going into the base to raise the collector voltage to mid way.
- examine the Rfb/Zi Ratio and expect gain to be about half at best if you are drawing enough current. This design is good for a gain of 20 if input and output impedance safe similar and up to 50 or more if you lower the input impedance relative to output such as shorting Re)
Here is a solution with a V gain of 22 and adjustable nFE from 100 to 800 with an adjustable Vin from 100mVpp and up. The Vpp probes indicate the signal levels so it is easy to see the Voltage gain with 100mVpp input.
BTW C1 is redundant.