Re: amplifier input is connected to transistor source?
Perhaps you are talking about common base operation? It is compatible with low input impedance.
However op amps are normally designed to have high input impedance, and high gain.
You would need to attach your input signal between the device's ground terminal, and supply ground (or neg.) rail. And then it would no longer work like a normal op amp. It's likely you need to make a custom arrangement, requiring that you design your own differential amplifier.