For my final project in my electronics course this semester, I need to design an amplifier with a specific set of parameters. Specifically, it needs to have a gain of 100, a voltage swing of at least 10 Volts peak-to-peak, an input resistance of 75 kilo-Ohms or greater and an output resistance of 100 Ohms or less. After designing a common-emitter amp with a gain of 100 and a voltage swing of at least 10 Volts, I calculated the input and output resistance, and determined that I needed common-source/common-drain buffers in order to get the input and output resistance that I need. After designing buffers that fulfil these requirements, I put them all into MultiSim for simulation. Each stage worked as expected, so I connected them all together. However, the gain of the overall amplifier was far, far less than expected. I checked the voltage being outputted by each stage, and I found that the first stage, a MOSFET common-drain amplifier, was outputting a signal of 2 mV when a 50 mV signal is applied. When I detached it from the rest of circuit, however, it began to output a 50 mV signal, as desired. The stages are coupled with capacitors to simplify the DC bias calculations.
Is there any reason why an amplifier would have a lower gain when coupled? What can I do to fix it?