Motion Sensor Design
The high gain is correct.
PIR sensors show a signal of 0.5 mV under the best conditions. Under the worst condition the signal is 0.01 mV roughly.
I actually don't know what you got in mind but, if you want to simulate the amplifier circuit, I suggest you to start from Zilog's circuit.
Texas Instruments has a very good application note on PIR sensors' circuits because they sell low power operational amplifiers designed for PIR sensors.
Murata has a good application note as well.
In the past I worked with this kind of sensors and I noticed that all front-end amplifier circuits are pretty much the same.