Using a simplified high-frequency small-signal equivalent circuit model for BSIM3 MOSFET, the fully differential two-stage folded-cascode CMOS operational amplifier is analyzed to obtain its small-signal voltage transfer function. As a result, the expressions for dc gain, five zero frequencies, five pole frequencies, unity-gain frequency, and phase margin are derived for op amp design using design equations. Then the analysis result is verified through the comparison with Spice simulations of both a high speed op amp and a low power op amp designed for the $0.13{mu}m$ CMOS process.