Design of 1V, 0.18Âľ Folded Cascode Operational Amplifier for Switch Capacitor Sigma Delta Modulator
229
SIMULATION RESULTS Differential Voltage Gain(Avd) The differential voltage gain of an opamp is defined as Avd 
Vod Vid
This is also the small signal ac. Voltage gain decreases with increasing frequency after the system encounters its first pole. Thereafter, this gain decreases at 20 dB/decade until it reaches another pole after which the drop rate becomes 40 dB/ decade and so on. Differential Gain Bandwidth Product GBW The differential gain bandwidth product (GBW) is equal to the unity gain frequency (W) since out system is a dominant pole system. The gain bandwidth product should ideally be equal at any frequency. In other words, it indicates that with the increase in the gain of system bandwidth suffers and vice versa. Phase Margin
Figure 7: Transient Response of FCOA For a system to be stable it is very important to maintain a certain phase margin A system having phase margin less than 45 degrees is considered unstable. Similarly a large phase margin like 100 degrees, though acceptable makes the system response slow
Figure 8: AC Response of FCOA