sharpsheep
Junior Member level 3
Hi,everyone
I've got some questions about cmfb stability in two-stage op amplifier.
1） Figure 1 is a schematic of two-stage opa with continuous cmfb circuit. I insert a device of iprobe (in analogLib library) to simulate the loop gain and phase of cmfb circuit. Figure 2 shows a voltage buffer using the two-stage opa. As we know, in order to fulfill the stability requirement, the cmfb circuit should be a negative feedback. However, when I simulate the loop gain and phase of cmfb circuit according to Figure1 and Figure2, the initial loop phase from 0Hz to 10 Khz shows to be “0” rather than 180. Does that mean the cmfb become positive feedback near DC?
2） When I break the feedback loop of voltage buffer in Figure 4 and do the same simulation, the result will be normal, the initial loop phase will be 180( In Figure 5).
Does anyone meet the same problem? How to solve it?
Thanks!
Lorry
I've got some questions about cmfb stability in two-stage op amplifier.
1） Figure 1 is a schematic of two-stage opa with continuous cmfb circuit. I insert a device of iprobe (in analogLib library) to simulate the loop gain and phase of cmfb circuit. Figure 2 shows a voltage buffer using the two-stage opa. As we know, in order to fulfill the stability requirement, the cmfb circuit should be a negative feedback. However, when I simulate the loop gain and phase of cmfb circuit according to Figure1 and Figure2, the initial loop phase from 0Hz to 10 Khz shows to be “0” rather than 180. Does that mean the cmfb become positive feedback near DC?
2） When I break the feedback loop of voltage buffer in Figure 4 and do the same simulation, the result will be normal, the initial loop phase will be 180( In Figure 5).
Does anyone meet the same problem? How to solve it?
Thanks!
Lorry
