analog circuit - what is phasemargin and gainmargin?

[electim2004]

analog circuit

what is phasemargin?gainmargin?

 

[gonewind]

Re: analog circuit

phase margin =180+phase(at which frequency the magnitude of feedback loopgain becomes 1)
gain margin=1-gain(at which frequency the phase of feedback loop gain becomes -180)
these two item are used to judge a system that whether it is stable and how a tent to which the system's stablity comes to.

 

[paladinzlp]

Re: analog circuit

Dear electim2004:
You can read the following paper.
I hope that helps

 

[moda]

Re: analog circuit

Lots of info at TI site

https://focus.ti.com/lit/an/slyt087/slyt087.pdf

Regards

Moda

 

[analog_design]

Re: analog circuit

a article about op-amp.

 

[eda_heat]

analog circuit

You also can get the "MOS_op-amp_design.pdf" at
hxxp://engr.smu.edu/ee/7321/MOS_op-amp_design.pdf

 

[vijay_nag]

analog circuit

hi!

The gain margin is defined as the change in open loop gain required to make the system unstable. Systems with greater gain margins can withstand greater changes in system parameters before becoming unstable in closed loop.
The phase margin is defined as the change in open loop phase shift required to make a closed loop system unstable.
i will also send a link with this reply. gives a very good idea
**broken link removed**

 

[bkat]

analog circuit

thank you
who can tell me how simulate phasemargin? and gainmargin?

 

[jake]

Re: analog circuit

thank you
who can tell me how simulate phasemargin? and gainmargin?

in Hspice for example, add an ac source in your amp input, and write an input file as follow:

.option probe
.temp 0
Vsupply VDD 0 3.3v
Vgnd gnd 0 0
Ibias ibias gnd 5u
vpi pi 0 1.65
Cl out 0 300f
.op
.ac DEC 10 1 500Meg
.probe ac vp(out) vdb(out) v(out)
.print ac vp(out) vdb(out) v(out)

.measure ac gain_Margin find vdb(out) when vp(out)=0
.measure ac phase_Margin find vp(out) when vdb(out)=0

 

[analog_design]

analog circuit

Dear jake:
Can you explain the real meaning about phasemargin?
Why the phasemargin must be 45~60?