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Active Elimination of the RHPZ

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How does the following method of Active Elimination of the RHPZ mathematically eliminate RHPZ ?

0fVQJkm.png
 

W/o grinding thru the algebra, one could use signal flow graph
to get at the T(s) almost by inspection, factor, and look at the
"buffered" case vs "normal" miller common source G stage.

But being a tad lazy this might answer your question - https://www.edn.com/miller-compensation-and-the-rhpz/


Regards, Dana.
 
The reverse component Ir is responsible for making Cc appear magnified by the Miller effect when reflected to node V1 , so it is desirable because it helps establish the dominant pole.
Click to expand...

1. How Ir helps in miller compensation ?

2. Why block only If ?

Note: the screenshots below are taken from the EDN article link in previous post above.

unity_gain_follower_RHPZ.png

bode_plot_RHPZ.png
 

1) Because Miller magnifies a small caps effects and makes it the dominant
pole in the response curve, eg. controlling phase margin. Stated another way
if no miller is done the parasitic poles due to stray C control the phase margin,
and leave us with pole locations close to axis with no + real damping loss.
eg. equal oscillation or partially damped oscillation. We could use a large
external C to modify response pole location, but then we have to supply large
current to slew that C, eg. to minimize OpAmp slew rate. But if we magnify the
small C, eg. miller effect, we only have to supply the unmagnified level of
current to slew that cap/gain stage.

2) The appendix has a good explanation....


Regards, Dana.
 

wait, you did not explain with regards to Ir (the reverse current) ?
 

1. How Ir helps in miller compensation ?

2. Why block only If ?
Click to expand...
The basic miller compensation circuit is build by connecting a capacitor across a gm cell. As explained very descriptively in the EDN link copied in post #3, it creates a wanted feedback action by current Ir and an unwanted feedforward action by current If.

"How Ir helps in miller compensation" - I'd say, Ir is the miller compensation.

"Why block only If" - Should be decomposed into "Why block If" - Because it causes unwanted feedforward effect, resulting in RHPZ. "Why not block Ir" - Because you want Miller compensation.
 

"How Ir helps in miller compensation" - I'd say, Ir is the miller compensation.
Click to expand...

Why Ir is the miller compensation ?
 

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