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diode connected capacitor?

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lucky8

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Guyz,

Need help to figure out the function of this diode connected capacitor (is it what it is called?).

Comparing the 2 cct attached, the only diff. is the diode connected at the capacitor. The capacitor basically acts as phase compensation from input stage to the output stage, but by adding the diode in between the cap, what is the effect of the diode to the whole cct?
 

hi,
i guess it help to increase slew rate, maybe reduce the rising time.

good luck.
jeff
 

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    lucky8

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Does the diode is actually a transistor with diode connection or its a real diode?
 

How can adding the diode improve the transient time?
Does it induce a voltage drop across the cap, etc..?
Can someone please explain?

Thanks.
 

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    lucky8

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i think the diode is level shift,the capacitor behavel frequency compensation.but i can't sure.
regard!
h.martin
 

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    lucky8

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This seems to be an op amp made from an old bipolar technology. And, this diagram seems to be an equivalent diagram. My hunch is that they are indicating this capacitor to be a depletion capacitor as a poly-poly cap cannot be realized in this process.

Did you confirm if there was indeed a diode in the layout in parallel with the cap? Or, is this just a cap realized using the depletion region of a pn diode(base-emitter or a collector-base diode?)
 

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    lucky8

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My opinion is that the capacitor is used as a battery
 

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    lucky8

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the diode resistance help to lead compensate the phase for better stability.refer ken martin text book pp. 240
 

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    lucky8

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