How to design a opamp for this kind of charge pump

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zhangljz

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Hello,

I am designing a charge pump for PLL, and this is the structure I want to use. But for this charge pump, it has 2 loops: positive and negative loop, to ensure the stability, the gain of negative loop should be larger than the positive loop gain.

But I have no idea how to design such am amplifier. Anyone can give me some clue?

Thanks you
 

I must confess, I don't get the idea of using an OP at first sight. The loop has only stable operation conditions if VCP is buffered by a capacitor, which isn't the usual charge pump operation. Otherwise it's permanently affected by switching transients.

Secondly, I don't see the need to get perfect current source matching for a charge pump. Current isn't but a PLL gain factor not more, no less.
 

It's not the amplifier's responsibility: you have to care for lower |gain| in the positive loop (M8 - VCP loop) by some asymmetry between the 2 gain loops: the VCP node (incl. its load) must have a lower impedance than the VR node.
 

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