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Buffer stage

Hi, with your help, i've designed an opamp folded cascode with a fixed capacitive load....now i want to make an output (mosfet) stage voltage follower-voltage follower....My output stage is composed by 2 voltage follower output transistor, each of them piloted by a mosfet current source (or sink) and a common drain...the gate of the 2 common drain are connected to folded cascode's capacitive load. I don't understand a strange problem that i've found.

1)If, before to connect output stage to folded cascode,i connect an ideal ac source to buffer stage and an output load, i obtain the desidered output stage's pole frequency 2)if i connect folded cascode's load (instead ac source), buffer pole has a lower frequency (folded cascode pole is unchanged) Why?? Thanks
Reply to
lionelgreenstreet
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It sounds like the input capacitance of the buffer stage is big enough to matter.

Reply to
MooseFET

How input capacitance can modifiy output pole?

Reply to
lionelgreenstreet

It is presenting a capacitive load on the high impedance point of the folded cascode (output), shifting the pole. I assume you mean the Bode response of the amp+buffer is difference from the amp alone.

Reply to
miso

No, i have an output buffer with capacitive load: if this output buffer is not connected to folded cascode, it presents the right pole; if instead is connected to folded cascode with its capacitive load, the buffer's output pole has a lower value (the output pole of folded cascode is the same with or without buffer stage). I don't understand how input capacitance can vary buffer's output pole.... thanks for the help

Reply to
lionelgreenstreet

Have you tried driving the buffer with a high impedance of some other type? Your buffer stage likely has a feedback from the output node to the input. The situation may look something like this:

Cfb -[Z]-----+----!!-------- ! ! ! ---- ! ---!+ \\ ! ! GM OO---++--- ---!- / ! ! ! ---- ! --- Cload --------------- --- ! GND

With a zero value for Z, the pole position is controlled by the resistance of GM and Cload forming a simple time constant. With non- zero Z, the non-inverting input of the GM has a smaller signal on it and hence the GM appears to be a higher impedance.

Reply to
MooseFET

Well, lets try to parse this.

OK, just who exactly has this pole, i.e. the buffer itself? And if it is in the right hand plane, why bother attaching it to the cascode?

This in itself seems unlikely since the buffer must present a bit of capacitance to the folded cascode, which should move the poles of the folded cascode a detectable but perhaps not significant amount.

I don't understand

Not seeing this buffer, I am assuming you go up a Pfet and down a Nfet. The fets have gate to source capacitance. Put them in a string (source of Pfet drives gate of Nfet) and connect the capacitive load, and you have capacitors in a string. CGS of P fet connects to CGS of Nfet connects to load capacitance. Hence the buffer has some inherent feedback path. Generally the buffer is of sufficient transconductance (at the output stage) that the feedback path is not significant, i.e. the dominant pole of the folded cascode would provide stability. Perhaps you need more current in the buffer output stage.

Reply to
miso

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