Hump in Closed Loop Frequency Response -- Root Cause ?

[EUVL]

> I think you are seeing problems that aren't really problems. +3dB @ 200kHz, so what?

Jan,

I knew somehow you were going to say that. : )

And I know I can just put in a 12p cap to have 250kHz -3dB. Square waves look OK, and I am not going to have stability problems.

And I agree with all of that.

But I have seen the same curve (freq plot with 2p cap) on 3 different topologies I have built, and I would love to understand where it came from. If in the end it is a design compromise which I cannot do better, then I am happy to solder in the 12p cap. But at least then I have understood.

Hope you see my point. : )

And no I do not have an input lowpass as yet. Will try this weekend.


Patrick

 

[jan.didden]

> I think you are seeing problems that aren't really problems. +3dB @ 200kHz, so what?

Jan,

I knew somehow you were going to say that. : )

And I know I can just put in a 12p cap to have 250kHz -3dB. Square waves look OK, and I am not going to have stability problems.

And I agree with all of that.

But I have seen the same curve (freq plot with 2p cap) on 3 different topologies I have built, and I would love to understand where it came from. If in the end it is a design compromise which I cannot do better, then I am happy to solder in the 12p cap. But at least then I have understood.

Hope you see my point. : )

And no I do not have an input lowpass as yet. Will try this weekend.


Patrick

I would probably think the same as you do ..

But changes of topologies can easily give that type of changes. Different parasitic capacitances on the pos input, different inductances and capacitances at high impedance nodes, etc.

The layout, pcb, wiring etc is all part of the schematic but you don't see that in the sim!

I usually use the sim for proof-of-concept but at a certain point you got to get your hands dirty.

Jan Didden

 

[EUVL]

Jan,

> I would probably think the same as you do ..



> I usually use the sim for proof-of-concept but at a certain point you got to get your hands dirty.

I don't use sim at all.

I measure my transistors under working conditions and then calculate everything from an excel sheet. That is the only calculations I do.

And then it is all hardware. My hands are quite dirty ......

: )


Patrick


PS I do not have the schematics with me right now, but I'll send you later.

It is basically similar (but not identical) to this :
http://www.diyaudio.com/forums/showthread.php?postid=925332#post925332