Re: Operational Amplifier Design

From: Kevin Aylward (salesEXTRACT_at_anasoft.co.uk)
Date: 11/09/04 

Date: Tue, 09 Nov 2004 07:44:03 GMT

Jim Thompson wrote:
> On 8 Nov 2004 15:57:30 -0800, Winfield Hill
> <whill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
>
>> Jim Thompson wrote...
>>>
>>> But I don't know of a single commercial amplifier that isn't set
>>> up that way.
>>
>> Doesn't adding a modest series resistor (along with an ESD network
>> after the resistor) give one a chance to both stabilize the loop,
>> and avoid ESD failure?
>
> Actually, most compensation caps _do_ have a series resistor (to
> introduce a zero).
>

Ahmmm. I disagree. If they do, its often a poor design. This particular
resister usually fails to do what it is idealised to do. It can only
work if the stage is not running out of steam at its zero frequency.
This is not often the case when you want to use such a trick. The cap
does a pole split, i.e. moves other poles further away, e.g low output
impedance driving the load. As soon as the resister starts coming in,
the poles start to migrate inwards again getting you back to where you
were without the Miller cap at all. i.e. loads of poles all over the
place.

Kevin Aylward
salesEXTRACT@anasoft.co.uk
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
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