[sdiy] sync circuit woes

[Neil Johnson]

Hi,

Andre Majorel <aym-htnys at teaser.fr> wrote:
> But isn't the GBP the consequence of the finite slew rate?

No, its an effect of the compensation network in the op-amp.

Slew-rate limiting is a different limitation.

> Cascade two x10 amplifiers and the second one will have lower
> bandwidth than the first because the excursion of its output
> voltage will be 10 times greater.

No, they'll have the same small-signal bandwidth as long as you don't hit slew-rate limitation.

> Am I missing something ?

GBP and slew-rate limitation are two separate issues in op-amp behaviour.  I think you've got the two mixed up together.

Tonight's homework reading is

http://www.analog.com/library/analogdialogue/archives/39-05/Web_Ch1_final_R.pdf

In particular, page 70 (marked "1.68") discusses slew rate and full power bandwidth, page 74 ("1.72") discusses gain bandwidth product.

Also, section 1.3, on page 33, describes op-amp structures, where you can find examples of the internal compensation capacitors (e.g. page 37).  The classic 741, shown here:

http://www.play-hookey.com/analog/inside_741.html

which clearly shows the 30pF internal compensation capacitor, rolling off the gain and giving rise to the 741's GBP.  This rather large capacitor gave rise to the 741's robust stability, eliminating external compensation networks in the majority of its applications and helping make it a very popular op-amp of the time.

Neil

--
http://www.njohnson.co.uk

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