Re: Op amp newbie

On Fri, 01 Jul 2005 04:42:34 +0100, Pooh Bear wrote:

> Mac wrote:
>
>> On Thu, 30 Jun 2005 07:28:34 +0100, Pooh Bear wrote:
>>
>> > Mac wrote:
>> >
>> >> On Wed, 29 Jun 2005 06:29:12 -0700, Yvan wrote:
>> >>
>> >> > Hi,
>> >> >
>> >> > I have a very simple question to ask. Say my required Gain-bandwidth
>> >> > product is 1kHz * 1 = 1k, what is the minimum gain-bandwidth product of
>> >> > the op amp in order to assume an infinite gain?
>> >> >
>> >> > In the book "sensors and signal conditioning", they gave an example of
>> >> > an application that had a GBW of 30 Khz. It mentioned that a opamp GBW
>> >> > of 5 Mhz was large enough to assume infinite gain. 5 Mhz/30khz=167
>> >> > times. Is there a rule of thumb would help in figuring if a given
>> >> > opamp is suitable for my application in terms of GBW?
>> >> >
>> >> > Thank you,
>> >> >
>> >> > Yvan
>> >>
>> >> It depends somewhat on what you are doing.
>> >>
>> >> 10 or even less might be good in some cases. I imagine that 1000 would be
>> >> good enough all the time.
>> >>
>> >> The problem is that as you get closer and closer to the limit, then there
>> >> will be less and less feedback available to linearize the circuit at
>> >> higher frequencies and you will have more and more distortion. If the
>> >> op-amp is followed by an aggressive low-pass filter, the distortion might
>> >> be tolerable, since the filter will knock it down quite a bit.
>> >
>> > Filter ?
>> >
>> > What filter ?
>> >
>> > Graham
>>
>> Graham, I said "if the op-amp is followed by an aggressive low-pass
>> filter, the distortion might be tolerable since the filter will knock it
>> down quite a bit."
>>
>> I don't know why I'm quoting it; it is still visible above. ;-)
>>
>> But anyway, does it make sense now? If not, I really don't know what to
>> say!
>
> It sounded like an odd thing to do. Can't say I've ever seen such an
> arrangement. Have you ?

Yes. In a sampled IF system. IF bandwidth is 50 MHz.

> The cost of a passive LP filter would be such that it
> would simply make sense to use a better op-amp I would have thought.
>

Well, since it is a sampled IF system, a brickwall anti-aliasing filter
is mandatory anyway. And finding op-amps which can drive 50 Ohms at 50
MHz with low distortion is not necessarily trivial.

> Graham

--Mac

.

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