Re: Summing Amplifier with non-ideal op-amp
From: Spehro Pefhany (speffSNIP_at_interlogDOTyou.knowwhat)
Date: 03/09/05
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Date: Wed, 09 Mar 2005 11:51:19 -0500
On Wed, 09 Mar 2005 09:22:51 -0700, the renowned Jim Thompson
<thegreatone@example.com> wrote:
>On Wed, 09 Mar 2005 11:20:33 -0500, Spehro Pefhany
><speffSNIP@interlogDOTyou.knowwhat> wrote:
>
>>On Wed, 9 Mar 2005 15:34:21 +0000, the renowned John Woodgate
>><jmw@jmwa.demon.contraspam.yuk> wrote:
>>
>>>I read in sci.electronics.design that Spehro Pefhany <speffSNIP@interlog
>>>DOTyou.knowwhat> wrote (in <n12u21hdrsh1h7hkl2bgqqipu0kob0v6ri@4ax.com>)
>>>about 'Summing Amplifier with non-ideal op-amp', on Wed, 9 Mar 2005:
>>>
>>>>The open-loop signal at the input is reduced by a factor of 1/(N+1)
>>>
>>>Sorry, where does 1/(N+1) come from? This 'open loop signal' is due to
>>>the op-amp open loop gain not being infinite?
>>
>>
>>Assume all the N summing inputs save one and the output of the op-amp
>>are grounded (the op-amp is doing nothing). With a voltage Vin applied
>>to the remaining summing input, the voltage at the inverting op-amp
>>input is Vin* 1/(N+1). That effectively degrades the Vos, GBW, DC gain
>>characteristics of the op-amp by a factor of ~1/(N+1). Or to look at
>>it from another direction, you have a passive ~1/N attenuator followed
>>by a non-ideal amplifier with gain ~-N (N>>1).
>>
>>
>>Best regards,
>>Spehro Pefhany
>
>Eh? Does this "summing" amplifier not include a feedback resistor?
When you close the loop, yes.
>If there IS a feedback resistor, the OpAmp maintains a virtual ground
>at the inverting input, so the gain is simply Rf/Rin.
Yes, -Rf/Rin, with an ideal input. Or -1 for a gain-of-one summing
amplifier from a given input to the output.
But shunt that virtual ground to ground through a relatively low-value
resistor and you degrade the performance. That's effectively what we
have here.
>The unused inputs have no effect on the gain... HOWEVER, they do add
>_offset_voltage_ terms.
>
> ...Jim Thompson
I think they do affect the non-ideal gain (and GBW) as measured from a
single input to the output with the other inputs at 0V.
Best regards,
Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
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