I read in sci.electronics.design that Larry Brasfield wrote (in ) about 'Summing Amplifier with non-ideal op-amp', on Wed, 9 Mar 2005:
I must say that I don't understand your *explanation* of this limit. If I have an op-amp with a feedback resistor of value R and N inputs also fed through resistors of value R, I don't see any deleterious loop gain effect that depends on N. In audio, the usual limitation of this type of circuit is noise.
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
The point is that with any opamp, the effective bandwidth starts dropping when you shunt the input to ground. So the question of how many inputs you can have (actually how low a shunt resistance you can tolerate) must include the gain bandwidth required, as well as the opamp in question.
-- John Popelish
Those two concepts should not be confused. The term "noise gain" refers to the closed loop gain for a signal arising between the amplifier inputs, while the term "loop gain" refers to the gain around the loop consisting of the forward path from amplifier input to output and the feedback path from that same output to that same input.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
I read in sci.electronics.design that Larry Brasfield wrote (in ) about 'Summing Amplifier with non-ideal op-amp', on Wed, 9 Mar 2005:
What do you mean by 'feedback gain'? The closed-loop gain is 1 for each input. If any number up to N-1 inputs are grounded, they make no difference to the gain from the active input(s) but can do nasty things to the noise.
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Simplified case, unity gain inverting summing amplifier, consider finite gain only.
Op-amp: Vo = -AoVin
|\| +----------|+\ | ___ | >----+----o Vo Vx o-----|-|___|-+ -|-/ | | _R_ | |/| | +-|___|-+ | | _R_ | ___ | +-|___|-+---|___|--+ | R | | | etc. N inputs | ___ | +-|___|-+ | R | | === GND
Vin = (Vo/R + Vx/R) * R/(N+1)
Vo = - AoVin => Vin = -Vo/A
-Vo/Ao = Vo/(N+1) + Vx/(N+1)
-Vo (1/Ao + 1/(N+1)) = Vx/(N+1)
Vo/Vx = -Ao/((N+1) + Ao) (closed loop gain) ~= -1 for Ao >> N+1
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
"Larry Brasfield" a écrit dans le message de news:HMCXd.4$ snipped-for-privacy@news.uswest.net...
There's a very simple and neat workaround for this: another opamp and 3 resistors.
Works also for the simple high gain inverter.
Now you'll have to guess how to connect them :-)
-- Thanks, Fred.
"John Woodgate" a écrit dans le message de news: snipped-for-privacy@jmwa.demon.co.uk...
.... and also to loop gain, which is I guess what he's reffering to.
-- Thanks, Fred.
I read in sci.electronics.design that Fred Bartoli wrote (in ) about 'Summing Amplifier with non-ideal op-amp', on Wed, 9 Mar 2005:
But that's the point; HOW is loop gain affected? Is this due to the op- map not being 'perfect' or what? If the summing junction really is a virtual ground, then there is no voltage across any of the N-1 resistors.
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
I read in sci.electronics.design that Spehro Pefhany wrote (in ) about 'Summing Amplifier with non-ideal op-amp', on Wed, 9 Mar 2005:
Sorry, where does 1/(N+1) come from? This 'open loop signal' is due to the op-amp open loop gain not being infinite?
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
It also depends on the required response - as the summing-point is *not*
0V, each input resistor affects the transfer function. conventional analysis ignores this, but if you calculate alpha and beta (a-la Dostal or Graeme) it shows up quite clearly. I recently completed a design that was a summing BPF, having 15 inputs, each fed from 100k. The feedback network was a bridged-T, and the difference between 1 and 15 inputs shifted the centre frequency by about 3%. I can show you how if you want- its not very hard, and implicitly deals with much of the non-ideal behaviour of the opamp.
Another design I did a few years back had 144 inputs (100k each) measuring a whole bunch of LED Vf's. The practical limit was actually 3 Vfs at once, given the gain for one Vf and the power supply limits (any more than 3 and the amp clipped at the +ve rail). In this app it was OK
- uP controlled LEDs so only did one at a time. Of course > 3 faulty LED drivers would make it saturate regardless of what the uP did, but it was a faulty LED detector, so that was acceptable.
So there's a couple more things, in addition to offset voltage & noise.
Cheers Terry
I read in sci.electronics.design that Spehro Pefhany wrote (in ) about 'Summing Amplifier with non-ideal op-amp', on Wed, 9 Mar 2005:
I think we are discussing different configurations. I presume a feedback resistor R, and N individual series input resistors, also R. The output is definitely NOT grounded; it has voltage V appearing on it. The voltage at the inverting input is zero for an ideal op-amp and is V/Ao for an open loop gain of Ao.
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
The resistance looking into the virtual ground is R/Ao. Since Ao is sort of big, you need an awful lot of grounded inputs to get to a shunt resistance equal to that! OK, if you are pushing a 741 op-amp to 20 kHz, there is a problem, but who does that these days .....? (;-)
-- Regards, John Woodgate, OOO - Own Opinions Only. The good news is that nothing is compulsory. The bad news is that everything is prohibited. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
Hi John,
that sounds like a pretty neat trick. Can you actually do it with a single summing amp? sounds like you would need to terminate then buffer each line, to prevent the summing-point impedance from screwing up your termination. And that would be the least of the problems....
Cheers Terry
"John Woodgate" a écrit dans le message de news: snipped-for-privacy@jmwa.demon.co.uk...
No, you're discussing the same configuration, a summing inverter with N equal gain inputs. From the POV of _one_ input, the equivalent schematics reduces to:
R1 R ___ ___ Vin -|___|------+-----|___|-. | | | | | |\ | +-----|-\ | | | >--+--Vout | .-|+/ .-. | |/ R1/(N-1) | | | | | === '-' GND | | === GND (created by AACircuit v1.28 beta 10/06/04
Obviously the gain is (about) -R/R1, but the noise gain, i.e. the loop gain, reduces from G(w)R1/(R1+R) for the one input case to G(w) R1/(R1+N R) for the N input case.
Suppose you use an OP77 (700kHz GBW product) you want gain=-2, have 5 inputs and need 1kHz BW, then your loop gain is only 700/11 = 63. Not that much, and easily further reduced as asked higher gain or with input numbers.
One cute trick if you have an opamp to spare, or you add another one because you want improved performance, is to add a negative resistor between the minus input and GND.
R1 R ___ ___ Vin -|___|------+-----|___|-. | | | | ___ | |\ | .-|___|---+------------+-----|-\ | | | | | >--+--Vout | /| | | .-|+/ | /+|---' .-. | |/ +--< | R1/(N-1) | | | | \-|---. | | === | \| | '-' GND | ___ | | '-|___|---+ | | === .-. GND | | | | '-' | | === GND (created by AACircuit v1.28 beta 10/06/04
You'll have to pay attention to some stability pb if you want to push the limit, but if you want a reasonable loop gain (say maybe 1/3 to 1/4 of the main opamp gain) this is easy to use.
-- Thanks, Fred.
"Fred Bloggs" wrote in message news: snipped-for-privacy@nospam.com...
[Cut a load of non-electronics related ad hominum crap.] [Cut a load of non-electronics related ad hominum crap.]I note that you have well developed name-calling skills, but are unable to answer the challenge I posed. I take this to mean that your "review OpAmps 101" suggestion is nothing but empty pretense.
You can take this as my response to your other two content-free posts.
-- --Larry Brasfield email: donotspam_larry_brasfield@hotmail.com Above views may belong only to me.
Larry Brasfield wrote: > "Fred Bloggs" wrote in > message news: snipped-for-privacy@nospam.com... >
That is a response to be expected from a pretentious little punk pseudo-intellectual like you. We know you are the PUNK born again xtian maggot from Washington state who has been plaguing the NG with his superficial trash for some months now.
You're quite unimpressive to look at your posting history. You spout a bunch of pseudo-theoretical elementary garbage, typical of a punk know-nothing 2nd rate EET student or wannabe, and soon evaporate once the thread becomes focussed. You certainly skulked out of the encoder divider thread without doing much of anything other than putting on airs about being an experienced motion controls designer- which we know now is a despicable fiction. You're a cowardly little hypocrite born-again xtian maggot and a hot air bag.
What a bunch of pseudo-intellectual garbage- and look at that damned complex math- and who the hell waits until loop gain gets near one to conclude they have an "accuracy" problem? What a pile of horse manure!
Consider? Consider?! You really are losing it. Who the hell are you to tell anyone to consider that pathetic, boring, and irrelevant pedantry- what a damned joke- stinking moron regurgitating trash from his textbook.
What a bunch of retarded crap- the moron is so damned dumb he/she/it thinks a working engineer with 60+ years of practice needs that low level minutia explained to him. What an unbelievable little wimp punk born-again xtian scumball.
Ahh- you did some homework on your own...whew-i-m-p-r-e-s-s-i-v-e.
[ snip boring pedantic drivel...] >Larry Brasfield wrote: [...snip usual pretentious Brasfield- fake name trash...]
You should talk about "content-free" posts, you little punk, superficial engineer wannabe. Hey!- Where's your "tricky" current source, maggot? Don't bother playing your trump card about having nothing to prove, because so far you're just a skulking little punk hot air bag.
So could this ever be considered a legitimate technique for gain / slew-rate adjustment? Or is it just too stupid?
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