Op Amp Calculations
in [Design]
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From: Jim Thompson on 27 Sep 2005 11:24 On Tue, 27 Sep 2005 06:34:10 +0100, John Woodgate <jmw(a)jmwa.demon.contraspam.yuk> wrote: >I read in sci.electronics.design that Jim Thompson ><thegreatone(a)example.com> wrote (in ><2sngj1pjheo5h7aifs3tklqatrf5ohdesn(a)4ax.com>) about 'Op Amp >Calculations', on Mon, 26 Sep 2005: > >>In the "interesting" cases the "divider" impedance is within the same >>order of magnitude as the first feedback resistor. > >In which case the 'bottom arm' is two resistors in parallel, and the >'first feedback resistor' is one resistor in series with two in >parallel. Not rocket science, jut the normal 'loaded potential divider'. Us old farts only understand loop and nodal analysis... short cuts in the head always wander off ;-) ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: John Woodgate on 27 Sep 2005 12:02 I read in sci.electronics.design that The Phantom <phantom(a)aol.com> wrote (in <6rnhj11j64e444iftv630grdfcu6his1gh(a)4ax.com>) about 'Op Amp Calculations', on Mon, 26 Sep 2005: >I would agree, we need to be "far, far below UGBW" to get a closed loop >gain of 1000 with a fairly small error from neglecting the shunt arm on >the minus input. I would really be reluctant to try to get a gain of 1000 out of one op-amp at today's prices. I suppose that sometimes there is no choice, but I think that's very rare. 31.62 is your friend. -- Regards, John Woodgate, OOO - Own Opinions Only. If everything has been designed, a god designed evolution by natural selection. http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
From: Terry Given on 27 Sep 2005 16:44 Fred Bloggs wrote: > > > Terry Given wrote: > >> >> I recently built about 50,000 of this circuit, with a feedback cap too >> (mathcad rather than mathematica, and a pencil to start with for the >> analysis), and 15 inputs thru 100k resistors. the effect of the 14 >> "grounded" resistors shifted the center frequency by about 10% - Aol >> was about 50. power consumption (and cost) constraints meant I couldnt >> use a faster opamp, so instead I stopped assuming and started >> calculating :) >> > > What was the transfer function you were shooting for, and which amp? > a summing band-pass (ish) filter. 40 x TLV274. I didnt want to AC-couple the inputs (that would have cost me 240 capacitors) so I used the bridged-T feedback network with an RC shunt to give a DC gain of about 1/16 - any DC is basically common-mode, and the next stage was AC coupled. 3 Rs and 2 Cs was a whole lot cheaper than an RLC. But 100k/14 = 7k in parallel with the -ve shunt arm, enough to move Fc 10% or so. SPICE clearly showed it, so I went back and re-did my opamp analysis using Dostals approach (originally I did it using the Woodgate approximation), and voila - out popped the same answer. Mr HP3577 also agreed with spice and mathcad. Dostals method also allowed me to directly calculate the phase margin. Since then, I have analysed all opamp circuits thusly - but I use the Woodgate approach with pencil & paper as a bullshit detector :) Cheers Terry
From: Jim Thompson on 27 Sep 2005 16:59 On Wed, 28 Sep 2005 08:44:46 +1200, Terry Given <my_name(a)ieee.org> wrote: [snip] >Dostals method also allowed me to >directly calculate the phase margin. [snip] >Cheers >Terry What IS Dostal's method? The Loop Gain & Phase analyser on my website is based on R.D. Middlebrook's laboratory technique, and is VERY accurate, since the loop is never actually broken. ...Jim Thompson -- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | | http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Terry Given on 27 Sep 2005 17:09
Jim Thompson wrote: > On Wed, 28 Sep 2005 08:44:46 +1200, Terry Given <my_name(a)ieee.org> > wrote: > > [snip] > >>Dostals method also allowed me to >>directly calculate the phase margin. > > [snip] > >>Cheers >>Terry > > > What IS Dostal's method? The Loop Gain & Phase analyser on my website > is based on R.D. Middlebrook's laboratory technique, and is VERY > accurate, since the loop is never actually broken. > > ...Jim Thompson Hi Jim, its the same one used by Jerald Graeme. Hideal(s) = -a(s)/B(s) a(s) = feedforward factor = signal at opamp -ve input when output grounded B(s) = feedback factor = signal at opamp -ve input when input grounded Hactual(s) = Hideal(s) ------------- 1+Aol(s)/B(s) I got Dostals book about 12 years ago, but until last year never used this method, as the -ve input = 0V method worked well enough. Its only when I started really pushing an opamp that it became necessary, mostly because I wasnt happy with simply twiddling component values in SPICE. I havent yet managed to get your SPICE trick to work in Simetrix :( but I did read RDMs paper. Cheers Terry |