Op Amp Calculations
in [Design]
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From: The Phantom on 27 Sep 2005 17:31 On Wed, 28 Sep 2005 08:44:46 +1200, Terry Given <my_name(a)ieee.org> wrote: >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 On what page(s) in Dostal is this approach described? >(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: The Phantom on 27 Sep 2005 17:56 On Wed, 28 Sep 2005 09:09:15 +1200, Terry Given <my_name(a)ieee.org> wrote: >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. There is a 2001 paper describing extensions of Middlebrook's method. It's somewhat obscure since it was published in one of the IEEE magazines rather than a regular journal. I've posted it over on ABSE with the subject "Small Signal Stability". > >Cheers >Terry
From: Terry Given on 28 Sep 2005 01:35 The Phantom wrote: > On Wed, 28 Sep 2005 09:09:15 +1200, Terry Given <my_name(a)ieee.org> wrote: > > >>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. > > > There is a 2001 paper describing extensions of Middlebrook's method. It's somewhat > obscure since it was published in one of the IEEE magazines rather than a regular journal. > I've posted it over on ABSE with the subject "Small Signal Stability". > > >>Cheers >>Terry aargh, my new news server (news.slingshot.co.nz) hasnt seen *any* new posts on abse for 5 days.... can you please email it to me... cheers terry
From: John Woodgate on 28 Sep 2005 02:51 I read in sci.electronics.design that Terry Given <my_name(a)ieee.org> wrote (in <1127855256.955918(a)ftpsrv1>) about 'Op Amp Calculations', on Wed, 28 Sep 2005: >B(s) = feedback factor = signal at opamp -ve input when input grounded How do you get any signal at the -ve input with the input grounded? -- 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 28 Sep 2005 04:03
John Woodgate wrote: > I read in sci.electronics.design that Terry Given <my_name(a)ieee.org> > wrote (in <1127855256.955918(a)ftpsrv1>) about 'Op Amp Calculations', on > Wed, 28 Sep 2005: > >> B(s) = feedback factor = signal at opamp -ve input when input grounded > > > How do you get any signal at the -ve input with the input grounded? in the simple case, a(s) = Vin(s)*Zf(s)/[Zin(s) + Zf(s)] B(s) = Vo(s)*Zin(s)/[Zin(s) + Zf(s)] H(s) = -Zf(s)/Zin(s) Cheers Terry |