Op Amp Calculations
in [Design]
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From: John Woodgate on 26 Sep 2005 14:24 I read in sci.electronics.design that The Phantom <phantom(a)aol.com> wrote (in <kfcgj1hadlo6jbiv02pt12jnesa30atfd2(a)4ax.com>) about 'Op Amp Calculations', on Mon, 26 Sep 2005: > (Refer to Jim Thompson's schematic in >alt.binaries.schematics.electronic) Instead of that, go back up the tread and see my ASCII art re-draw of the feedback as a simple potential divider across the output, with the feedback resistor taken from the tap. It's FAR easier to analyse. -- 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: Jim Thompson on 26 Sep 2005 14:48 On Mon, 26 Sep 2005 19:24:53 +0100, John Woodgate <jmw(a)jmwa.demon.contraspam.yuk> wrote: >I read in sci.electronics.design that The Phantom <phantom(a)aol.com> >wrote (in <kfcgj1hadlo6jbiv02pt12jnesa30atfd2(a)4ax.com>) about 'Op Amp >Calculations', on Mon, 26 Sep 2005: >> (Refer to Jim Thompson's schematic in >>alt.binaries.schematics.electronic) > >Instead of that, go back up the tread and see my ASCII art re-draw of >the feedback as a simple potential divider across the output, with the >feedback resistor taken from the tap. It's FAR easier to analyse. But considering it so low an impedance, that you call it just a divider, is a degenerate case... and certainly NOT the most useful one. ...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 26 Sep 2005 15:40 I read in sci.electronics.design that Jim Thompson <thegreatone(a)example.com> wrote (in <7hggj1ldkmifftc03sd4c47e2dcjss59r5(a)4ax.com>) about 'Op Amp Calculations', on Mon, 26 Sep 2005: >But considering it so low an impedance, that you call it just a >divider, is a degenerate case... and certainly NOT the most useful one. Would you care to enlarge on that? It's always a divider, for finite resistor values, so I don't see your point. -- 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: Jim Thompson on 26 Sep 2005 16:53 On Mon, 26 Sep 2005 20:40:00 +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 ><7hggj1ldkmifftc03sd4c47e2dcjss59r5(a)4ax.com>) about 'Op Amp >Calculations', on Mon, 26 Sep 2005: > >>But considering it so low an impedance, that you call it just a >>divider, is a degenerate case... and certainly NOT the most useful one. > >Would you care to enlarge on that? It's always a divider, for finite >resistor values, so I don't see your point. In the "interesting" cases the "divider" impedance is within the same order of magnitude as the first feedback resistor. ...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 26 Sep 2005 17:13
John Woodgate wrote: > I read in sci.electronics.design that Terry Given <my_name(a)ieee.org> > wrote (in <1127713562.620391(a)ftpsrv1>) about 'Op Amp Calculations', on > Mon, 26 Sep 2005: > >> John Woodgate wrote: >> >>> I read in sci.electronics.design that Roger Lascelles >>> <despam_rklasl(a)aanet.com.au> wrote (in >>> <1127358972.f51b6a91d2505dd8d1089bfe370aa720(a)teranews>) about 'Op Amp >>> Calculations', on Thu, 22 Sep 2005: >>> >>>> Looking at the circuit open loop, for a high gain amp, you want R4 >>>> >>>>>> R1, so most of the input signal hits the opamp minus input, but >>>> >>>> you end up with the opposite. >>> >>> The inverting op-amp circuit works by having NO signal on the - >>> input. That's what 'virtual earth' means. >> >> >> Hi John, >> >> obviously your "no signal" is quite a bit larger than my "no signal" > > > I don't know what that means. my "no signal" is zero. > >> >> thats a convenient approximation, but it is most surely an approximation. > > > Yes, it assumes that the op-amp has infinite open-loop gain. Not a bad > approximation at frequencies very much smaller than the unity-gain > bandwidth. indeed. at 1% of the UGBW, OL gain is 100, far from infinite > >> > >> to prove it, place a 10 Ohm resistor from the -ve input to 0V, and >> watch the circuit perform differently. >> > > You can 'prove' all sorts of things by changing a circuit. If you > measure the signal voltage on the inverting input, you find, unless you > are using the op-amp at an unwisely-high frequency, that the voltage > there is TINY compared with the input voltage. Not 'most of the input > signal'. We both disagree with Roger then. 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 :) Cheers Terry |