Op Amp Calculations
in [Design]
|
From: John Woodgate on 22 Sep 2005 13:44 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. -- 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: Roger Lascelles on 22 Sep 2005 23:29 "John Woodgate" <jmw(a)jmwa.demon.contraspam.yuk> wrote in message news:Gu57MsI21uMDFwV2(a)jmwa.demon.co.uk... > > The inverting op-amp circuit works by having NO signal on the - input. > That's what 'virtual earth' means. > -- > 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 Hello John - also Graham (Pooh Bear). You people are making me defend my not very academic grasp of opamps! I can see people don't like me talking about the voltage divider at the input, but it is there when you open the feedback loop and therefore affects noise, offset and drift. I first saw it when calculating signal to noise ratios for audio amplifiers using opamp stages. For example : _______R2______ | - | o----R1--+---|\ | | \_________|__ out | / Vn --|/ + R1 = R2 for gain = 1. Vn is noise, offset & drift referred to input and has gain of (1 + R2/R1) = 2. So signal to noise is half as good as the opamp can do with the signal going straight in. Now, short the output to ground and look - the signal IS halved by the R1 R2 divider in exactly that 2 to 1 ratio. The ratio holds for whatever R1 and R2 you choose. R1 and R2 are dividing down the input signal before it hits the opamp input terminal. I know this continues to be correct when I close the loop, because I continue to see the same signal to noise ratio at the amplifier output - although gain changes. Makes sense really - simple feedback does not change the S/N performance the network had before you added feedback. Certainly you don't see much voltage at the virtual earth point when you close the feedback loop, and I see that I needed to explain what I meant by 'input attenuation". So when I see R2 < R1, I immediately think "input attenuation", poor noise, offset and drift. Just like the T feedback circuit we were discussing. If you can get R1 >> R2 you can really tap into available opamp performance. ------------------ The other issue is that the T feedback circuit runs with less loop gain for the same overall gain than a simple two resistor amplifier. This means less bandwidth and accuracy. If anyone is interested, I can post the maths, but the 'input attenuation" line of thought gives it to you for free - the input signal is attenuated, but you get the same overall gain, so you must have sacrificed loop gain. Roger Lascelles
From: Jim Thompson on 23 Sep 2005 14:57 On Fri, 23 Sep 2005 13:29:48 +1000, "Roger Lascelles" <despam_rklasl(a)aanet.com.au> wrote: [snip] >------------------ > >The other issue is that the T feedback circuit runs with less loop gain for >the same overall gain than a simple two resistor amplifier. This means less >bandwidth and accuracy. If anyone is interested, I can post the maths, but >the 'input attenuation" line of thought gives it to you for free - the input >signal is attenuated, but you get the same overall gain, so you must have >sacrificed loop gain. > >Roger Lascelles > Roger, I take issue with your comment on "T" feedback circuits. If you choose the more common situation where you looking to reduce the feedback impedance, say by a factor of 10X, the difference is barely noticeable. I simulated two configurations both with GBW=10MHz and GDC=100K. The difference in high-end roll-off response is less than 1dB. If you add a capacitor in series with the R to ground, in the "T" configuration, you can knock down Vos sensitivity by nearly a factor of 10X. I will post the results on A.B.S.E shortly. ...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: Jim Thompson on 23 Sep 2005 15:03 On Fri, 23 Sep 2005 11:57:08 -0700, Jim Thompson <thegreatone(a)example.com> wrote: >On Fri, 23 Sep 2005 13:29:48 +1000, "Roger Lascelles" ><despam_rklasl(a)aanet.com.au> wrote: > >[snip] >>------------------ >> >>The other issue is that the T feedback circuit runs with less loop gain for >>the same overall gain than a simple two resistor amplifier. This means less >>bandwidth and accuracy. If anyone is interested, I can post the maths, but >>the 'input attenuation" line of thought gives it to you for free - the input >>signal is attenuated, but you get the same overall gain, so you must have >>sacrificed loop gain. >> >>Roger Lascelles >> > >Roger, > >I take issue with your comment on "T" feedback circuits. If you >choose the more common situation where you looking to reduce the >feedback impedance, say by a factor of 10X, the difference is barely >noticeable. > >I simulated two configurations both with GBW=10MHz and GDC=100K. The >difference in high-end roll-off response is less than 1dB. > >If you add a capacitor in series with the R to ground, in the "T" >configuration, you can knock down Vos sensitivity by nearly a factor >of 10X. > >I will post the results on A.B.S.E shortly. > > ...Jim Thompson See... Newsgroups: alt.binaries.schematics.electronic Subject: Op Amp Calculations (from S.E.D) - MultiPathFeedback.pdf Message-ID: <d6k8j1l0hrju6f8mhuhkkg9fv17kmc04mn(a)4ax.com> ...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: Roger Lascelles on 23 Sep 2005 22:57
"Jim Thompson" <thegreatone(a)example.com> wrote in message news:qak8j1tkkfnhejnitafai3qrdbq1p3sr80(a)4ax.com... > See... > > Newsgroups: alt.binaries.schematics.electronic > Subject: Op Amp Calculations (from S.E.D) - MultiPathFeedback.pdf > Message-ID: <d6k8j1l0hrju6f8mhuhkkg9fv17kmc04mn(a)4ax.com> I don't seem to have access to alt.binaries.schematics.electronic - perhaps someone can tell me how. I do take your point Jim, that the T circuit is not "bad". Engineering is about making "how much" judgements. I don't think I wrote off the circuit, in fact I suggested using unequal resistances each side of the tap to minimise the penalty. My point is - use the ratio of resistances at the opamp minus node to ESTIMATE what you are doing to noise, offset, drift and bandwidth. Applies for any opamp circuit, not just the T. Most of the time you have extra performance to burn. The T feedback can be evil. Easy as pie to peel 6 or 10db off your equipment spec and make it worse than the competition. The original post disturbed me because it advertised the "T" as a free lunch and seemingly equivalent to a feedback resistor. Roger Lascelles |