Timeline for Op-amp unexpected behaviour; amplification regime breakdown
Current License: CC BY-SA 3.0
13 events
| when toggle format | what | by | license | comment | |
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| Oct 20, 2016 at 12:52 | vote | accept | Vincz777 | ||
| Oct 20, 2016 at 12:07 | comment | added | Spehro 'speff' Pefhany | @Vincz777 The output resistance is inside a feedback loop so you have to multiply the apparent output resistance times (1 + A*b) where A is the open-loop gain at the test frequency and b is the feedback factor. | |
| Oct 20, 2016 at 9:54 | comment | added | Vincz777 | With V+=3V and V-=0V, when I put a 10 kOhm resistor at the output, I have 3V. When I put a 100 Ohm resistor, then I have 1.3 V. Which would mean that output resistor is 207 ohms? | |
| Oct 20, 2016 at 9:27 | comment | added | Vincz777 | Also is it correct to say the output resistor can be modeled as a resistor plugged directly in series at the output of an ideal op-amp? | |
| Oct 20, 2016 at 7:07 | comment | added | Vincz777 | Sorry I meant with R2 = 1 kOhm, C1 = 100 nF, gain max is 27 dB, whatever R1 is... | |
| Oct 19, 2016 at 21:17 | comment | added | Spehro 'speff' Pefhany | Now increase R1/R2 by another 10:1. | |
| Oct 19, 2016 at 20:43 | comment | added | Vincz777 | I just put 100 nF and now the gain is topped at 27 dB, whatever R2 is. This is mind boggling! | |
| Oct 19, 2016 at 20:23 | comment | added | Spehro 'speff' Pefhany | If C1 is too small you'll have reduced gain at the frequency of interest (and significant phase shift). Try going up to my suggested values (you can use 100nF rather than 39nF) and see what happens. You need to worry about loading when the output resistance is of similar order to your feedback network, then C1 starts to load your output in a frequency-dependent manner. Output resistance is maybe 100 ohms for a normal op-amp that draws a mA but yours works on 600nA so much higher- try measuring it with SPICE (you can do it open-loop in SPICE and sweep a voltage source with (say) 10K series. | |
| Oct 19, 2016 at 20:20 | comment | added | Vincz777 | In addition, when I increase R2, gain increases and reaches a maximum value (41 dB), then decreases again. I don't understand why! | |
| Oct 19, 2016 at 20:17 | comment | added | Vincz777 | I took R2=1kOhm and C1=10nF and it works great, I couldn't reproduce the "breakdown" effect. Could you please detail how the feedback impedance and the output resistance are related? When is it an important parameter when choosing an Op-Amp? Thanks if you can explain, because I've been struggling with op-amp recently. For C1, my goal is to have a gain of 1 for DC input, so I guess it's okay to have a small reactance at the frequency of interest. | |
| Oct 19, 2016 at 20:05 | history | edited | Spehro 'speff' Pefhany | CC BY-SA 3.0 |
added 46 characters in body
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| Oct 19, 2016 at 20:00 | history | edited | Spehro 'speff' Pefhany | CC BY-SA 3.0 |
added 46 characters in body
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| Oct 19, 2016 at 19:48 | history | answered | Spehro 'speff' Pefhany | CC BY-SA 3.0 |