Op-amp imperfections, sine wave generation, and linear transistor amplifier - Page 1
Forum section: electronics / beginners

author: renzoms
author_rank: Regular Contributor
timestamp: December 12, 2021, 08:22:36 pm
content: I made this post to ask about how to measure op-amp imperfections. While the datasheet says VOS = 2mV and IB = 80nA, I can't/don't know how to measure these effects on my SDS 1104.I set up this buffer sort of circuit to measure the effect of VOS and IBhere1. I set up the circuit and measured the output with an oscilloscope, and got voltages rapidly jumping ~60mV-130mV. Close but I'm not sure if this is the same noise I see when the oscilloscope is just on and the probe is plugged in. Said more clearly, I expected a clear 0V output to jump up to a mean value of ~80mV. So maybe you have expert insight.I also want to summarize I've got on-going posts abouta) an npn linear amplifier circuit (last to-do)andb) generating a sine wave (next deep-dive)These are all interconnected.I wanted to producea sine waveto superimpose on a DC Biased lineartransistor amplifier.In regards to,a) The npn amplifier is producing unexpected outputs. I assumed a hFE value which may explain this, so I will get back to testing this, last, once I've got a nice sine-wave/op-amp/oscilloscope measuring understanding.b) I successfully made what looks like a sine wave.Here, I'll set it up and take a picture.Wrapping this up, as I have to go work at Papa John's in 39 minutes and I haven't had lunch, I will make this/these in SPICE to look at input/output currents versus max rated currents, and said hurriedly, I hope you can provide me insight. I look forward to seeing replies, geniuses.ThanksMight've forgot something, idk.

author: gamalot
author_rank: Super Contributor
timestamp: December 12, 2021, 08:36:00 pm
content: This video may help

author: renzoms
author_rank: Regular Contributor
timestamp: December 12, 2021, 08:53:37 pm
content: Thanks gamalot, that seems more about what VOS and IB are and how they affect Voutput, but I was asking about how to measure them with an oscilloscope/noise measured (I may've have written the post too poorly)

author: Kleinstein
author_rank: Super Contributor
timestamp: December 12, 2021, 09:50:08 pm
content: For the measurment one usually sets up the OP under test to already produce some of the gain. So for the offset voltage this would be something like a gain of 100. So 1 mV of offset would give out 100 mV to measure with the scope or DMM. The circuit should include capacitors to reduce the effect of EMI. A point sometimes overlooked is to have a resistor at the output so there is no large capacitive load to the OP that may make some OP oscillate. Also use a shield, so there is no offset from rectified superimposed AC.The amplifier circuit to measure the offset could also be used to measure or at least get a glimbs on the noise.For the bias the idea is to switch in a large (e.g. 1 M range - depends on the OP to test) resistor at one of the inputs at a time. Especially for very low currents (e.g. < 10 pA) one may also use a capacitor (e.g. 1 nF range) and than watch how fast the voltage goes up.

author: rstofer
author_rank: Super Contributor
timestamp: December 12, 2021, 11:09:16 pm
content: Offset voltage can be in the low number of uV and you aren't going to see that on a scope. The lowest range on my DS1054Z is 10 mV per division. A DMM is the way to go but only if it can measure uV and my BM786 will only read in terms of 100 uV. So, the idea is to amplify the value with an instrumentation amplifier like in Figure 3 herehttps://www.analog.com/media/en/training-seminars/tutorials/MT-037.pdfOr use a bench style DMM like the HP3478A which will measure down to 0.1 uV or the GW Instek 8351A which will measure down to 1 uV. Lead length and fluorescent lighting probably don't help.For measuring bias current, things are different and it seems that integration on a capacitor is one techniquehttps://www.analog.com/media/en/training-seminars/tutorials/MT-038.pdfIntegration on a capacitor is fundamental to analog computing, one of my hobbys.

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2021, 11:05:21 pm
content: National application note 24, included below, covers this exact subject.Volume 1 Number 3 Analog Dialogue September 1967(!) also covers it.For open loop gain testing, also check out:https://www.analog.com/media/en/training-seminars/tutorials/MT-044.pdfhttps://www.ti.com/lit/an/snaa047a/snaa047a.pdf