OP-AMP voltage swing help please.  - Page 1
Forum section: electronics / beginners

author: cvriv
author_rank: Frequent Contributor
timestamp: April 12, 2016, 12:31:59 pm
content: So I've begun to learn OP-AMPS and don't quite understand why the book I'm learning from had done something in one of the example problems. I attached three pics from the book related to this problem.I know it says that if the load resistance is greater than or equal to 10K that I need to add and subtract 1V from the DC sources, AKA limits. I know that if the load resistance is between 2K and 10k and I have to add and subtract 2V from the limits. It doesn't say anything about a load resistance below 2k though. It says refer to the graph, which I did. It says the limit for a 1.5K load is 25Vpp, assuming that the DC sources are 15 and -15V. Fine. But then it says that the source voltages for the circuit are well below the limit for 1.5k and that we can safely assume that the load will not restrict the output voltage swing. Thus....... im going to pull a 2v from my arse to be added and subtracted from the limits and not explain how that came to be.What does it mean that the load will not restrict the swing because the source voltages are well below the limits?So what im gathering is that IF the source voltages for the circuit were 15V and -15V, a 1.5k load will have a maximum output of only 25Vpp?Why am I adding and subtracting the 1V or the 2v, depending, from the limits? I know that the output cant exceed the source limit, 15V and -15V, because it will clip or does it clip when it exceeds the limit after I added and subtracted the 1V or 2V?Can someone please shed some light on this for me.

author: danadak
author_rank: Super Contributor
timestamp: April 12, 2016, 12:41:34 pm
content: The fundamental problem with output swing is related to transistorVcesat for the device pulling to the negative rail, and in ability tosat to + rail if NPN, if PNB Vcesat again the problem.RRIO CMOS OpAmps can pull all the way to the rail, with no load.In any event load and intrinsic semiconductor operating junction voltagesprimary issues.Here is a book that should help (more info than you want to know) -http://www.ti.com/lit/an/sboa092a/sboa092a.pdfGraeme wrote and co-authored several books, here is just one of them, on thefoundations of OpAmps.https://archive.org/details/ApplicationsOfOperationalAmplifiers-3rdGenerationTechniquesRegards, Dana.

author: Lee Leduc
author_rank: Regular Contributor
timestamp: April 12, 2016, 01:12:45 pm
content: What book are you using? Would you post the Title and Author (or ISBN)?Thank you.

author: danadak
author_rank: Super Contributor
timestamp: April 12, 2016, 01:34:00 pm
content: Besides the two links I provided in prior post this is the original in the seriesbook -http://www.amazon.com/Operational-Amplifiers-Design-Applications-Tobey/dp/0070649170Regards, Dana.

author: Dave
author_rank: Super Contributor
timestamp: April 12, 2016, 01:42:48 pm
content: I think the main problem is that the right graph doesn't show the output voltage swing for ±4V supply voltage. For all you know, the opamp might not work at all at that voltage.Flawed exercise.

author: cvriv
author_rank: Frequent Contributor
timestamp: April 12, 2016, 07:50:38 pm
content: The book I'm using is called Electronics Technology Fundamentals. Conventional Flow VersionISBN-13# 978-0-13-504874-0ISBN-10#0-13-504874-0I totally get this now!!! Holy hell this book is horrible with explanations! I figured all this out using Multisim. So I learned the output voltage clips at the swing voltage and NOT the source limit. The output voltage will never be or exceed the source limit, at least with the 741, because of the voltage drop across the op-amp.So, if the load is 10k or greater, the swing is 28Vpp. If the load is between 2k and 10k, then the swing is 26Vpp. I dont know how to calculate the swing if the load is between 2k and 10k or greater than 10k if the input voltages are not 15V, but I know how to use an oscilloscope!So for the circuit with the 1.5k load and the 4V input voltages, I know the swing is less than 4V. I dont know that the swing is 4Vpp as the book says it is or might be. After messing around with multisim it turns out the the swing is not 4Vpp as the book is trying to have me believe. It's more like 7Vpp. That' a huge difference and book has no explanation as to how it came up with 4Vpp. So I figured out that the swing is around 7Vpp because after entering the appropriate input voltage, the output voltage doesn't clip.So what I've learned from this is that I need to know the swing voltage for a give load so that I can calculate the MAXIMUM input voltage. How easy is that! That's all the book had to say!

author: danadak
author_rank: Super Contributor
timestamp: April 12, 2016, 09:25:43 pm
content: Also keep in mind output swing also affected by slew rate limiting at higherinput frequency.Regards, Dana.

author: cvriv
author_rank: Frequent Contributor
timestamp: April 13, 2016, 12:26:06 am
content: Ok. I'm about to cover slew rate and everything else in this lovely book of mine. Thank you.