Help with Programmable / Variable gain amplifier theory - Page 1
Forum section: electronics / beginners

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: April 28, 2021, 11:10:19 am
content: Hello everyone,I am currently working on my dissertation project. The topic of the project has to do with studying programmable gain amplifiers for the readout of CMOS pixel sensors.I have had a very hard time trying to find information on different PGA / VGA topologies and could not find a single book that went over both topologies or discussing them in depth.So far my bibliography includes the following books:1) Title: Op Amp Applications Handbook ISBN: 0-7506-7844-52) Title: Operational Amplifiers & Linear Integrated Circuits: Theory and Application ISBN: 978-17968568973) Title: Op Amps Design Application and Troubleshooting ISBN: 97800805130894) Title: Design With Operational Amplifiers ,and Analog Integrated Circuits, 4Th Edn ISBN: 9780072320848 || 97800711217365) Title: Op-amp circuits manual ISBN: 9780434912070Despite all of these being great books about operational amplifiers in general, they don't get to the details of PGA or VGA topologies.Does anyone have experience or know any books that analyze programmable gain or variable gain amplifiers? (Especially for on-chip CMOS applications)Thank you in advance for everyone's help!If more information is needed please feel free to ask!

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: May 02, 2021, 07:07:04 pm
content: Thank you very much for your reply.A few of the papers I found on the topic mentioned the use of charge amplifiers for sensing.This is what I am trying to wrap my head around. Some papers use a charge amplifier with switched capacitors to alter the gain, others use a gilbert cell topology and control the current in order to create a transconductance amplifier, and others use differential mosfet pairs.I just cannot figure out the distinct topologies since most of the papers seem to use a different topology according the application (no surprise there).

author: Terry Bites
author_rank: Super Contributor
timestamp: May 03, 2021, 11:12:26 am
content: Charge cannot be directly measured at any instant in time. This means it first has to flow into an integrator for a fixed time and the result measured. Integrators of the op-amp type can be liner linear or time discrete, seehttp://www.ecircuitcenter.com/Circuits/SWCap_Int/SWCap_Int.htm.See AD8488 and ACF2101 chips for the analogue approach used in real world devices. For High speed the charge is simply allowed to accumulate on a grounded capacitor which is just read with a hi-Z high speed amplifier. Seehttps://www.analog.com/en/analog-dialogue/articles/integrated-solutions-for-ccd-signal-processing.htmlYou may also find this interesting.https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjX6sz3q63wAhXIGuwKHZ-2BMs4ChAWMAR6BAgNEAM&url=https%3A%2F%2Fwww.osti.gov%2Fservlets%2Fpurl%2F1422600&usg=AOvVaw3noLULYHorh-sgJW604fkzA gilbert cell, as far as I can tell, is not used as a charge amplifier per-se. Though you can view it as a current controlled multiplier. It is used to create voltage-controlled voltage gain which you might need to add post integration. PGAs are usually set with internal or external digitally selected gain resistors. This gives you pression set gain ranges. VGAs generally use an improved gilbert cell or Blackmer cell. Their setting are not as stable or accurate as PGAs . VGAs have great advantages in dynamic gain control. Continuous and fast control of gain can be very handy. The control input can be linear or log scaled. For PGA and VGA topologies visit TI or AD websites where these products abound. seehttps://user.eng.umd.edu/~neil/EE408D_02/Design_Ex/Mixer/mixer.htmlSee this as wellhttp://www-physics.lbl.gov/~spieler/ICFA_Morelia/text/Front_End_Electronics.pdfhttps://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwin5di5uK3wAhXFxqQKHRznAX0QFjAAegQIBRAD&url=https%3A%2F%2Fwww.analog.com%2Fmedia%2Fen%2Ftechnical-documentation%2Ffrequently-asked-questions%2Fvga-design-faqs.pdf&usg=AOvVaw162P4xm97nHIIgBxYM2eIn

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 03, 2021, 02:33:02 pm
content: The variable gain thing has nothing to do with the opamps themself but with some kind of variable circuitry surrounding it, and there are many different approaches to do such a thing.A very old fashioned way is to use an LDR with a LED (or even a light bulb) to vary the resistance of the LDR. This was used in the '70-ies for audio, but it tended to be too noisy.The topology of a "multiplying DAC" is another way. You can also have a look at "IC potentiometers". These usually are a long string of chip resistors with mos switches in between them.Yet another way is to use a J-fet with a low gate voltage. This method is sometimes uses to vary the gain of a Wien-Bridge oscillator to stabilize the amplitude.PGA's are also a common part of ADC's. Take a look at the datasheet of the ADS1118 for example. For such an application you want to KNOW what your attenuation is, so a few fixed steps (Usually by a power of 2) are preferable, and that's just a handful of resistors and CMOS switches.

author: ChrisGreece52
author_rank: Frequent Contributor
timestamp: May 03, 2021, 03:58:51 pm
content: Quote from: Terry Bites on May 03, 2021, 11:12:26 amSorry for the late reply to the thread.I cannot thank you enough for all the resources! I have been going over them since your post. The website with the charge amplifier topology and the other that explained Gilbert Cell, in addition to yourexplanation were extremely helpful. To fully understand and interpret everything tough I'll have to do a lot more studying.Quote from: Doctorandus_P on May 03, 2021, 02:33:02 pmThank you very much for the clarification! My confusion derived from some papers not distinguishing between the gain control circuitry and the amplifier configuration.So I assumed that every amplifier design was operating differently.I have a lot of research to do on the designs you mentioned. Most of these techniques and applications I did not even know before your post.Thank you very much for your explanations since they helped me categorize everything in my head. I will take today to review and research everything I am unfamiliar with. It's a whole new world for me, since I have not dived into the specifics of op-amps for years now.