Amplifier Help - Page 1
Forum section: electronics / beginners

author: ekong
author_rank: Newbie
timestamp: November 18, 2021, 10:42:59 pm
content: I started out with a common source amplifier on a n channel CMOS transistor.Output swing looks great but the stability is lacking (+/-100mV needs to be around +/-37mV).470K drain resistanceMicrochip TN5335K1-G5K source resistanceI'm looking to add feedback to this circuit to hopefully add some stability.Any help would be appreciated.

author: t1d
author_rank: Super Contributor
timestamp: November 19, 2021, 02:22:13 pm
content: Hi, ekong. I see that you are new to the forum, welcome!Posting your schematic and scope traces will likely help you get some replies. As is, the brevity of what you posted is not so likely to garner any participation. HTH

author: ekong
author_rank: Newbie
timestamp: November 19, 2021, 02:42:40 pm
content: Fair enough.I have attached the schematic and the output wave form.The input signal is a triangle wave ~0-5V @ 1HzThe output is inverted as expected ~2.3V-274V @ 1HzLet me know if there's other information that would help.

author: tooki
author_rank: Super Contributor
timestamp: November 19, 2021, 07:26:07 pm
content: Quote from: ekong on November 18, 2021, 10:42:59 pmWhat do you mean by “stability” anyway? Stability in electronics generally means that a circuit resists getting into oscillation or other undesirable states. It’s impossible to evaluate stability using an oscilloscope trace of less than a single cycle.If you are saying that you’re expecting ±37mV of noise but are getting ±100mV of noise, then I think your result is well within the manufacturing tolerances of the components and the abilities of the test gear. 0.1V is 0.03% of 300V. A typical digital oscilloscope only has 8 bits (256 levels), so a single-bit error at 300V full-scale is already 0.3%, or nearly 32 times your stated 37mV!That kind of precision is in the realm of a 5.5 digit multimeter at minimum.Just a little FYI: You mean “MOSFET”. The C in “CMOS” stands for “complimentary”, referring to the technique of using a complimentary pair of MOSFETs (one n-channel, one p-channel) to perform logic functions. A single MOSFET can’t be CMOS because it’s just one.

author: rfclown
author_rank: Frequent Contributor
timestamp: November 19, 2021, 07:51:07 pm
content: The word stability refers to a lack of unwanted oscillations in the amplifier. It isn't clear what you mean by "stability". If it is a precise output vs input voltage relationship, you will not achieve that without some type of feedback (which will need more active devices, or an op-amp). From the datasheet, you can see that Vgs(th) is specified as 0.6v min, 2.0v max, so it can be anywhere in that range. FETs do not have well controlled threshold voltages part to part. You'll also see that Vgs(th) varies -4.5mV/C, so there is a temperature dependency also.

author: ekong
author_rank: Newbie
timestamp: November 19, 2021, 10:27:56 pm
content: Yes I did mean noise.I wasn't expecting +/-37mV of noise. That is the application requirement. I'm getting +/-100mV of noise and I would like to reduce it.With regards to the data provided, I was asked to post a scope trace and the schematic to help get replies. You are correct, the oscilloscope was insufficient to measure noise.The data was collected from 6 digit multimeter. I can show data tables/graphs of voltage outputs showing the noise at various levels.Input 0.008958 0.32751 0.62567 0.93791 1.2492 1.5742 1.8523 2.1811 2.499 2.8108 3.1218 3.4396 3.7517 4.0625 4.375 4.6698Expected Output 299.462 280.349 262.459 243.725 225.048 205.548 188.862 169.134 150.06 131.352 112.692 93.624 74.898 56.25 37.5 19.812Actual Output 273.78 270.554 254.114 230.992 206.272 180.192 157.866 131.328 106.2624 81.5728 57.5116 32.9856 9.63228 2.18944 2.4571 2.3761273.988 270.752 254.138 230.754 206.512 180.132 157.74 131.64 106.3304 81.6568 57.6596 33.3056 9.48228 2.34144 2.4771 2.2901273.782 270.678 254.178 231.08 206.294 180.216 157.738 131.604 106.0884 81.6948 57.3476 33.0576 9.42628 2.42344 2.5151 2.3361273.802 270.868 254.208 230.708 206.198 180.292 157.7 131.356 106.2364 81.7268 57.5896 33.3216 9.74228 2.22944 2.3431 2.5081273.874 270.622 254.006 230.722 206.204 180.326 157.824 131.464 106.3064 81.5768 57.7016 33.2736 9.48828 2.54544 2.4131 2.2501274.168 270.832 254.172 230.89 206.468 180.412 157.832 131.368 106.3824 81.6168 57.4356 32.9956 9.58828 2.50744 2.4771 2.2141273.802 270.592 253.912 231.054 206.322 180.12 157.958 131.63 106.3804 81.8368 57.4076 33.1116 9.53428 2.28344 2.2571 2.1941273.896 270.506 254.056 230.8 206.286 180.024 157.682 131.592 106.1844 81.6248 57.6396 32.9456 9.61828 2.55744 2.2491 2.3501274.08 270.866 253.936 230.908 206.548 180.028 157.67 131.666 106.4104 81.5048 57.4196 33.0456 9.78228 2.50344 2.3391 2.3421273.922 270.776 254.166 230.89 206.386 180.326 157.986 131.48 106.3424 81.7548 57.3536 33.0316 9.76428 2.37744 2.2311 2.5241274.094 270.862 253.86 230.69 206.256 180.182 157.66 131.396 106.4304 81.6828 57.4696 33.0756 9.63628 2.45344 2.2171 2.2781273.788 270.57 254.182 230.8 206.484 180.25 157.89 131.36 106.3804 81.8668 57.5176 33.0876 9.57828 2.21544 2.2131 2.4801274.036 270.602 253.862 230.868 206.278 180.244 157.714 131.49 106.2784 81.7868 57.6596 33.1496 9.45228 2.18344 2.3291 2.4201Average 0.114769231 0.117727811 0.112710059 0.10383432 0.102177515 0.090532544 0.09547929 0.104686391 0.076284024 0.085846154 0.102982249 0.096710059 0.093846154 0.11964497 0.092568047 0.085136095Deviation(sorry about the formatting)I've order several op-amps but I would like guidance on how I should configure the feedback loop.I've attached another schematic.

author: rfclown
author_rank: Frequent Contributor
timestamp: November 20, 2021, 05:14:01 am
content: The "feedback" you have draw isn't feedback for the circuit; it's just a buffered input. But before going down that path (because feedback might not be what you need)... what is it you are trying to do? What is the application? What is this noise requirement coming from? The "noise" you are seeing could be the amplified noise from your input, or an artifact of the measurement (scope etc), or something else.

author: HB9EVI
author_rank: Frequent Contributor
timestamp: November 20, 2021, 11:33:26 am
content: please explain what you consider as 'noise'?I'm not getting clever what you mean by that.

author: ekong
author_rank: Newbie
timestamp: November 21, 2021, 05:07:46 pm
content: You are absolutely correct. I understand the "feedback" on the first stage is to just make the Op-amp act as a buffer.I'm looking to add feedback from the output of the second stage to the input of first stage. I'm just unsure how and would very much appreciate any help.The application is driving some sensitive electro-static devices.The noise requirement comes from having 12 bit precision.300V is the maximum. 300V/4096 steps is 73mV per step 73mV/2 = +/-37mVI had not considered the noise was coming from the source. I will get back to you on that.

author: ekong
author_rank: Newbie
timestamp: November 21, 2021, 05:12:31 pm
content: I get that you're not trying being clever and you're just asking for clarification.What I mean by "noise":The input signal gets amplified by some amount. The output's average value is X but there's some +/- ripple around X. I would like that +/- ripple around X to be within 37mV. If I'm using the wrong terms please let me know.