Amplifier Help

[ekong]

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 resistance
Microchip TN5335K1-G
5K source resistance

I'm looking to add feedback to this circuit to hopefully add some stability.
Any help would be appreciated.

[t1d]

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

[ekong]

Fair enough.

I have attached the schematic and the output wave form.
The input signal is a triangle wave ~0-5V @ 1Hz
The output is inverted as expected ~2.3V-274V @ 1Hz

Let me know if there's other information that would help.

[tooki]

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 resistance
Microchip TN5335K1-G
5K source resistance

I'm looking to add feedback to this circuit to hopefully add some stability.
Any help would be appreciated.

What 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.

[rfclown]

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.

[ekong]

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   4.6698
Expected 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      19.812
Actual 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   2.3761
            273.988   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   2.2901
            273.782   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   2.3361
            273.802   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   2.5081
            273.874   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   2.2501
            274.168   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   2.2141
            273.802   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   2.1941
            273.896   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   2.3501
            274.08   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   2.3421
            273.922   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   2.5241
            274.094   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   2.2781
            273.788   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   2.4801
            274.036   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   2.4201
                                                
Average 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   0.085136095
Deviation   

(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.

[rfclown]

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.

[HB9EVI]

please explain what you consider as 'noise'?
I'm not getting clever what you mean by that.

[ekong]

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 = +/-37mV

I had not considered the noise was coming from the source. I will get back to you on that.

[ekong]

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.