Mostly at higher frequencies, above the useful range of frequency response in the feedback loop. I saw a board design that used an op amp and an n-FE T as a linear regulator. The power source to the pass FET was a switcher t o prevent a lot of dissipation and the op amp was powered from the 12V inpu t so there was plenty of drive to the gate. The reference voltage was the same as the output voltage, so with the FET in a common source configuratio n there was no gain other than the op amp which has a 1 MHz gain-BW product . So in the frequency range above 100 kHz say, I would expect the control loo p to have minimal impact on noise from the switcher. However, even though the FET configuration has no gain from gate to source, it should have gain to minimize the transmission of noise from the drain to the source. Raisin g the drain voltage will increase the current raising the source voltage. Of course that reduces the gate-source voltage which acts to prevent the so urce rise... however, that is without considering the FET capacitances. The gate-source and gate-drain capacitances will bring the gate up to allow the drain noise to transfer through to the source. Is there a reasonable way to mitigate this effect? Someone suggested adding capacitance to the g ate to swamp out the effect of the gate-drain and gate-source capacitances. If that can be done without impacting the response of the control loop, i t seems like it might work. Or is this doomed to fail because it *will* im pact the control loop at a level before does what is intended? -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code -

Usually, a regulator can be made quiet by an output capacitor. If stability is more important than accuracy, you can downsize the output capacitor and just use a slow op amp and transistor follower. Bipolar transistors have nicely low output Z as long as they're driving significant current, and a base capacitor gets capacitance-multiplied.

What limits noise in voltage regulators?

Jan 28, 2021 Last reply: 5 years ago 87 Replies

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Steve Wilson 5 years ago

Bandgaps are noisy. Recall the LM723, which had a buried zener and was recognized as one of the lowest noise regulators available.

I spent considerable time looking for a zener model that included noise without success. However, a zener has to be quieter than a bandgap.

Here is a circuit that uses a 1N751 in a variable output voltage regulator.

It has decent frequency response, -28dB at 1mHz, with a slight peak at 6 Hz,

-80dB at 120 Hz, -165dB from 100 KHz to 3 MHz, then falls from there. It probably would be much quieter than the Motorola TL431, and uses standard junk box quality parts.

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The best designs are no accident - sw

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Steve Wilson 5 years ago

I made a very significant improvement by moving the source for the main capacitance multipler from Vin to the emitter of the first stage.

The attenuation is now -26dB at 1mHz, -40dB at 10 Hz, -110dB at 120 Hz, and >-160dB from 10KHz to 3Mhz. The effect of sneak paths is reduced by having two transistors in series, and the effect of zener noise is reduced by bypassing with a 100 uF capacitor.

The capacitors are a new product by KEMET: Aluminum Organic Polymer,

100uF 25V, ESR=14mOhms ESL=2.5nh Dia = 6.3mm Height = 5.7mm Part # A768EB107M1ELAE036 $0.47 Qty 1 at Mouser and Digi-Key. Both have stock.

Polymer is the way to go.

The LTspice files are at

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The best designs are no accident - sw

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Simon S Aysdie 5 years ago

All your zip files have viruses.

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Rick C 5 years ago

It's not that they did away with it, but they don't properly account for newsgroups not being Google groups. So it is disabled because of inaction by the group administrator as there isn't one. Something about not having permission to view email addresses. Hell, I've not been able to get view email addresses to work for years!

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Rick C 5 years ago

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Simon S Aysdie 5 years ago

very cool!

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Rick C 5 years ago

Yeah, I like the line, "If you won't take care of us, won't you please, please let us be?"

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Steve Wilson 5 years ago

I download each zip and check it before posting.

The files are all LTspice ASC and PLT files.

Pretty hard to get a virus in a text file.

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