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mixed_signal Senior Member Offline Posts: 183	Keithley low noise source vs potentiometer bias: Noisier? Apr 22^nd, 2014, 7:19am Hi , Which biasing will be more noisier? Resistor biasing using a potentiometer (acting as current source) External Keithley low noise source. Since resistor is passive , it should be low noise than keithley, right?
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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Keithley low noise source vs potentiometer bias: Noisier? Reply #1 - Apr 22^nd, 2014, 3:32pm A current source is always noisy due to high impedance characteristics. You want ultra low noise? Use a battery.
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mixed_signal Senior Member Offline Posts: 183	Re: Keithley low noise source vs potentiometer bias: Noisier? Reply #2 - Apr 22^nd, 2014, 9:36pm Hi loose-electron, You mean resistor for biasing source and battery for Vdd?
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loose-electron

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Re: Keithley low noise source vs potentiometer bias: Noisier?
Reply #3 - Apr 23^rd, 2014, 2:56pm

back when I was characterizing some ultra low noise things, we found that using a battery as the power source, and fixed resistor dividers with filter capacitance got it done.

Resistors/Temperature low enough in value that thermal noise did not rise up, and the capacitor such that the BW was like 0.01Hz or something similar.

We were looking at flicker noise at the time and we could actually see the transitions associated with single electrons from a CMOS amplifier.

Neat stuff, electricity is discrete units of charge if you look close enough. 8-)

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mixed_signal

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Re: Keithley low noise source vs potentiometer bias: Noisier?
Reply #4 - Apr 23^rd, 2014, 9:48pm

Hi loose electron,

Honestly speaking it is my dream to have that experience of observing noise from a single electron. I shall definitely try and let you know .

I want to bias a baseband buffer (30MHz) at the end of a receiver. The buffer is a source follower to drive 50 ohm. I want to use attached biasing network. I dont want to use resistive voltage divider because MOS threshold may vary and cause problems in onchip implementation. Do you agree? is my scheme ok?

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loose-electron Senior Fellow Offline Best Design Tool = Capable Designers Posts: 1638 San Diego California	Re: Keithley low noise source vs potentiometer bias: Noisier? Reply #5 - Apr 24^th, 2014, 4:15pm should work, but I expect the main source of noise in Vout is the active load (NMOS current source) used in the output amplifier. Do some noise simulations and determine what your noise source are.
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