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How does negative impedance cobverter enable impedance matching ?

I was about to make the same argument, until I re-realized that LNAs I made for some beam position pick-ups in some accelerators here use feedback around the pre-amp to make a synthetic 50 Ohm input impedance that is quieter than a real 50 Ohm resistor. A beam position pick-up is very much like an antenna, exposed to the EM field of the passing beam.

OK, it's not a negative impedance, but it uses the same principles and it *does* make a better [lower noise] amplifier. My amplifier has an effective noise temperature of 30K or so, despite being at room temperature.

Jeroen Belleman

Reply to
Jeroen Belleman
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No, it's not the same thing. NIC uses positive feedback to cancel the capacitance of a short antenna. You are using negative feedback to force a specific input impedance. If you used positive feedback the noise would increase.

Rohde published a paper some time ago on the use of negative feedback to reduce amplifier noise. I cannot supply a reference, but some of his equipment have a dynamic range of 170 dB. Utterly amazing.

Reply to
Steve Wilson

[Snip!]

We could argue about this, and I can't afford the time, but I believe you are wrong. The way I see it is that it's not the feedback that reduces the noise. Rather, the feedback makes a grossly mis-matched resistor look like 50 Ohms when seen from the amp's input. The fact that it's mis-matched makes that its thermal noise couples poorly, which is what was sought.

In physics detectors, they go one step further: The pre-amps are usually integrating, so the 50 Ohm input impedance is obtained by feedback through a capacitor, which makes even less noise. Look up publications by Veljko Radeka, if you care.

Jeroen Belleman

Reply to
Jeroen Belleman

I wish you could stick around. I learn a lot from you.

Integrating uses negative feedback. This reduces noise as Rohde has showed.

I do care about noise. I found two publications by Radeka:

Detector Signal Processing

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LOW-NOISE TECHNIQUES IN DETECTORS

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cl_part_sci_1988.pdf

Radeka uses uses negative feedback and integration. Integration reduces noise.

Can you supply links to show how to obtain a 50 ohm input impedance with feedback through a capacitor? I can invision generating a 50 ohm input impedance by using a 50 ohm resistor in series with the negative input of an op amp, but I don't see the need for a capacitor. Please enlighten me.

I'm very interested in methods to reduce thermal noise.

Reply to
Steve Wilson

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