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dynamic range of simple LC filters compared to opamps and transistors

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neazoi

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Hello,
Ignoring the losses, i would like to find out if in general an LC filter (audio for example) has a greater dynamic range than an opamp, transistor or other active device.
Also that is the limitation in dynamic range in an LC?
 

jiripolivka

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Hello,
Ignoring the losses, i would like to find out if in general an LC filter (audio for example) has a greater dynamic range than an opamp, transistor or other active device.
Also that is the limitation in dynamic range in an LC?

You are basically wrong in ignoring loss. Loss is the only limitation to the power applied to a passive filter. If the current is too high, it heats up or melts the L wires, or breaks down the C by an overvoltage. Active elements do have lower limits due to their design. Amplifiers have P-1 dB limit to their linearity.
On the other end of the dynamic range, active elements generate noise power which is higher han that of a matched resistor. On LC filters, only their loss resistance generates noise while reactive elements do not.
 
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neazoi

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You are basically wrong in ignoring loss. Loss is the only limitation to the power applied to a passive filter. If the current is too high, it heats up or melts the L wires, or breaks down the C by an overvoltage. Active elements do have lower limits due to their design. Amplifiers have P-1 dB limit to their linearity.
On the other end of the dynamic range, active elements generate noise power which is higher han that of a matched resistor. On LC filters, only their loss resistance generates noise while reactive elements do not.

I am thinking of using high dynamic range amplifiers and combine them with passive LC circuits to achieve the highest dynamic range and reduce the losses on the LC in a system. I am thinking more of light load systems like receivers, than high powered amplifiers.
For example, is it better in terms of dynamic range, to include a high dynamic range preamplifier (100db) followed by a passive LC filter, or using just an opamp filter?
 

jiripolivka

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This depends on the frequency of interest. I do not know what is your frequency and the high-power limit. Also consider that LC filters reflect back the out-of-band signals, and the amplifier must be ready to absorb it.
A high-dynamic range amplifier with the gain of 100 dB will rather amplify its noise than anything else. I would rather udse small-gain amplifier before a LC filter, then another stage and another filter.

Good opamps also offer a reasonable dynamic range including a low nose and a high output.
 

Dan Mills

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Also, the inductors (in particular) are often not really linear at high level, and building a LC filter that does not limit the IMD3 of the system can become surprisingly tricky in a high dynamic range design.

Regards, Dan.
 

jiripolivka

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Only inductors with ferrite or other cores exhibit saturation or non-linearity. For precise applications use only air-core inductors.
Some capacitors, mostly with ceramic or plastic dielectric, also exhibit polarization and non-linearity. Make sure yours are well selected.
 

Dan Mills

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Air cores are of course linear, but there are practical value limits and the increase in the amount of wire required can raise the resistance sufficiently to be problematic, fringing fields can also be an issue.

Outside very low impedance or VHF and up applications, air core is usually a poor trade off IMHO.
 

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