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Micro Channel Plate (MCP) audio amplifier

I normally use MCP's for electron gain in low light imaging CCD's for research where faint light signals need to be amplified. MCP are also used in military night vision goggles. They fit between the front objective lens and eyepiece so you get a sense of how small they are. The MCP is basically an array of microcapillary glass tubes coated with a semiconductor so that when biased at high potential exhibit photo electron gain. The advantage is they are very compact and use very little power. For optical purposes they are coupled with a photocathode that emits electrons when illuminated. For electrical signals, we can skip the photocathode stage. MCP's also exhibit a remarkable property of acting as a high speed gate as fast as nanoseconds. As an amplifier, there is nothing simpler as the gain occurs in one stage with level of gain proportional to bias voltage. I had the idea that this could be used as an audio amplifier or preamplifier stage. A quick Google search shows a patent exists but I wonder if anyone here has tried it or has heard one in person.

MCP's are expensive but may offer some unique advantageous as a vacuum based very high gain device.

Here is the patent:
Patent US6895096 - Microchannel plate audio amplifier - Google Patente

Typical MCP's are produced by companies like Hammamatsu.
MCP (microchannel plates) | Hamamatsu Photonics

Anyhow, was curious if anyone has tried this.
 
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Seems a rather expensive method, and considering you aren't intensifying images, the micro channels would be rather wasted on such a thing...

Secondary emission has been looked at before, like EFP60. The advantage (higher gain per part) is about balanced with the greater transit time (more distance for the electron beam to travel --> the signal gets smeared out at high frequencies, keeping gain-bandwidth about constant), so the increased supply voltage and current, and device complexity, isn't worth the bother.

Beyond that, I don't know what "advantages" might be intended. Who needs just gain? Who needs gain-bandwidth? What's it matter "what it sounds"? Such a thing will never be made commercially if it cannot compete with semiconductors. The only applications where these sorts of things remain are where the physics prevents direct integration, like image intensifiers and particle detectors.

Tim
 
The current on a small MCP is not very much as it is designed to multiply tens of photo electrons to a million. A million electrons is not much current. However, I was thinking it would be a good pre amp stage for very weak signals like a phono cartridge or certain condensor mics. You can always use a larger area MCP for more current.

Another idea for election gain that is solid state is called Electron Multiplication (EM) that is used in low light CCD's as give charges are transferred from the pixel to the readout charge amp. By using a special semiconductor channel that had HV (500v) bias applied, small single electron signals can get gain of up to a million as it passes through the HV bias region. This is made by both TI and EEV. Wonder why the same gain tech has not been applied to a straight "wire" with EM gain? That would be a much more compact and low cost device. it's different than a transistor or a MOSFET because there is no "gate" and the signal is moved via charge transfer from bucket to bucket at rates up to 30MHz - so should lend itself to high speed analog to digital converter with gain. Or vice versa.
 
My guess is that phono cartridges could produce more current than the output of a photomultiplier type device already, but if their noise figure is superior, that would raise interest.

This HV thing about secondary emission multiplication, avalanching etc., started me thinking in a different, but somewhat similar direction. Magnetic material (square hysteresis) can avalanche magnetic domains too when the square hysteresis "knee" is exceeded. If one could pre-charge magnetic material just up to the knee, then a small incremental signal could push it over the "edge" and induce current in a low impedance winding. If one could arrange for a steady supply of pre-charged magnetic material, then one could have a continuous amplifier.

With that in mind, suppose one had a toroid of such material, constantly rotated around its major axis by a motor/turntable. Then a cut-out C core could fitted from edge to edge of the toroid (with tiny air gaps), and would have a pick-up winding on it. Then another cut-out C core with strong neodymium magnets would be fitted edge to edge to fully magnetize the toroid across the width ahead of that. An intervening cut-out C core, between those two, would have a winding on it with sufficient DC current to nearly, but not quite reverse the field over the hysteresis edge. An additional small DC signal in series would provide the final push, to push the field over the edge, causing a big flux change.

I suppose it would take two of these units then (still using the same rotating ring, just opposite sides) with opposite DC field polarity to get an AC output signal.

Does seem to have a problem with continuous flux change polarity occuring in the same pick-up core though.

So maybe two of these toroid rings, each with opposite DC flux, would have to pass through the same pick-up unit. Then AC flux changes could be induced in the pick-up (by flipping the field in either ring at one time). No doubt some much more elegant way to do this. Just idle dreaming at this point.

There is actually an existing device that can do just this. It is just a DC generator with signal applied to it's field winding. Rather noisy output however with a commutator. It is possible to machine the armature pole pieces so as to shape the commutated waveforms slightly, for noiseless commutation. (just some HF glitches left, could filter them out)
 
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jcx

Member
2003-02-17 7:38 pm
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avalanche multiplication is very noisy - and you have the impedance problem too

other than wanting vacuum tech I don't see any circuit need in audio

"magnetic amplifiers" are a thing - mostly replaced with power semiconductors

another weird solution looking for a problem is the ac exited diode bridge parametric amplifier - a early Philbrick op amp tech