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- Jun 21, 2012
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- 4,886
What I do find encouraging is that the engineers and scientists who actually design BJTs, and other semiconductor devices, do know what is going on. However, once the packaged device leaves the fab, it becomes a "black box" and it is open season for all sorts of "explanations" for what is going on inside.
You want to test your "knowledge" of BJTs? Try building a temperature-compensated logarithmic amplifier with six decades of response, say from one nanoampere to one milliampere of input current, using two BJTs and a few low bias-current op-amps. You might want to start with Linear Technology LT1008 op-amps or equivalent and carefully match, and temperature-control, your BJTs. Back-to-back epoxy bonding of the two BJTs and a small temperature-controlled oven will help. When I was building log amps in the 1970s, I could purchase matched NPN transistors built on an internally heated substrate, all in a nice TO-5 metal can. That was almost too easy, but I haven't been able to find any lately. The 24-bit delta-sigma ADC pretty much killed any further interest in precision log amps for data acquisition, although vanilla log-amps are still used for analog signal processing.
It is advisable to use ultra-clean ceramic circuit boards and Teflon insulation to control leakage currents. I had to go with FR4 boards, Teflon insulated solid wire, Teflon insulated stand-off terminals, and Teflon-insulated sockets. I don't think I ever quite reached six decades without substantial drift on the low end, but what I built was good enough to monitor a PIN photo-diode output.
It was an interesting one-off design experience with no follow-up. The effect they were looking for with the photo-diode (and also with a photo-multiplier tube or PMT) was a time delay in the Faraday rotation of linearly polarized light passing through a liquid sample, said delay to be characteristic of its chemical structure. Apparently the effect does not exist, or at least was not reproducible with our equipment.
If interested, you can Google the Allison Magneto-optical Effect. It is an example of pathological science. Fred Allison (July 4, 1882 - August 2, 1974) went to his grave still firmly believing he had discovered a unique and simple way to identify trace elements and compounds. AFAIK, the U.S. Air Force funded the last experiments trying to prove that Allison's apparatus (carefully replicated in a lab in Area B at Wright-Patterson AFB) worked and could be used to find trace contaminants in hydraulic fluid and engine lubricating oils. It would have been worth millions if that had panned out. The apparatus was dirt cheap compared to conventional spectroscopic analysis of fluid contaminants. I have a copy of the final report.
Part of that research funding payed my minuscule technician salary for a few months, along with a couple of PhD professors, a graduate-student engineer-employee, another full-time electrical engineer, and who knows how many civil servants and Air Force officers "supervising" the program over the course of a year or so. But that's the cost of basic research. Sometimes you spend big bux and it leads to a dead end.
Maybe someone will resurrect Allison's experiments with more modern electronics and better electro-optics than what we had available in the 1970s, which was not much more than what Allison used in the 1930s... some Lecher lines with trolleys to hold hand-wound coils containing the sample and a "reference" sample; a spark-gap discharge to send an electrical pulse down the Lecher lines; a pair of rotate-able, crossed, Nicol linear polarizing prisms to extinguish the light from a visible red-light HeNe laser equipped with a high-speed shutter (Allison used a spark gap illumination source); some photo detectors (Allison used his Mark I eyeball); and some Tektronix oscilloscopes with film cameras to record the traces. The laser shutter was synchronized to open with the spark-gap discharge that pulsed the Lecher lines.
You started out with two coaxial "reference" samples, typically carbon disulfide that has a known Verdet constant and "zeroed" the Nicol prisms, one in front of the light source and the other at the other end of the Lecher lines, for extinction of the laser beam. Then, pulsing the Lecher lines, the laser beam polarization is rotated when it passes through the sample in the first coil. After a time-of-flight delay the beam passes through the second coil which rotates the polarization back to its original orientation to extinguish the beam passing through both coils in succession. So far, so good. Just a complicated demonstration of Faraday rotation in a liquid.
You can demonstrate Faraday rotation with just one coil and DC excitation of the coil by observing how far the second (analyzing) prism has to be rotated to extinguish the beam again when there is current (and a magnetic field) in the coil. The Verdet "constant" is usually measured this way. What Allison was claiming was (1) a delay in the onset of the Faraday effect, (2) that his Lecher-line apparatus could measure this delay, and (3) the measured delay was characteristic of the elemental composition of the sample. It is said that Ernest O. Lawrence, the inventor of the cyclotron, visited Allison's lab one day, peered into the telescope, adjusted the Lecher lines, then turned around and said, "Gentlemen, this man is perpetrating a fraud!" That should have ended Allison's career, but he was adamant that his apparatus worked and spent the rest of his life trying to prove it.
Fast forward now to the 1970s. Fred is dying but he has the ear, and the funding, of someone willing to try his apparatus one more time. Now you build his apparatus and replace one of the samples with an unknown sample and measure the trolley positions where extinction occurs. Allison claimed these positions (there were supposed to be several) were indicative of sample composition... right down to parts per billion sensitivity. Problem was, it was difficult to find the extinction point while manipulating the sample trolley along the Lecher lines. You were looking for something that wasn't there, viz., the absence of light at each extinction position. Multiple observers reported different extinction points. Instrumentation with photo-diodes and PMTs yielded no conclusive results, although measurements were difficult because of the large RF field around the pulsed Lecher lines. Maybe better shielding was needed?
It is a simple experiment using simple technology, but I sure won't be involved in resurrecting it. I still have yet to build my cold-fusion design that will forever free me from the local power grid. Maybe the profits from that would allow me to revive a search for the Allison Effect. And I don't mean audio acoustics, which is the first thing a Google search turns up..
So, if construction of a log-amp is too much trouble, and five decades of input range is good enough for your application, purchase a Maxim MAX4206 with all the design done for you. But at least try to play around with log-amp circuits when you are ready for it. They would make a nice LDR interface to a microprocessor ADC even without temperature compensation. Your spiffy LDO will help you see what is going on. Enjoy your electronics adventure. Have fun!
Hop
You want to test your "knowledge" of BJTs? Try building a temperature-compensated logarithmic amplifier with six decades of response, say from one nanoampere to one milliampere of input current, using two BJTs and a few low bias-current op-amps. You might want to start with Linear Technology LT1008 op-amps or equivalent and carefully match, and temperature-control, your BJTs. Back-to-back epoxy bonding of the two BJTs and a small temperature-controlled oven will help. When I was building log amps in the 1970s, I could purchase matched NPN transistors built on an internally heated substrate, all in a nice TO-5 metal can. That was almost too easy, but I haven't been able to find any lately. The 24-bit delta-sigma ADC pretty much killed any further interest in precision log amps for data acquisition, although vanilla log-amps are still used for analog signal processing.
It is advisable to use ultra-clean ceramic circuit boards and Teflon insulation to control leakage currents. I had to go with FR4 boards, Teflon insulated solid wire, Teflon insulated stand-off terminals, and Teflon-insulated sockets. I don't think I ever quite reached six decades without substantial drift on the low end, but what I built was good enough to monitor a PIN photo-diode output.
It was an interesting one-off design experience with no follow-up. The effect they were looking for with the photo-diode (and also with a photo-multiplier tube or PMT) was a time delay in the Faraday rotation of linearly polarized light passing through a liquid sample, said delay to be characteristic of its chemical structure. Apparently the effect does not exist, or at least was not reproducible with our equipment.
If interested, you can Google the Allison Magneto-optical Effect. It is an example of pathological science. Fred Allison (July 4, 1882 - August 2, 1974) went to his grave still firmly believing he had discovered a unique and simple way to identify trace elements and compounds. AFAIK, the U.S. Air Force funded the last experiments trying to prove that Allison's apparatus (carefully replicated in a lab in Area B at Wright-Patterson AFB) worked and could be used to find trace contaminants in hydraulic fluid and engine lubricating oils. It would have been worth millions if that had panned out. The apparatus was dirt cheap compared to conventional spectroscopic analysis of fluid contaminants. I have a copy of the final report.
Part of that research funding payed my minuscule technician salary for a few months, along with a couple of PhD professors, a graduate-student engineer-employee, another full-time electrical engineer, and who knows how many civil servants and Air Force officers "supervising" the program over the course of a year or so. But that's the cost of basic research. Sometimes you spend big bux and it leads to a dead end.
Maybe someone will resurrect Allison's experiments with more modern electronics and better electro-optics than what we had available in the 1970s, which was not much more than what Allison used in the 1930s... some Lecher lines with trolleys to hold hand-wound coils containing the sample and a "reference" sample; a spark-gap discharge to send an electrical pulse down the Lecher lines; a pair of rotate-able, crossed, Nicol linear polarizing prisms to extinguish the light from a visible red-light HeNe laser equipped with a high-speed shutter (Allison used a spark gap illumination source); some photo detectors (Allison used his Mark I eyeball); and some Tektronix oscilloscopes with film cameras to record the traces. The laser shutter was synchronized to open with the spark-gap discharge that pulsed the Lecher lines.
You started out with two coaxial "reference" samples, typically carbon disulfide that has a known Verdet constant and "zeroed" the Nicol prisms, one in front of the light source and the other at the other end of the Lecher lines, for extinction of the laser beam. Then, pulsing the Lecher lines, the laser beam polarization is rotated when it passes through the sample in the first coil. After a time-of-flight delay the beam passes through the second coil which rotates the polarization back to its original orientation to extinguish the beam passing through both coils in succession. So far, so good. Just a complicated demonstration of Faraday rotation in a liquid.
You can demonstrate Faraday rotation with just one coil and DC excitation of the coil by observing how far the second (analyzing) prism has to be rotated to extinguish the beam again when there is current (and a magnetic field) in the coil. The Verdet "constant" is usually measured this way. What Allison was claiming was (1) a delay in the onset of the Faraday effect, (2) that his Lecher-line apparatus could measure this delay, and (3) the measured delay was characteristic of the elemental composition of the sample. It is said that Ernest O. Lawrence, the inventor of the cyclotron, visited Allison's lab one day, peered into the telescope, adjusted the Lecher lines, then turned around and said, "Gentlemen, this man is perpetrating a fraud!" That should have ended Allison's career, but he was adamant that his apparatus worked and spent the rest of his life trying to prove it.
Fast forward now to the 1970s. Fred is dying but he has the ear, and the funding, of someone willing to try his apparatus one more time. Now you build his apparatus and replace one of the samples with an unknown sample and measure the trolley positions where extinction occurs. Allison claimed these positions (there were supposed to be several) were indicative of sample composition... right down to parts per billion sensitivity. Problem was, it was difficult to find the extinction point while manipulating the sample trolley along the Lecher lines. You were looking for something that wasn't there, viz., the absence of light at each extinction position. Multiple observers reported different extinction points. Instrumentation with photo-diodes and PMTs yielded no conclusive results, although measurements were difficult because of the large RF field around the pulsed Lecher lines. Maybe better shielding was needed?
It is a simple experiment using simple technology, but I sure won't be involved in resurrecting it. I still have yet to build my cold-fusion design that will forever free me from the local power grid. Maybe the profits from that would allow me to revive a search for the Allison Effect. And I don't mean audio acoustics, which is the first thing a Google search turns up..
So, if construction of a log-amp is too much trouble, and five decades of input range is good enough for your application, purchase a Maxim MAX4206 with all the design done for you. But at least try to play around with log-amp circuits when you are ready for it. They would make a nice LDR interface to a microprocessor ADC even without temperature compensation. Your spiffy LDO will help you see what is going on. Enjoy your electronics adventure. Have fun!
Hop
