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First, the AMT3000 is not a “serious” transmitter for long range broadcasting UNLESS it is used with the external base-loaded antenna. Using the internal tuning network and wire antenna will yield a good signal inside a house, but won’t go beyond about 200 ft., if you are lucky. So, it is only fair to compare efficiency and range when you are talking about the AMT3000 wired to use the external base-loaded antenna.
Although the Wenzel circuit and AMT3000 circuit are based on the same concept, Wenzel did not invent the circuit, nor did I. Back in the 70’s there were “Operational Transconductance Amplifier” (OTA) ICs that were capable of producing very linear AM modulation, but were not particularly suitable for even 100mW RF circuits without a lot of extra components. The innards of these things were similar to the Wenzel and AMT3000 circuit, but much more sophisticated. They used “current mirrors” which are hard to duplicate in discrete circuits. The Wenzel and AMT3000 circuits are similar but are different ways of implementing an OTA using discrete components. The concept works very well.
When the AMT3000 is set up for the external base loaded antenna, Resistor R18 is removed. The AMT3000 circuit matches the low impedance of the series connected loading coil and antenna capacitance by means of an L-net circuit. The Wenzel 1 circuit was puzzling in this respect. The Wenzel 2 circuit corrects Wenzel 1 problems by using the pot core L to series resonate with the antenna. A low-end tap on the coil matches the collector impedance instead of the L-net used in the AMT3000. The AMT3000 advantage is that the small pot core inductor is replaced by a much more efficient 3.5″ loading coil.
The Wenzel 2 design was published well after the AMT3000 was put on the market. The AMT3000 was not “based” on the Wenzel 1 design. Rather, the two designs were independent and both based on older technology.
Here are relevant links:
Wenzel 1 and 2:
A Carnegie Mellon University EE lab project in Postscript. I have a pdf version if anyone wants it. Just email me at “info at sstran dot com.”
Operational Transconductance Amplifier information:
My previous Part15.us post describing the AMT3000 RF output circuit:
Just wanted to try to clear things up a bit!