Posted on Aug 5, 2012 6885
This transmitter consists of a keyed crystal oscillator/driver and a high efficiency final, each with a TMOS Power FET as the active element. The total parts cost less than $20, and no special construction skills or circuit boards are required. The Pierce oscillator is unique because the high 4ss of the final amplifier power FET, 700 -1200 pF, is used as part of the capacitive feedback network. In fact, the oscillator will not work without Q2 installed.
The MPF910 is a good choice for this circuit because the transistor is capable of driving the final amplifier in a switching-mode, while still retaining enough gain for oscillation. To minimize cost, a readily-available color burst TV crystal is used as the frequency-determining element for Ql. An unusual 84% output efficiency is possible with this transmitter. Such high efficiency is achieved because of the TMOS power FET"s characteristics, along with modification of the usual algorithm for determining output matching.