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 Crss 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 Q1.
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. |