** Q2 (as an RF Power Amplifier) ** Although Q2 is also a 2N2222 it has a larger package (SOT-89) than Q1 (SOT-23). The larger size dissipates heat more efficiently. Its second function, as an RF power amplifier during transmit, can cause Q2 to dissipate as much as half a watt when poorly matched to an antenna. When matched, it still needs to dissipate as much as 300 milliwatts. Any general purpose transistor in a SOT-89 package should work well in the KnightSMiTe. Only 3 components are required to make up the RF power amplifier in the KnightSMiTe. These are L2, L3 and Q2. ** L2 and L3 ** The function of L2 and L3 remain as described in the receive mixer description. ** Q2 ** The function of Q2 is only slightly altered by virtue of shorting its emitter to ground during transmit. This effectively removes C5 and R4 from the circuit as well as the redundant bias path introduced by R5 in series with switching diode D1. When keyed, the only components surrounding Q2 that remain active are L2, L3 and R3. With the emitter of Q2 at ground potential, maximum collector current flows when its base-emitter junction is forward biased by the positive excursions of the oscillator. The resultant high collector current is an amplified replica of the oscillator signal and is coupled via C6 to the antenna port J3, completing the transmitter portion of the KnightSMiTe. ** Historical Note ** Q2 biased in this manner, serving both as an RF Power amplifier and a receive mixer is the "elegance" of Oleg's (RV3GM) design. The result is a combined high performance receive mixer and RF power amplifier at low cost. Oleg's approach also avoids T/R switching losses encountered in most classical transceiver designs. It's only shortcoming is that it suffers from LO radiation during receive much like a regenerative receiver. ** R3 ** R3, although not required during transmit, provides a small base-emitter bias thru Q2 which remains cut off because most of the bias is shorted around the base emitter junction by L2. R3's presence has negligible effect during transmit. See the receive mixer discussion for its function in the KnightSMiTe.
If you are interested in the Knightlites and Amateur Radio contact Paul at