Lc Sine-Wave Generator
Posted on Nov 4, 2012 12427
This compact LC oscillator offers a frequency range of about 1 kHz to almost 9 MHz and a low-distortion sine-wave ou
tput. The heart of the circuit is series-resonant circuit L1/C2/C3 in the feedback loop of amplifiers T1/T2. Transistor T2, which is connected as an emitter follower, serves as impedance converter, whereas Tl, connected to a common base circuit, is a voltage amplifier whose amplification is determined by the impedance of LI in its collector circuit and the emitter current. The feedback loop runs from the collector of Tl via the junction of capacitive divider C1/C2, source-follower BS170, and the input impedance is formed by R1/C4, The whole is strongly reminiscent of a Colpitis circuit. The signal is also taken to the output terminal via C5. Of particular interest is the amplitude control by the current source. The signal is rectified by two Schottky diodes, smoothed by C9, then used to control the current through T3. The gain of amplifier Tl is therefore higher at low input levels than at higher ones. This arrangement ensures very low distortion, because the amplifier cannot be overdriven. With values as shown, it extends from 863 Hz ( = 10 H) to 8.630 MHz (^=100 H). The unit can be used to measure the Q of inductors. To that end, a potentiometer is connected in parallel with LI and adjusted so that the current through the amplifier is doubled.