| 摘要 |
PURPOSE:To prevent previously the cuoff of a carrier wave signal for an amplitude modulating circuit which is suitably used when the reproduction signal of a magnetic video recording/reproducing device is supplied to a TV receiver, by controlling the modulation factor after detecting the variance of the input signal which is supplied to a modulating circuit. CONSTITUTION:When a reproduction signal VIN is lowered less than the reference voltage V2, an overmodulation preventing circuit 11 is actuated to turn off transistors Q4 and Q6. Then a current I2 is flowed from a +VCC power supply via a TRQ5 and a resistance R6. Then a TRQ7 is turned on by the voltage V2, and therefore a current route of a current I3 is formed from the +VCC power supply via a TRQ8 or Q9 and TRsQ7 and Q2. In this case, I2<I3 is satisfied. When the signal VIN is reduced less than the V2, the current I3 flows to a load resistance RL. Thus the voltage drop of the load RL is set at I3.RL which is larger than I2.RL. Therefore an output signal VOUT is slightly less than the highest level while the signal VIN is lower than the V2. In other words, the cutoff of a carrier wave signal fv is prevented by flowing forcibly the current I3. |
