| 摘要 |
PURPOSE:To make direct coupling possible by suppressing the DC potential of a signal output to a constant value by integrating the output of the multiplex MPX demodulating circuit of a FM tuner and by negatively feeding the integral output back to the output line of a FM detecting circuit. CONSTITUTION:An IF signal is converted by FM detecting circuit 1 into a composite signal, which is directly coupled with post amplifier 2, whose amplifier output is input to MPX demodulating circuit 3 by direct coupling. Circuit 3 separates the output into right and left audio signals, which are directly coupled with the next-stage audio amplifier. The outputs of circuit 3 are mixed by resistances R3 and R4 by adding and the sum is negatively fed back to the output line of circuit 1 via DC negative feedback circuit 4. Circuit 4, consisting of operational amplifier OP2 and the integrating circuit of capacitors C1 and C2, controls switch SW by the control signal of channel selection detecting circuit 5 in channel selection to decrease the integral time constant. Consequently, the DC potential variation of the demodulation output is suppressed and DC variation in channel selection can directly be coupled without being transmitted to the audio amplifier stage. |
