PLL CIRCUIT

PURPOSE:To increase the value of mubeta and at the same time to ensure impression of the bias voltage suited to a VCO by setting an amplifier after an LPF and also setting optionally the action point of the amplifier. CONSTITUTION:A transistor TRQ serves as a circuit which produces the voltage VX and can be varied by a variable resistance R2. Here VO=-(RF/RX)VX+ (1+ RF/RX)v1 is satisfied between the output v1 of an LPF and the output VO of an amplifier 5. Then the control sensitivity beta of a VCO can be equivalently increased through amplification of the amplifier 5. In this case, the term (-RF/RX) VX functions to secure the coincidence between the bias voltage of the VCO and a center frequency. Thus it is possible to keep the oscillation frequency of the VCO at 4fSC by controlling the resistance R2 or by maintaining v1=0.