A balanced modulating circuit receiving an input signal to be modulated for performing balanced modulation of said input signal under control of a control signal having a logical level varying at a carrier wave frequency to output a modulated signal as an output signal, said balanced modulating circuit comprising:
- a signal input terminal receiving said input signal,
- a control signal input terminal receiving said control signal,
- a reference voltage source for providing a reference voltage of a predetermined level,
- a signal output terminal,
- an operational amplifier having positive and negative inputs and an output connected to said signal output terminal,
- a first resistor portion connected to said negative input of said operational amplifier,
- a second resistor portion connected to said positive input of said operational amplifier and having the same resistance as said first resistor portion,
- a first switch connected in series with said first resistor portion and provided between said signal input terminal and said negative input of said operational amplifier for turning itself on and off as a function of said control signal,
- a second switch provided between said reference voltage source and said first resistor portion for turning itself on and off as a function of said control signal,
- a third switch provided between said signal input terminal and said second resistor portion for turning itself on and off as a function of said control signal,
- a fourth switch provided between said reference voltage source and said second resistor portion for turning itself on and off as a function of said control signal, said first to fourth switches having the same internal construction, said control signal being applied to said first to fourth switches such that the ON/OFF state of said first and fourth switches is constantly opposite to that of said second and third switches,
- a third resistor portion provided in a feedback path from said output of said operational amplifier to said negative input thereof, the resistance ratio of said third resistor portion to said first resistor portion being N where N is a natural number,
- a first corrective switch portion including corrective switches connected in said N-number in series and connected in series with said third resistor portion in said feedback path, said N-number of corrective switches having the same internal construction as said first to fourth switches and being constantly set to an ON-state under the same conditions as said first to fourth switches in the ON-state,
- a fourth resistor portion provided in a reference voltage positive input path from said positive input of said operational amplifier to said reference voltage source, the resistance ratio of said fourth resistor portion to said second resistor portion being N, and
- a second corrective switch portion including corrective switches connected in said N-number in series and connected in series with said fourth resistor portion in said reference voltage positive input path, said N-number of corrective switches having the same internal construction as said first to fourth switches and being constantly set to the ON-state under the same conditions as said first to fourth switches in the ON-state.