Transconductance varying circuit of transconductor circuit, varying bandwidth filter circuit using the same and digital tuning circuit of transconductor-capacitor filter

A tuning circuit of filter having a transconductor outputting current proportional to input voltage and varying capacitor being connected with output nodes and between grounds of said transconductor and having capacitance varying in accordance with level of control signal, said tuning circuit comprising:

• a transconductor outputting current proportional to applied input voltage;
• a comparator comparing voltage applied to input node to output signal to up-signal output node if the voltage of said input node is higher and to output signal to down-signal output node otherwise;
• a counter being connected with said up-signal output node and said down-signal output node of said comparator, increasing or decreasing the level of output signal by a predetermined amount in response to said up-signal and said down-signal, said output signal being inputted as control signal to varying capacitor of said filter;
• a varying capacitor being connected with output node of said transconductor and between grounds of said tuning circuit, capacitance thereof varying in accordance with the level of the output signal of said counter;
• means for maintaining the output voltage of the transconductor of said tuning circuit as substantial zero for first period;
• means for inputting said input voltage to said transconductor of said tuning circuit for second period;
• and
• means for inputting output voltage of said transconductor of said tuning circuit to the input node of said comparator for third period.