Method for time constant tuning of gm-C filters

A time constant tuning circuit, comprising:

• a) a transconductor connected to a first pair of capacitors and a second pair of capacitors through a set of switches,
• b) said transconductor input connected to a first input voltage through a first input switch and a second input voltage through a second input switch,
• c) said transconductor producing a first output current resulting from the first input voltage and a second output current resulting from the second input voltage,
• d) said first output current charging a first capacitor of the first pair of capacitors to a first voltage during a first clock period and said first voltage held on said first capacitor of said first pair of capacitors for a second clock period and a third clock period,
• e) said first output current charging a second capacitor of the first pair of capacitors to a second voltage during a third clock period and said second voltage held on said second capacitor of said first pair of capacitors for a fourth clock period and said first clock period,
• f) said second output current charging a first capacitor of the second pair of capacitors from a reference voltage to a third voltage during a second clock period and said third voltage held on said first capacitor of said second pair of capacitors for said third clock period and said fourth clock period,
• g) said second output current charging a second capacitor of the second pair of capacitors from said reference voltage to a fourth voltage during a forth clock period and said fourth voltage held on said second capacitor of the second set of capacitors for said first clock period and said second clock period,
• h) said first voltage and said second voltage combining to provide a first positive DC input to the positive input terminal of a differential amplifier,
• i) said third voltage and said fourth voltage combining to provide a second positive DC input to the negative input terminal of said differential amplifier,
• j) output of said differential amplifier controls said transconductor in a feedback loop to produce equal voltages at the inputs of the differential amplifier, and
• k) said transconductance of the transconductor set by length of said clock periods used to charge capacitors.

A transconductance tuning circuit for time constant control and tuning, comprising:

• a) an input to a transconductor connected to a first input voltage through a first input switch and a second input voltage through a second input switch,
• b) current output of said transconductor connected to a current summing operational amplifier through a first output switch,
• c) a first capacitor connected in the feedback path of the current summing operational amplifier,
• d) said first capacitor reset to zero volts during a first time period of a clock by a first feed back switch connected across said capacitor,
• e) said first capacitor charged by a first current during a second time period of the clock as a result of the first input voltage connected by the first input switch to said transconductor,
• f) a second capacitor connected to output of said current summing operational amplifier during a third time period of the clock to sample and hold a first voltage of the first capacitor,
• g) said first capacitor reset by a reference voltage during a fourth time period of the clock,
• h) said first capacitor charged by a second current during a fifth time period of the clock as a result of the second input voltage connected by the second input switch to said transconductor
• i) a third capacitor connected to output of said current summing operational amplifier during a sixth time period of the clock to sample and hold a second voltage of the first capacitor,
• j) voltage of the second capacitor and voltage of the first capacitor compared in a differential amplifier and controlled to be the same by said differential amplifier in a feedback loop controlling transconductance of the transconductor, and
• k) changing frequency of the clock changes charging time of the first capacitor and changes the transconductance of the transconductor needed to produce a current necessary to charge the first capacitor to a defined voltage level.

A method for time constant tuning of gm-C filters, comprising:

• a) resetting a first capacitor to zero volts,
• b) connecting a first input voltage to a transconductor,
• c) charging said first capacitor for one clock period to a first voltage with a first output current of said transconductor,
• d) disconnecting the transconductor from the first capacitor and holding the first voltage on said first capacitor,
• e) sampling and holding said first voltage with a second capacitor,
• f) resetting said first capacitor to a reference voltage,
• g) connecting a second input voltage to said transconductor,
• h) charging first capacitor for one clock period to a second voltage with a second output current of said transconductor,
• i) disconnecting the transconductor from the first capacitor and holding the second voltage on said first capacitor,
• j) sampling and holding said second voltage with a third capacitor,
• k) comparing said first voltage with said second voltage to produce a control voltage,
• l) controlling transconductance of said transconductor with said control voltage in a feedback loop to make the first voltage and the second voltage equal and establishing a circuit time constant,
• m) looping back to step "a" to maintain circuit time constant, and
• n) changing frequency of said clock to set a new circuit time constant and looping back to step "a" to create and maintain new circuit time constant.