Transconductor circuits
View Patent ↗A transconductor circuit, such as a gyrator filter, comprises an arrangement of balanced class AB transconductors, capacitors and floating MOS resistors formed by MOS transistors operating in their triode region. Tuning of the filter is effected by varying a common supply rail voltage. The circuit includes a means for producing a voltage offset from the common mode voltage of the class AB transconductors. The offset voltage is supplied to a parallel arrangement of a class AB transconductor having a transconductance and the source-drain path of a MOS transistor emulating a MOS resistor. The current output of the parallel arrangement is integrated and supplied as a control voltage to the gate electrode of the MOS transistor. By loop action the control voltage is adjusted and supplied to the floating MOS resistors.
1. A transconductor circuit comprising a transconductor ( 30 , 35 ) coupled to a supply rail voltage source (V dda ) and at least one MOS resistor (R) comprising a MOS transistor ( 84 , 86 ) whose source-drain path is coupled to the transconductor, resistance tuning means ( 96 ) coupled to the supply voltage rail and having an output for supplying a control voltage (cntrl) to a gate electrode of the MOS transistor, whereby the resistance value of the MOS resistor tracks changes in the transconductance of the transconductor due to tuning effected by altering the supply rail voltage (V dda ).
2. A transconductor circuit including a balanced transconductor comprising a first and a second single ended transconductors ( 30 , 35 ), each having an input ( 40 , 45 ) and an output ( 20 , 25 ), MOS resistors respectively comprising MOS transistors ( 84 , 86 ) having their source-drain paths coupled between respective first and second input terminals ( 10 , 15 ) and the inputs ( 40 , 45 ) of the first and second single ended transconductor ( 30 , 35 ), a differential transconductance ( 200 ) coupled between the first and second input terminals ( 10 , 15 ) and the inputs ( 40 , 45 ) of the single ended transconductor ( 30 , 35 ), a source of a common supply rail voltage (V dda ), and resistance tuning means ( 96 ) coupled to the common supply rail voltage source for supplying a control voltage (cntrl) to gate electrodes of the MOS transistors for tuning the resistance of the MOS resistors ( 84 , 86 ).
3. A circuit as claimed in claim 1 , characterized in that the or each MOS resistor comprises a NMOS transistor.
4. A filter circuit including a balanced transconductor comprising a first and a second single ended transconductor means ( 30 , 35 ), each having an input ( 40 , 45 ) and an output ( 20 , 25 ), first and second MOS resistors respectively comprising first and second MOS transistors ( 84 , 86 ) having their source-drain paths coupled between respective first and second input terminals ( 10 , 15 ) and the inputs ( 40 , 45 ) of the first and second single ended transconductor means ( 30 , 35 ), a differential transconductance ( 200 ) coupled between the first and second input terminals ( 10 , 15 ) and the inputs ( 40 , 45 ) of the single ended transconductor means ( 30 , 35 ), and a source of a common supply rail voltage (V dda ), frequency tuning means for tuning the filter by adjusting the common supply rail voltage and resistance tuning means ( 96 ) coupled to the common supply rail voltage source for supplying a control voltage (cntrl) to gate electrodes of the first and second MOS transistors for tuning the resistance of the MOS resistors ( 84 , 86 ).
5. An integrated circuit having a transconductor circuit as claimed in claim 1 .
6. An electronic device having a transconductor circuit as claimed in claim 1 .