Cascode bias of power MOS transistors
There is disclosed a driver circuit for a power amplifier of class D type having a segmented architecture with at least one current branch which can be powered down in a low power mode of operation of the circuit. The branch comprising a switch with a cascode MOS transistor, the circuit further comprises a bias circuitry adapted for dynamically generating a dynamic bias control signal so as to cause the cascode MOS transistor of the switch to be ‘Off’ in the low power mode.
1. Driver circuit for a power amplifier of class D type comprising:
at least a first output terminal;
a high power supply rail receiving a high power supply voltage and a low power supply rail receiving a low supply voltage;
at least first and second current branches arranged in parallel between the high power supply rail and the low power supply rail, each comprising at least a first switch and being adapted for driving power to the output terminal, and said second current branch being powered in a first configuration of the driver circuit and being powered down in a second configuration of the driver circuit;
wherein:
the first switch of the first branch comprises a switching Metal Oxyde Semiconductor, MOS, transistor of a first conduction type, N or P, controlled by a first switching signal, in series with a cascode MOS transistor of said first conduction type, controlled by first static bias control signal;
the first switch of the second branch comprises a switching MOS transistor of the first conduction type, controlled by a second switching signal, in series with a cascode MOS transistor of said first conduction type, controlled by a first dynamic bias control signal; and,
the driver circuit further comprising a first bias circuitry adapted for dynamically generating the first dynamic bias control signal from the first static bias control signal and from at least one additional control signal enabling operation of the driver circuit in the second configuration, said first bias circuitry being further adapted to cause the cascode MOS transistor of the first switch of the second branch to be ‘Off’ in said second configuration of the driver circuit.
2. The driver circuit of claim 1 , wherein the first bias circuitry is adapted for causing a gate terminal of the cascode MOS transistor of the first switch of the second branch to be connected to the lower voltage between a voltage at the output terminal and a voltage of the first static bias control signal.
3. The driver circuit of claim 1 , wherein the first conduction type is the N conduction type, whereby the switching MOS transistor of the first switch of the second branch is a NMOS transistor of a given deep submicron CMOS technology, and the cascode MOS transistor of said first switch of the second branch is a also NMOS transistor.
4. The driver circuit of claim 3 , wherein the first bias circuitry is adapted for causing the gate terminal of the cascode MOS transistor of the first switch of the second branch to receive the first static bias control signal if the voltage at the output node goes above the voltage of said first static bias control signal plus a threshold voltage of NMOS transistors in the CMOS technology, or to be connected to the output terminal if the voltage at the output node goes below the first static bias control signal minus said threshold voltage.
5. The driver circuit of claim 1 , further comprising a first additional circuitry adapted for controlling a voltage at a common source node of the switching MOS transistor and the cascode MOS transistor of the first switch of the second branch, so that said voltage is kept at the voltage of the first static bias control signal.
6. The driver circuit of claim 1 , wherein:
the first current branch further comprises a second switch having a switching MOS transistor of a second conduction type, opposite to the first conduction type, controlled by a second switching signal, in series with a cascode MOS transistor of a second conduction type, opposite to the first conduction type, controlled by a second static bias control signal,
the second current branch further comprises a second switch having a switching MOS transistor of a the conduction type, controlled by the second switching signal, in series with a cascode MOS transistor of said second conduction type, controlled by a second dynamic bias control signal,
the driver circuit further comprising a second bias circuitry adapted for dynamically generating the second dynamic bias control signal from the second static bias control signal and from the at least one additional control signal enabling operation of the driver circuit in the second configuration, said second bias circuitry being further adapted to cause the cascode MOS transistor of the second switch of the second branch to be ‘Off’ in said second configuration of the driver circuit.
7. The driver circuit of claim 6 , wherein the second bias circuitry is adapted for causing a gate terminal of the cascode MOS transistor of the second switch of the second branch to be connected to the upper voltage between a voltage at the output terminal and a voltage of the second static bias control signal.
8. The driver circuit of claim 6 , wherein the first conduction type is the N conduction type and the second conduction type is the P conduction type, whereby the switching MOS transistor of the second switch of the second branch is a PMOS transistor of a given deep submicron CMOS technology, and the cascode MOS transistor of said second switch of the second branch is a also PMOS transistor.
9. The driver circuit of claim 8 , wherein the second bias circuitry is adapted for causing the gate terminal of the cascode MOS transistor of the second switch of the second branch to receive the second static bias control signal if the voltage at the output node goes below the voltage of said second static bias control signal minus a threshold voltage of PMOS transistors in the CMOS technology, or to be connected to the output terminal if the voltage at the output node goes above the first static bias control signal plus said threshold voltage.
10. The driver circuit of claim 6 , further comprising a second additional circuitry adapted for controlling a voltage at a common source node of the switching MOS transistor and the cascode MOS transistor of the second switch of the second branch, so that said voltage is kept at the voltage of the second static bias control signal.
11. Power amplifier of class D type comprising a driver circuit according to claim 1 .
12. Integrated Circuit realized in a deep submicron CMOS technology comprising a digital part and an analog part, wherein the analog part comprises the power amplifier of claim 11 .
13. Wireless product, comprising the Integrated Circuit of claim 12 .