Capacitor-coupled level shifter with duty-cycle independence and supply referenced bias configuration
A circuit architecture, or topology, that provides a level shifter which is substantially independent of the duty cycle of an input signal includes an H-bridge arrangement of field effect transistors, a pair of capacitively coupled inputs terminals connected to the gates of the high-side (i.e., connected to the positive power supply) transistors and a pair of voltage dividers to set the bias voltage at the gates of the high-side transistors, wherein one side of each voltage divider is coupled to the power supply node and the other side of each voltage divider is cross-coupled to the output node of the opposite side of the H- bridge.
1. A circuit, comprising:
an H-bridge arrangement of field effect transistors, including a pair of high-side transistors connected to a power supply node, the H-bridge having a pair of output nodes, each output node disposed on an opposite side of the H-bridge;
a first pair of input terminals, each of the first pair of input terminals capacitively coupled to a gate electrode of a respective one of the high-side transistors; and
a pair of voltage dividers, wherein one side of each voltage divider is coupled to the power supply node, an intermediate node of each voltage divider is coupled to the power supply node through at least one circuit element, and the other side of each voltage divider is cross-coupled to the output node of the opposite side of the H-bridge;
wherein the at least one circuit element coupled to a first intermediate node of a first one of the pair of voltage dividers is operable to change the voltage at the first intermediate node so as to provide voltage tracking with the power supply node, and the at least one circuit element coupled to a second intermediate node of a second one of the pair of voltage dividers is operable to change the voltage at the second intermediate node so as to provide voltage tracking with the power supply node.
2. The circuit of claim 1 , wherein the at least one circuit element comprises a zener diode.
3. The circuit of claim 2 , wherein each high-side transistor is a PFET, and wherein the H-bridge includes a pair of low-side transistors, each low-side transistor connected to ground.
4. The circuit of claim 3 , wherein each low-side transistor is an NFET.
5. The circuit of claim 1 , wherein each voltage divider comprises at least three resistive elements.
6. The circuit of claim 1 , wherein the resistive elements are resistors.
7. A level-shifter circuit, comprising:
a first PFET coupled source-to-drain between a first power supply node and a first output node;
a first NFET coupled drain-to-source between the first output node and a second power supply node;
a second PFET coupled source-to-drain between the first power supply node and a second output node;
a second NFET coupled drain-to-source between the second output node and the second power supply node;
a first resistor coupled between the first power supply node and a gate electrode of the first PFET;
a second resistor coupled between the gate electrode of the first PFET and a first intermediate node;
a third resistor coupled between the first intermediate node the second output node;
a fourth resistor coupled between the power supply node and the gate electrode of the second PFET;
a fifth resistor coupled between the gate electrode of the second PFET and a second intermediate node;
a sixth resistor coupled between the second intermediate node and the first output node;
a first zener diode coupled between the first intermediate node and the power supply node; and
a second zener diode coupled between the second intermediate node and the power supply node.
8. The level-shifter of claim 7 , further comprising:
a first capacitor having a first terminal coupled to the gate electrode of the first PFET; and
a second capacitor having a first terminal coupled to the gate electrode of the second PFET.
9. The level-shifter of claim 8 , wherein a second terminal of the first capacitor is coupled to a first signal source; and a second terminal of the second capacitor is coupled to a second signal source; and wherein the magnitude of the voltage at the first power supply node is greater than the magnitude of input signals provided by the first signal source and the second signal source.
10. The level-shifter of claim 8 , wherein the first zener diode is a 6.8 volt zener diode, and the second zener diode is a 6.8 volt zener diode.
11. The level-shifter of claim 9 , wherein the first resistor has a first resistance; the second resistor has a second resistance; and the second resistance divided by the first resistance is equal to one less than the zener voltage of the first zener diode divided by magnitude of first signal source.
12. The level-shifter of claim 11 , wherein a load is connect across the first and second output nodes, and the load comprises at least a speaker.
13. A Class D amplifier suitable for audio amplification applications, comprising:
a first PFET coupled source-to-drain between a first power supply node and a first output node;
a first NFET coupled drain-to-source between the first output node and a second power supply node;
a second PFET coupled source-to-drain between the first power supply node and a second output node;
a second NFET coupled drain-to-source between the second output node and the second power supply node;
a first resistor having a first resistance coupled between the first power supply node and a gate electrode of the first PFET;
a second resistor having a second resistance coupled between the gate electrode of the first PFET and a first intermediate node;
a third resistor having a third resistance coupled between the first intermediate node the second output node;
a fourth resistor having a fourth resistance coupled between the power supply node and the gate electrode of the second PFET;
a fifth resistor having a fifth resistance coupled between the gate electrode of the second PFET and a second intermediate node;
a sixth resistor having a sixth resistance coupled between the second intermediate node and the first output node;
a first zener diode having a first zener voltage coupled between the first intermediate node and the power supply node;
a second zener diode having a second zener voltage coupled between the second intermediate node and the power supply node;
a first capacitor having a first terminal coupled to the gate electrode of the first PFET; and
a second capacitor having a first terminal coupled to the gate electrode of the second PFET.
14. The Class D amplifier of claim 13 , wherein the second resistance divided by the first resistance is equal to one less than the first zener voltage divided by magnitude of first signal source.
15. A method, comprising:
providing a bias network, the bias network having an output node, and operable to provide a bias signal at the bias network output node;
providing a first switching element, the first switching element having a control terminal coupled to the bias network output node, the first switching element further having a terminal coupled to a first power supply node, the first power supply node providing a first supply voltage; and
capacitively coupling a signal onto the control terminal, the signal having a first voltage swing, and further having a first duty cycle;
wherein the bias signal varies such that an output of the first switching element is substantially independent of the first duty cycle; and wherein the bias signal further varies such that an output of the first switching element is substantially independent of power supply fluctuations.
16. The method of claim 15 , wherein the first voltage swing and the first supply voltage are different.
17. The method of claim 15 , wherein the first switching element is a transistor.
18. The method of claim 17 , wherein the transistor is a bipolar transistor.
19. The method of claim 17 , wherein the transistor is a field effect transistor.
20. The method of claim 15 , wherein the bias network comprises three or more resistive elements.