Operational transconductance amplifier input driver for class D audio amplifiers

A class D audio amplifier, comprising:

• an operational transconductance preamplifier, having first and second inputs and first and second outputs;
• first and second resistors, connected to first and second inputs of the operational transconductance preamplifier, for receiving a capacitively coupled input signal;
• and
• an audio output amplifier, comprising:
• an integrating amplifier, having inputs coupled to the first and second outputs of the operational transconductance preamplifier;
• a pulse-width modulating amplifier, having inputs coupled to outputs of the integrating amplifier, for generating a pulse-width modulated output signal;
• and
• first and second feedback resistors, each connected between an output of the pulse-width modulating amplifier and an input of the integrating amplifier,
• wherein the operational transconductance preamplifier comprises:
• a differential operational amplifier, for presenting a differential signal at first and second outputs responsive to a differential signal at first and second inputs;
• a first output transistor, having a gate coupled to the first output of the differential operational amplifier, and having a source/drain path;
• a second output transistor, having a gate coupled to the second output of the differential operational amplifier, and having a source/drain path;
• first bias current circuitry connected in series with the source/drain path of the first output transistor;
• second bias current circuitry connected in series with the source/drain path of the second output transistor;
• a first feedback path connected between the drain of the first output transistor and the first input of the differential operational amplifier;
• and
• a second feedback path connected between the drain of the second output transistor and the second input of the differential operational amplifier;
• wherein the first and second outputs of the operational transconductance preamplifier are at drains of the first and second output transistors.

A method of amplifying a signal in an audio channel, comprising the steps of:

• coupling an input signal through a capacitor in series with a resistor, to an input of an operational transconductance amplifier;
• conducting a bias current through the source/drain path of at least one output MOS transistor;
• operating the operational transconductance amplifier to drive the gate of the at least one output MOS transistor responsive to the coupled input signal, to generate an output current;
• coupling a drain voltage from the at least one output MOS transistor as feedback to at least one input of the operational transconductance amplifier;
• applying the output current to at least one input of an integrating amplifier;
• applying the output of the integrating amplifier to a pulse-width modulating amplifier to generate a pulse-width modulated output signal.