Operational transconductance amplifier input driver for class D audio amplifiers
First Claim
1. A preamplifier circuit, comprising:
- a differential operational amplifier, for presenting a differential signal at first and second outputs responsive to a differential signal at first and second inputs;
first and second resistors, connected to the first and second inputs of the differential operational amplifier, respectively;
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.
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Accused Products
Abstract
An audio preamplifier (10) based on an operational transconductance amplifier, in combination with a class D audio output amplifier (12) is disclosed. The input signal is coupled to the preamplifier (10) through a capacitor (14) in series with a resistor (17) that sets the transconductance of the preamplifier. The preamplifier (10) includes a differential operational amplifier (20) that drives output MOS transistors (22a, 22b), which are biased by current sources (24a, 26a; 24b, 26b). Feedback from the drain nodes of the output MOS transistors (22a, 22b) to the inputs of the differential operational amplifier (20), along with the series capacitor (14) and resistor (17) input coupling, ensures minimum offset voltage and current at the output of the preamplifier (10). Common mode feedback control amplifiers (25, 29) ensure proper bias of the components into the saturation region.
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Citations
21 Claims
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1. A preamplifier circuit, comprising:
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a differential operational amplifier, for presenting a differential signal at first and second outputs responsive to a differential signal at first and second inputs; first and second resistors, connected to the first and second inputs of the differential operational amplifier, respectively; 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. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
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9. A class D audio amplifier, comprising:
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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. - View Dependent Claims (10, 11, 12, 13, 14, 15)
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16. A method of amplifying a signal in an audio channel, comprising the steps of:
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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. - View Dependent Claims (17, 18, 19, 20, 21)
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Specification