Amplifiers – With semiconductor amplifying device – Including particular biasing arrangement
Reexamination Certificate
1999-10-28
2001-08-14
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including particular biasing arrangement
C330S085000, C330S098000
Reexamination Certificate
active
06275112
ABSTRACT:
TECHNICAL FIELD
This invention relates generally to the field of electronic amplifier circuits, and more particularly to a microphone bias amplifier.
BACKGROUND OF THE INVENTION
The recent trend towards the miniaturization of electronic circuits is driven by consumer demand for smaller and light-weight electronic devices such as cellular phones and portable computers. Often, the heaviest component in an electronic device is the battery. Smaller batteries are able to provide less power. As batteries become smaller, integrated circuits (ICs) need lower working voltages and power consumption to prevent the battery from discharging too rapidly.
An amplifier is a linear electronic circuit that may be used to amplify an input signal and provide an output signal that is a magnified replica of the input signal. Amplifiers are used in a variety of electronic circuit design applications. As appliances and circuit designs continue to decrease in size and increase in speed, the need for low power, low noise, current efficient amplifier circuitry increases.
Amplifiers have various performance requirements depending on the function they are used for in a circuit. A microphone bias amplifier should have the following attributes: 1) low noise, 2) high power supply rejection ratio (PSRR); 3) low quiescent current, or rather, low overall current usage; 4) the ability to drive high current levels with an output voltage as close to the power supply rail as possible to obtain a good acoustical gain; 5) low output impedance for rejection of any coupled noise; and 6) the use of as little silicon area as possible.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as a microphone bias amplifier having low noise, high PSRR, high output current relative to the quiescent current, and high output voltage relative to the power supply rail. The output stage includes a source-follower transistor coupled to a feedback loop that keeps the source-follower transistor current constant. The action of the output stage is very linear and works well in applications having a high voltage swing, relative to the power supply.
In one embodiment, disclosed is an output stage for an amplifier circuit, including a first transistor coupled to a feedback loop. The feedback loop includes an amplifier coupled to the first transistor, and a current source powering a second transistor and coupled to the amplifier. The amplifier drives a third transistor powered by a voltage source. The first and second transistors are coupled together. The third transistor of the feedback loop provides the entire source current for the first transistor.
In another embodiment, disclosed is a microphone bias amplifier circuit, including an input stage coupled to an output stage. The output stage includes a first transistor coupled to a first feedback loop. The first feedback loop includes a first amplifier coupled to the first transistor, and a first current source powering a second transistor and coupled to the first amplifier. The first amplifier drives a third transistor powered by a voltage source. The first and second transistors are coupled together, and the third transistor provides the entire source current for the first transistor.
Further disclosed is a method of biasing a microphone with an amplifier circuit having an input stage coupled to an output stage. The output stage has a feedback loop coupled to a first transistor. The method includes the step of controlling the current through the first transistor with the output stage feedback loop.
Advantages of the present invention are an amplifier circuit having a low quiescent current or overall current usage, having the ability to drive high current levels relative to the quiescent current. The output voltage may approach, or be approximately equal to, the voltage of the power supply. The circuit has a low output impedance for rejection of coupled noise and uses very little silicon area. Furthermore, the output stage has a high transconductance compared to circuits of the prior art, so large capacitive loads may be driven at low current levels while maintaining stability.
REFERENCES:
patent: 4042889 (1977-08-01), Baker
patent: 4459553 (1984-07-01), Diller
patent: 4573021 (1986-02-01), Widlar
patent: 5025226 (1991-06-01), Taylor
Brady III W. James
Choe Henry
Hernandez Pedro P.
Pascal Robert
Telecky , Jr. Frederick J.
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