Microphone with soft clipping circuit

An apparatus includes a microelectromechanical system (MEMS) device having a diaphragm and a back plate; a clipping circuit coupled to the MEMS device, wherein the clipping circuit is configured to clip an output signal of the MEMS device so that the maximum signal drawn by a buffer is substantially constant over a temperature range; and an integrated circuit coupled to the clipping circuit, the integrated circuit including the buffer.

FIELD OF THE INVENTION

This application relates to microphones, and more specifically to clipping circuits used within these microphones.

BACKGROUND OF THE INVENTION

Microphones are typically constructed of two main components: Micro-Electro-Mechanical System (MEMS) devices that receive and convert incoming sound into electrical signals, and Application Specific Integrated Circuits (ASICs) that take the electrical signal from the MEMS device and perform post processing on the signal and/or buffering the signal for the following circuit stages in a larger electronic environment. The following circuit stages may be disposed in cellular phones, personal computers, or tablets to mention a few examples.

However, these previous microphones have limits as to how much signal the microphone can take before distorting the incoming signal. These distortions come from both the MEMS device and the ASIC and they are typically independent of each other.

Signal distortion in the ASIC generally happens when the input and/or the output electrical signal is too large a magnitude. A general way to control the input distortion is to have a circuit to clamp/soft clip the signal. This is to ensure that the signal will not be distorted in an abrupt and undesirable way but in a controlled way.

Previous clipping circuits sometimes used diodes. However, the problem with these previous circuits was that the diode had a very large temperature coefficient. This caused different clipping points to occur at different temperature.

The problems of previous approaches have resulted in user dissatisfaction with these previous approaches.

Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION

The present approaches provide a soft clipping circuit with temperature compensation. In one aspect, as temperatures vary the maximum signal drawn by a buffer in a microphone is constant or substantially constant. Because the maximum signal is constant, large signal distortion is also compensated and the problems associated with large signal distortion are avoided.

Referring now toFIG. 1, one example of a digital Microelectromechanical system (MEMS) microphone100is shown. The microphone100includes a MEMS device102, a clipping circuit104, and an application specific integrated circuit (ASIC)106(including a buffer108). The ASIC106may include other circuit elements such as differential amplifiers, and analog-to-digital converters. Other examples of additional components are possible.

The MEMS device102is any type of MEMS microphone device that converts sound energy101into an analog electrical signal. The MEMS device102may also include a diaphragm and back plate that form a capacitance that varies with the acoustic energy received to produce an analog electrical signal. The analog electrical signal is fed to the buffer104, which buffers the signal for later processing (by the ASIC106, or other devices inside or outside the microphone100).

The clipping circuit104performs a soft clipping on output signal110of the MEMS device102so that the maximum signal drawn by a buffer108is constant or substantially constant over a temperature range. Because the maximum signal is constant over a temperature range, large signal distortion is also compensated or the problems associated with large signal distortion are avoided. In one example, the clipping circuit104is disposed at the ASIC106. In other examples, the clipping circuit104is a separate integrated circuit that is not disposed at the ASIC106.

The buffer108holds data received from the ASIC106. This data may be further processed by other functional elements at the ASIC106.

Referring now toFIG. 2, one example of a clipping circuit200(in one example, the clipping circuit104ofFIG. 1) is described. The circuit200includes a first diode202(D1), a second diode204(D2), a third diode206(D3), a fourth diode208. The first diode202(D1) is coupled to a reference voltage (VR). V0is coupled to the output of a MEMS device (e.g., the MEMS device102ofFIG. 1). The fourth diode208(D4) is coupled to voltage VC, which is one example is connected to ground. The circuit200produces a constant current210. V0is also coupled to a buffer212, which in one example is disposed on an ASIC.

In one example of the operation of the circuit200, V1is the resultant voltage of VR minus two forward bias voltage of a diode (e.g., diodes202and204). The input signal214starts to clip when V0is larger than V1by a forward bias voltage of a diode. The input signal214also clips when V0is smaller than VC by a forward bias voltage of a diode (e.g., diodes202and204).

When the temperature of the circuit200varies, the forward bias voltage of diodes206(D3),208(D4) will also change, but this change in voltage across these diodes will be compensated for by the changing voltage V1. The voltage V1changes because the forward bias voltage of the diodes202(D1),204(D2) matches the voltage changes occurring across the diodes206(D3),208(D4). In other words, the voltage changes of the voltage V1will be same as the changes on the diodes206(D3),208(D4).

For example and at cold temperatures, the voltage across the diodes206(D3),208(D4) becomes larger. At the same time, the voltage V1will move further away from VR by the same magnitude due to the changing voltages across the diodes202(D1), and204(D2).

If the soft-clipping point of the circuit200is temperature compensated, the maximum signal entering the buffer212will be constant (or substantially constant) across a temperature range and this is how the large signal distortion is compensated for in the present example. In other words, a large temperature swing will not affect (or will not substantially affect) the magnitude of the voltage being received by the buffer212and beneficial clipping will still be performed.

In one example, the input signal214is soft clipped by the circuit200. As shown inFIG. 2, the magnitude of the top of the incoming sinusoidal waveform assumes the shape222so that the input signal is attenuated or modified in this portion of the waveform214. In contrast, hard clipping refers to eliminating the top portion of the waveform as in the shape224. The present approaches although preferably directed to soft clipping may in some instances be used with hard clipping and/or combinations of soft clipping and hard clipping.

It will be also appreciated that the circuit ofFIG. 2is one example of an approach whereby soft clipping with temperature compensation is performed. Other examples of circuit arrangements may include other circuit elements (e.g., additional diodes), or other circuit elements arranged in other configurations (e.g., additional diodes arranged in other configurations).