Low noise amplifier for MEMS capacitive transducers

Abstract

This application relates to amplifier circuitry for amplifying a signal from a MEMS transducer. A super source follower circuit (40) is provided which includes a feedback path from its output node (Nout) to a control bias node (BC) in order to provide a preamplifier signal gain that may be greater than unity. A first transistor (M1) is configured to have its gate node connected to an input node (NIN) for receiving the input signal (VIN) and its drain node connected to an input node (X) of an output stage (A). The source node of the first transistor is connected to the output node (NOUT). A current source (I2) is configured to deliver a current to the drain node of the first transistor (M1), wherein the current source (I2) is controlled by a bias control voltage (VBC) at the bias control node (BC). A feedback impedance network (Z1) comprising a first port connected to the output node (NOUT) and a second port connected to the bias control node (BC) is provided.

Description

technical field [0001] The present invention relates to methods and apparatus for amplifying signals from MEMS capacitive transducers, and in particular to the use of super source follower based low noise preamplifiers. Background technique [0002] figure 1 A MEMS microphone 2, an associated amplifier 4 and bias circuitry 6 are shown. boosted by charge pump 8 from the supply voltage V DD derived regulated voltage V R , to utilize an appropriate voltage V CP , such as 12V, to bias the capacitive transducer 2 (C MEMS ) of a terminal. The other terminal of the capacitive transducer 2 is biased via a very high impedance 10 (for example of the order of 10 GΩ) to another fixed voltage, usually ground. One example of the type of component that can be used to create this impedance is a polysilicon diode network. Acoustic waves incident on the capacitive transducer 2 will cause one electrode plate to displace, thus changing its separation from the other electrode plate, thus ...

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Problems solved by technology

However, this example is not ideal and wastes power in distorting electrical linearity

Furthermore, the signal component of the current delivered to the current connected to the output load impedance of the amplifier is provided by a similar modulation of the current flowing through the MOS transistor, resulting in a signal dependent change in the gate-source voltage of the MOS transistor

This produces a load-dependent attenuation of the output signal, which also becomes significantly non-linear for large output signal swings, resulting in distortion of the output signal

In other words, the non-zero output impedance of the source follower produces a load-dependent gain and the signal dependence of this output impedance introduces distortion of the large output signal

The output impedance can be reduced by using a higher bias current, but this causes undesirably higher power dissipation

[0005] Although simpler and lower power than amplifiers that include differential input stages, conventional source follower amplifiers have an inherent fixed gain close to 1 or less

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