A sound pressure detected by the microphone element will cause its membrane to move and consequently change the capacitance of the capacitor formed by its membrane and a so-called back plate of the microphone element. When the charge on the capacitor formed by these two members is kept constant, the voltage across the two capacitor members will change with the sound pressure acting on the membrane. As the charge on the microphone capacitor has to be kept constant to maintain proportionality between sound pressure and voltage across the capacitor members, it is important not to load the microphone capacitance with a resistive load. A resistive load will discharge the capacitor and thereby degrade or ruin the capacitor's performance as a microphone.
Therefore, in order to pick up a microphone signal from the capacitor, amplifiers configured with the primary objective of providing high input resistance are preferred in order to buffer the capacitor from circuits which are optimized for other objectives. The amplifier connected to pick up the microphone signal is typically denoted a preamplifier or a buffer amplifier or simply a buffer. The preamplifier is typically connected physically very close to the capacitor—within a distance of very few millimeters or fractions of millimeters.
For small sized microphones the active capacitance is very small (typically 1 to 10 pF). This further increases the requirement of high input resistance and capacitance. Consequently, the input resistance of preamplifiers for small sized microphones has to be extremely high—in the magnitude of Giga ohms. Additionally, the input capacitance of this amplifier has to be very small in order to achieve a fair sensitivity to sound pressure.
Especially so-called telecom microphones with an integrated preamplifier are sold in high volumes and at very low prices. As cost of an amplifier for a telecom microphone is directly related to the size of the preamplifier chip die it is important, for the purpose of reducing price, that the preamplifier chip die is as small as possible. Therefore extraordinary attention is drawn to compact circuits and such circuits are in very high demand. However, it is important in this respect to provide circuits with a low noise level. Low noise is important as noise can be traded for area—i.e. if the circuit has low noise and a noise lower than required, this noise level can be traded for lower chip die area and it is thus possible to manufacture the preamplifier at lower cost.
When designing a preamplifier in CMOS technology for a microphone there is normally three noise sources. These sources are noise from a bias resistor, 1/f noise from an input transistor, and white noise from the input transistor. We assume that input transistor noise dominates. Both white noise and 1/f noise can be minimized by optimizing the length and the width of the input transistor(s). This applies for any input stage e.g. a single transistor stage or a differential stage. The noise from the bias resistor can also be minimized. If the bias resistor is made very large then the noise from the resistor will be high-pass filtered and the in-band noise will be very low. This has the effect though that the lower bandwidth limit of the amplifier will be very low. This can be a problem as the input of the amplifier will settle at a nominal value only after a very long period of time after power up. Additionally, signals with intensive low frequency content arising form eg slamming of a door or infra sound in a car can overload the amplifier. Another related problem is small leakage currents originating from mounting of the die inside a microphone module. Such currents will due to the extreme input impedance establish a DC offset. This will reduce the overload margin of the amplifier.
In addition to the above there is a demand for digital microphones comprising a microphone element and an integrated circuit with a preamplifier and an analogue-to-digital converter to provide a digital output signal. Since typically a telecom microphone is integrated in a consumer electronics device where a substantial amount of digital signal processing is performed by a mainly digital integrated circuit chips it is in general preferred that signals from sensors (such as microphones) are provided as digital signals. This introduces new challenges in respect of signal processing in the integrated circuits embedded with the microphones—and especially in respect of distortion in the digital domain.
In recent years so-called sigma-delta modulators have become very popular for implementing A/D converters. They exhibit many virtues, which among others are: no need for high precision components; high linearity; and for so-called single loop modulators also the advantages of small die area, low voltage operation and possibly very low power consumption. These are advantages which makes sigma delta modulators very suitable for single chip implementations.
A special class of sigma delta modulators are 1-bit quantized sigma delta modulators. This type of modulators are especially suited for low cost implementations as the complexity of the analogue part of the A/D converter is minimal compared to other types of A/D converters. A complete 1-bit sigma delta converter consists of a 1-bit analogue sigma delta modulator and a digital decimation filter only. The normally required higher order anti-aliasing filter can be implemented by a simple RC-filter. This is due to the fact that heavy over-sampling is used and thus the digital decimation filter performs the job of anti-aliasing filtering.
1-bit sigma delta modulators are very simple to implement in the analogue domain. Thus they are very suitable for low cost miniature digital microphones. Unfortunately they do also have disadvantages. Especially 1-bit sigma delta modulators exhibit the so called idle mode tones, which are low level tones in the audio band caused by low frequency or DC levels at the input of the modulator. This is the reason why 1-bit sigma delta modulators has been abandoned by many despite of its many virtues. One can use dither to remove this problem or design chaotic modulators: but all of these solutions has the effect that the complexity of the design increases dramatically. Thus both power consumption and die area increases dramatically.
This idle-mode-tones effect has caused sigma delta modulators less suitable for high quality audio applications. Apparently, this may seem to be of little concern in consumer/telecom applications. But as the demand for low cost digital microphones increases higher demands of performance, which may almost equal the performance of high quality audio, will follow. Consequently, the idle-mode-tones effect will become an increasing problem also for telecom applications.
In order to achieve high performance from the digital microphone, the preamplifier of the digital microphone ASIC has to have as high performance as possible i.e. low noise, low distortion, high dynamic range etc. According to presently available technology, CMOS technology is a prerequisite to achieve low noise performance and it can be shown that the input stage of the amplifier can be optimized in respect to noise. Also the input impedance should be as large as possible in order to minimize the noise. This is especially dominant for new and thinner types of telecom microphones which has a much lower sensitivity and cartridge capacitance than previously experienced.
Unfortunately this has the consequence that the preamplifier becomes capable of amplifying low frequency signals arising from the sound pressure of a door slamming, car rumbling or just changes in sensitivity of the microphone element due to humidity changes. This adds to the above explained problem of idle tone modes if a 1-bit sigma delta modulator is used. In fact also 2-bit and modulators with even more levels will exhibit such behaviour when exposed to such low frequency signals.
Additionally, these low frequency signals reduces the dynamic range and creates inter-modulation distortion as the low frequency signals can be excessive in amplitude.
The problem is worsened as the telecom microphones are becoming smaller and thinner and thus more gain is required from the preamplifier. However, normally the disturbing low frequency signals do not become smaller in amplitude. Thus the relative effect of the disturbance will increase.
So there is a need for a configuration of a preamplifier and an A/D converter which is suited for thin ECM cartridges with a very low cartridge sensitivity and capacitance. Additionally, the configuration should provide a very high performance on noise, dynamic range and distortion. Moreover, it shall be feasible to implement the configuration on a single chip die with a very small area in combination with few or none external components.
In the below description, the term audio band is used. In the prior art this term have various definitions depending on its context. However, in the below it will be used to designate a frequency band which typically has a lower corner frequency of 20 Hz to 500 Hz and an upper corner frequency of 5 KHz to 25 KHz. The specific definition of the band represents a design criterion, but for the below description it should be read with this broad definition.