The demand for microphones for mobile equipment such as mobile telephones, headsets and cameras tends to follow the growing demand for mobile equipment—for instance mobile telephones.
The demand has for many years been rather simple in that the demand was for microphones with extremely low costs and for microphones suitable for production in very high volumes. The performance of such microphones was comparable from one manufacturer to another and was at a level comparable to that of telephony systems. In recent years, however, the demand has changed to also be for microphones with a performance above that of telephony systems. Today there appears to be a trend in the demand heading towards so-called high-fidelity (hi-fi) quality.
Use of integrated digital processors, with ever increasing performance, in the different types of mobile equipment has also brought attention to the performance of the more peripheral links of the signal processing chain from pick-up of a signal over transmission and/or storage to reproduction of the signal. Such more peripheral link is for instance the microphone or the circuitry embodied with a microphone transducer in a microphone capsule. A microphone capsule—also denoted a microphone element—can include shock mounts, acoustic isolators, protective covers and a semiconductor die, with integrated circuitry, in addition to the microphone transducer. The microphone transducer and the integrated electronic circuit is embodied in the microphone capsule converts acoustic energy to electrical energy so as to provide an electrical microphone signal.
It has been discovered that integrated digital processors can be configured to repair some damages to a microphone signal occurred due to inadequate signal conditioning in the microphone capsule. But in general it is far more efficient not to disregard aspects of signal conditioning in peripheral links of the signal processing chain to thereby avoid destroying the microphone signal and consequently being able to provide far better repair of the signal if needed at all. The microphone signal may be destroyed by disregarding a noise source and/or by overloading an amplifier (in the capsule).
Thus, there is a demand for high quality microphones, but unfortunately the demand for low price seems to persist. As cost of a semiconductor die is directly related to the size of the die it is important, for the purpose of reducing price, that the electronic circuit integrated on the die is as small or compact as possible. Therefore, very simple circuits are desired, with a due regard to a desired (high) performance.
It has been discovered that meeting high performance demands is not simply a question of providing a more robust or conservative design. Due to the important cost issues and signal conditioning aspects it is found that there is no single one fixed signal conditioning circuit that is able to provide high performance in various acoustic situations. Such various situations could be described as a voice signal with/without loud/quiet background noise, a loud or quiet voice signal or combinations thereof. Thus, the signal conditioning needed to provide high performance is different from one situation to another.
It has therefore been proposed—despite the additional cost of a more complex semiconductor die in the capsule—to provide the semiconductor die with means for the circuit on the die to adapt to a given acoustic situation. Thereby, high performance can be achieved in different acoustic situations. In some designs of microphones it may be equitable to provide the adaptation to different acoustic situations by a control loop embodied entirely on the semiconductor die, but in other designs it may be equitable to provide the control loop to provide a control feedback from a circuit external to the microphone capsule. Thereby it is necessary to configure the semiconductor die for an external circuit to make the circuit in the semiconductor die adapt to a situation. To this end cost is generally an obstacle simply to have one or more additional pad(s) for receiving such an external feedback.
Thus, since high quality microphones are sought after, more complex circuitry is inherently needed which—all other things being equal—has a higher power consumption. Since the mobile equipment is battery powered current consumption of the device including portion thereof is subject to be minimized as far as possible. This adds an additional and important dimension to the demand.
The microphone is based on the principle of a capacitor, which is formed by a movable member that constitutes a membrane of the microphone and another member, e.g. a so-called back plate of the microphone. One of the members of the microphone, preferably the membrane, is charged by a constant electrical charge. The charge is either provided as an electrostatic charge captured on one of the members or provided by a voltage source e.g. a charge pump or voltage step-up circuit on the semiconductor die.
A sound pressure detected by the microphone will cause the membrane to move and consequently change the capacitance of the capacitor formed by the membrane member and the other member. 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 incoming sound pressure level. 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 any resistive load. A resistive load will discharge the capacitor and thereby degrade or ruin the capacitors performance as a microphone. A capacitive load will reduce sensitivity of the microphone transducer.
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 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 millimetres or fractions of millimetres.
For small sized microphones only a very limited amount of electrical charge can be stored on one of the microphone members. This further emphasizes the requirement of high input resistance. 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.
Traditionally, this preamplifier has been implemented as a simple JFET. The JFET solution has been sufficient, but demands in the telecom industry call for ever smaller microphones—with increased sensitivity. This yields a contradiction in terms since sensitivity of the microphone capacitor drops as size goes down. All other things being equal, this will further reduce the sensitivity of the microphone and the buffer in combination. The demands in the telecom industry are among other things driven by market trends which encompass hands free operation of different types of small-sized equipment and more widespread application of microphones in e.g. camera applications.
So obviously, there is a need for microphone preamplifiers with gain and very low input capacitance, and lowest possible preamplifier die area. Additionally, low noise is important. Low noise is important as noise, during design of the microphone, can be traded for area—i.e. if the circuit has low noise and a noise lower than required, this noise level overhead can be traded for lower chip die area and it is thus possible to manufacture the preamplifier at lower cost.
When designing a preamplifier 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. Typically, 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 e.g. 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.
Microphones is typically manufactured with a yield of approximately 80-90% i.e. 80-90% of the total number of produced microphones satisfy specifications on their performance. Unfortunately, 10-20% of the production is discarded since for instance sensitivity of the microphone does not satisfy the specification. A solution to reducing the discarding rate would be highly appreciated by the industry.
Another problem of e.g. electret microphones is the ageing phenomena's in which the electret microphone might change sensitivity over time thereby leading to a discrepancy between the electret microphone output and the gain of the buffer amplifier.
A microphone subjected to a background noise comprising low frequency sound at high amplitudes, e.g. from a motorized vehicle, may be prone to the problem of e.g. clipping the sound signal from the microphone. In case a voice signal is present in combination with such a background signal, the information in the voice signal may be lost since the sound pressure results in the corresponding electrical signal being clipped. The clipping of the microphone signal may occur when the amplitude of the low frequency background superposed on the voice signal overload the amplifier amplifying the signal from the microphone e.g. by exceeding a maximum sound pressure that the microphone and amplifier may handle e.g. 110 dB SPL. Minor overloading of the amplifier may result in signal clipping while severe overloading of the amplifier may yield a period of time, e.g. in the order of seconds, where the amplifier has ceased to operate as an amplifier.