Design of a Small Preamplifier for Electret Microphone

With the rapid development of China's communications industry, the demand for electret microphones is increasing. At present, although some small electret microphones can integrate field effect tubes into the microphone, the high price of high-end products and the inability to guarantee the accuracy and sensitivity of low-end microphones, coupled with the fact that the traditional preamplifier is too large. Therefore, it is of great significance to design a preamplifier with the smallest possible volume, low cost and excellent performance.

1 Overview of the principle of electret microphones

A microphone is an electroacoustic transducer that converts an acoustic signal into a corresponding electrical signal. Electret microphones are a new type of microphone made of electret materials. It has the advantages of simple structure and high sensitivity, and is widely used in language pickup, acoustic signal detection and other aspects.

The interior of an electret microphone mainly includes two parts: acoustic-electric conversion and impedance conversion. The acoustic-electric conversion part includes three parts: a diaphragm, a plate and a gap. The key element of sound-to-electric conversion is the vibration membrane, which is an extremely thin plastic membrane. A layer of pure gold film is evaporated on one side. After being electret in a high-voltage electric field, opposite charges are stationed on both sides. The evaporated gold side of the membrane faces outward and is connected to the metal shell. The other side of the membrane is separated from the metal plate by a thin insulating liner, so that a capacitor is formed between the evaporated gold membrane and the metal plate. When sound is transmitted, the diaphragm vibrates with the movement of the sound wave, and the capacitance between the diaphragm and the fixed electrode also changes with the sound. Thus, an alternating voltage signal that changes with the sound wave is generated, thus completing the process of converting sound into an electrical signal. The magnitude of the voltage change reflects the strength of the external sound pressure, and the frequency of this voltage change reflects the frequency of the external sound. The capacitance between the diaphragm and the plate of the electret microphone is relatively small, generally tens of pF. Therefore, the output impedance of this electrical signal is very high and weak. Therefore, the output of the electret microphone cannot be directly connected to the audio amplifier. The field effect transistor has the characteristics of extremely high input impedance and low noise factor. Therefore, a junction field effect transistor with extremely high input impedance is usually connected inside the microphone to amplify the voltage signal generated by the electret capacitor, and at the same time output the signal at the source S or drain G with relatively low impedance to achieve impedance transformation, as shown in Figure 1.


As shown in Figure 1, UOUT1 or UOUT2 is the output signal of the microphone. Since UOUT1 will not be affected by the power supply noise VDD and has a strong ability to resist power supply noise interference, UOUT1 is connected to the preamplifier for amplification.

2 Design and analysis of preamplifier circuit

The function of the preamplifier is to pre-amplify the signal output by the capacitor microphone head on the one hand, and to convert the high output impedance of the capacitor head into a low impedance output on the other hand. The circuit of the small preamplifier mainly includes two parts, one of which is the impedance conversion circuit composed of field effect tubes, and the other is the amplifier circuit that will be analyzed in detail below.

2.1 Simplified model of amplifier circuit

The preamplifier circuit of the microphone is shown in Figure 2. The operational amplifier in the figure uses the microphone preamplifier MAX4465 of Maxim Corporation of the United States. The MAX4465 is a 5-pin SC70 package with low cost and micro power consumption. The principle of this circuit is simplified and explained below. To facilitate the analysis of the circuit, let Z1 = R1 + 1 / (jωC1), Z2 = R2 / / 1 / (jωC2) = R2 / (1 + jωR2C2). According to the characteristics of virtual short and virtual open of the ideal operational amplifier, the transfer function of the circuit can be obtained as follows:

　
It can be seen from equation (1) that when ω→∞ or ω→0, the transfer function Au→1 of the circuit.


2.2 Estimation of the mid-band passband gain

In the frequency band of the speech signal (20 Hz ~ 20 kHz), select appropriate R2, C2 values ​​to make R2C2≈0, then 1+jωR2C2≈1, if 1+jωR1C1≈jωR1C1, then substitute it into the transfer function of formula (1), and we can get Au≈1+R2/R1. If R2=10R1, then Au=1+R2/R1≈R2/R1.

2.3 Estimation of the upper cutoff frequency

When the signal frequency is high, that is, the ω value is large in the passband, and R2=10R1, formula (1) can be changed to:

　
From the above formula, we can see that ω=1/(R2C2), that is, f=1/(2πR2C2) is the upper cutoff frequency corresponding to the circuit.

2.4 Estimation of the lower cutoff frequency

When the signal frequency is low, that is, when the ω value is small in the passband and R2=10R1, then 1+jωR2C2≈1, and formula (1) can be changed to:


From the above formula, we can see that when ω=1/(R1C1), f=1/(2πR1C1) is the lower cutoff frequency of the circuit.

2.5 Simulation results of preamplifier circuit

During the design of the circuit, we used circuit simulation software to perform simulation verification. The simulation results are shown in Figure 3.


As can be seen from Figure 3, the above estimation results are basically consistent with the simulation results. At the same time, the actual debugging results of the preamplifier circuit are also basically consistent with the above analysis.

3 Small preamplifier structure characteristics

The diameter of the preamplifier circuit board designed according to the above principle is about 10 mm (1/2 inch). Its own small volume can greatly reduce the total volume of the entire microphone system when combined with the high-sensitivity 1/2 inch electret microphone, so as to better meet the strict requirements of the microphone volume in complex situations.

4 Summary

The microphone preamplifier circuit designed in this paper has the advantages of small size, low cost, high input impedance, and strong anti-interference performance. In the process of circuit processing, a high-precision digital multimeter is used to carefully screen the components to ensure the consistency between different preamplifiers in the same batch. In addition, the preamplifier circuit can also be powered by a 3-18 V voltage source as needed to meet the engineering needs under different conditions. At present, the 1/2 inch electret microphone preamplifier has been well applied in engineering practice.