CN102571009A - Feedforward automatic gain control circuit working in extremely low voltage current mode - Google Patents

Abstract

The invention provides a feedforward automatic gain control circuit working in an extremely low voltage current mode, and relates to the technology of sensors. The feedforward automatic gain control circuit comprises a fully differential operational amplification unit, a capacitive-resistance device, a resistor array, a control switch, rectifier, a filter, a controller, a digital reference table, a digital comparator and an add-subtract counter, wherein the fully differential operational amplification unit is used for correspondingly amplifying voltage signals of an output end of a microphone according to variation of the resistance array; the rectifier is used for converting the voltage signals into current signals and rectifying current; the filter is used for filtering the current output by the rectifier and carrying out envelop detection on input signals; the digital controller is used for comparing current output by the filter with the reference current to obtain digital control signals in the digital reference table; the digital comparator is used for comparing output of the add-subtract counter with the digital control signals, when the sound volume is in different levels, the amplifying multiple of the fully differential operational amplification unit is adjusted, so that the hearing of the auditorily handicapped is protected; when the sound volume is in a normal state, the amplifying multiple of the fully differential operational amplification unit is restored to the original value, so that the automatic gain control on the different sound volumes is realized.

Description

Current Mode Feedforward Automatic Gain Control Circuit Working at Very Low Power Supply Voltage

technical field

The invention relates to the technical field of microphone sensors, relates to a high-precision readout circuit chip suitable for a microacoustic electric system (MEMS) microphone sensor, and is an automatic gain control circuit for a digital hearing aid microphone readout circuit chip.

Background technique

With the continuous advancement of micro-acoustic electric system (MEMS) technology, micro-acoustic electric hearing aid microphone sensors have emerged at the historic moment. It has the advantages of small size, low power consumption, small distortion and strong anti-noise ability. The corresponding readout circuit become a research hotspot in the industry.

This kind of sensor can usually be equivalent to a relatively ideal voltage source: a small resistance on the ideal voltage source string. However, the signal output by the microelectromechanical sensor of the microphone is very weak, generally only in the order of μV ~ mV, which puts forward very strict requirements for the design of the readout circuit. The noise level and precision of the readout circuit determine the minimum signal amplitude it can detect. Low noise and high-precision readout circuit design is the key to high precision, especially in the case of extremely low power supply voltage for digital microphones.

At present, the automatic gain control design of the microphone readout circuit in the world can be mainly divided into analog feedback control op amp forward open-loop gain, that is, analog feedback control op amp bias current (reference: Jones, Martinez: "A CMOS hearing aid ", Analog Integrated Circuits and Signal Processing, 21, 163-172 (1999); Baker: 'A Low-Power Single- and Dual-Loop Automatic Gain Control for Artificial Ears', Solid State Circuits, SC-41(9), pp .1983-1996, 2006) and analog feedback to control the gate-source voltage of the equivalent resistance of the controllable MOS tube (reference: Hörmann: "A low-noise CMOS automatic gain control technology", solid-state circuits, SC-27(7), pp.974-981, 1992; Jin Shuyong: "A high-efficiency front-end analog circuit for a sub-1V digital hearing aid", Solid State Circuits, SC-41(4), pp.876-882, 2006.) and current mode feedback control MOS Tube transconductance (Reference: Grales: "Low Voltage Subthreshold Exponential Amplification and Automatic Gain Control", Circuits, Devices and Systems, Vol.152, No.1, Feb 2005) Two schemes. The above automatic gain control circuit is widely used in the field of analog hearing aids. But the disadvantage of the former is that its linearity will be limited by the process and power supply voltage, especially when the power supply voltage is low. The advantage of analog feedback is that its signal has strong continuity. But the difficulty of analog feedback lies in the realization of the output accuracy of the low-voltage op amp. Usually, the magnification of the operational amplifier has achieved ideal precision under the control of analog feedback, but it is extremely difficult to achieve high precision in integrated circuits, and usually requires complex compensation circuits or very large power consumption. The disadvantage of the current mode feedback method is that the operational amplifier itself does not have a negative feedback structure, so the accuracy of the system for signal processing is low. The digital hearing aid requires the readout circuit to have very good readout accuracy, which poses a challenge to the design of the readout circuit.

Contents of the invention

The purpose of the present invention is to disclose a current-mode feed-forward automatic gain control circuit that works on an extremely low power supply voltage, which is a microphone readout circuit for current-mode feed-forward, so as to overcome the deficiencies in the prior art and not only effectively realize automatic gain control The function, and realize high-precision readout under extremely low voltage by using passive resistor array. At the same time, it has the function of reducing power consumption as the volume increases under low power supply voltage to achieve high-efficiency utilization of power consumption. The invention provides a high-precision, low-voltage automatic gain control readout circuit for the digital hearing aid microphone.

For achieving the above object, technical solution of the present invention is:

A current-mode feed-forward automatic gain control circuit that works on an extremely low power supply voltage and is used for hearing aids; it includes: a fully differential operational amplifier unit, a capacitive resistance device, a resistor array, an array control switch, a rectifier, a filter, a digital reference meter, Digital controllers, digital comparators and up and down counters;

A fully differential operational amplifier unit for amplifying the microphone output signal;

The two input terminals of the fully differential operational amplifier unit are respectively connected to the microphone output terminal and the ground through two capacitive resistance devices to form a high-pass filter for the input signal;

Two resistor arrays are respectively connected between the two output terminals and the two input terminals of the fully differential operational amplifier unit to determine the gain value of the fully differential operational amplifier to the signal;

The two resistor array control switches are respectively connected in parallel to one side of the two resistor arrays to control the gain value of the fully differential operational amplifier unit;

A rectifier is used to rectify the small output signal of the microphone;

The rectifier and a filter form an envelope detection circuit, which is used to filter the rectified signal to obtain the energy of the output signal of the detection microphone;

a digital controller, which compares the output value of the peak detection circuit with the reference level;

A digital reference table, used to look up the corresponding digital reference code;

An up-down counter, used for automatic gain control to compress and restore the signal;

A digital comparator controls the addition and subtraction counter to perform addition and subtraction operations, and compares the output code of the addition and subtraction counter with the reference code to output the enabling logic of compression and restoration.

Described current mode feed-forward automatic gain control circuit, its specific circuit is as follows: the first and the second input terminal of fully differential op-amp unit are connected with the microphone output terminal through the first and the second capacitive resistance device respectively, the first The resistor array is connected across the first input terminal and the second output terminal of the fully differential operational amplifier unit, and the second resistor array is connected across the second input terminal and the first output terminal of the fully differential operational amplifier unit; the first resistor array controls the switch Connected across the first resistor array and an output end of the fully differential operational amplifier unit, the second resistor array control switch is connected across the second resistor array and the other output end of the fully differential operational amplifier unit;

The input end of the rectifier is connected to the output end of the microphone, the order of the rectifier is electrically connected to one input end of the low-pass filter, the digital controller, the digital reference meter, and the digital comparator, and the other output end of the digital controller is electrically connected to the fully differential operational amplifier unit;

The input terminal of the addition and subtraction counter is electrically connected with the two clocks and the two output terminals of the digital comparator, and the output terminal of the addition and subtraction counter is respectively electrically connected with the first array control switch, the second array control switch, and the other input end of the digital comparator.

In the current-mode feedforward automatic gain control circuit, the fully differential op-amp unit adjusts the bias current according to the magnification, so that the power consumption of the op-amp is reduced when the volume is high; wherein, M1 and M2MOS tubes work in the sub-threshold region, so that they have a constant bandwidth.

In the current mode feedforward automatic gain control circuit, the rectifier and filter rectify and filter the weak signal above 100uV output by the microphone under the extremely low power supply voltage of 0.8V, and the rectifier and filter The transmission signal is a current mode signal; the M8 and M9 MOS tubes of the filter circuit work in the sub-threshold region.

In the current-mode feedforward automatic gain control circuit, the digital controller performs logic control output on the weak signal above 100uV output by the microphone under the extremely low power supply voltage of 0.8V.

A current-mode feed-forward automatic gain control circuit of the present invention, which works on an extremely low power supply voltage, is an automatic gain control readout circuit that can be used for a current-mode feed-forward of a digital hearing aid. The energy is converted into a digital control signal, and the digital control circuit controls the resistor array to control the gain of the operational amplifier, which effectively solves the problems of continuity and precision. The circuit of the present invention is suitable for very low operating power supply voltage (such as, below 0.8V), which helps to provide a low voltage, low power consumption and high precision readout circuit used in portable digital hearing aids, and realizes automatic control Gain digital feed-forward method and corresponding circuit working at low voltage.

Description of drawings

Fig. 1 is the circuit diagram of the feed-forward automatic gain control of current mode feedforward working in the very low supply voltage of the present invention, wherein, the main component label description:

Fully differential operational amplifier unit 1 The first capacitive resistance device 2

The second capacitive resistance device 3 The first resistor array 4

The second resistor array 5 The first resistor array control switch 6

The second resistor array controls the switch 7 rectifier 8

Filter 9 Numerical Reference Table 10

Digital Controller 11 Digital Comparator 12

Addition and subtraction counter 13;

Fig. 2 is a circuit diagram of a fully differential operational amplifier working at an extremely low power supply voltage in the present invention;

Fig. 3 is the circuit diagram of a rectifier working at a very low power supply voltage in the present invention;

Fig. 4 is the filter circuit diagram that works in very low supply voltage among the present invention;

Fig. 5 is a circuit diagram of a digital controller operating at an extremely low power supply voltage in the present invention.

Detailed ways

The present invention provides a current mode feed-forward automatic gain control circuit that works at extremely low power supply voltage, and effectively solves the existing problems in the prior art by adopting a circuit module that works at low power supply voltage, current mode feedforward and digital circuit control. non-linear problem.

In the present invention, capacitive resistance devices 2 and 3 perform high-pass on the microphone DC signal, filter the microphone DC signal, and read out the weak signal of the microphone by the fully differential operational amplifier circuit 1; at the same time, the envelope detection circuit composed of the rectifier 8 and the filter 9 detects The energy of the weak microphone signal, the digital controller 11 processes the energy to obtain a digital control signal, the digital reference table 10 finds the corresponding gain control code according to the digital control signal, applies it to the digital comparator 12, and obtains the enable signal to realize the gain control code. The addition and subtraction counting function of the down counter 13 realizes the function of signal compression and restoration. Up and down counting are synchronized with clocks 22 and 33 respectively. Different sound volume levels correspond to different gain control codes, and the final operational amplifier gain is realized by controlling the resistor arrays 4, 5 through the gain control codes through the resistor arrays 4, 5 and the resistor array control switches 6, 7.

The circuit realizes high-precision microphone signal readout under low power supply voltage by adopting the method of resistor array, and at the same time realizes automatic gain control under low power supply voltage through current feedforward and digital control.

Fig. 1 is the current mode feed-forward automatic gain control circuit of the present invention working in extremely low power supply voltage. The automatic gain control circuit is divided into the following parts, including: fully differential operational amplifier unit 1, capacitive resistance device, resistor array, array control switch, rectifier 8, filter 9, digital reference table 10, digital controller 11, digital comparator 12 plus and minus counters 13;

The first and second input terminals of the fully differential operational amplifier unit 1 are connected to the microphone output terminal via the first and second capacitive resistance devices 2 and 3 respectively, and the first resistor array 4 is connected across the first fully differential operational amplifier unit 1. The input terminal and the second output terminal, the second resistor array 5 is connected across the second input terminal and the first output terminal of the fully differential operational amplifier unit 1, and the first resistor array control switch 6 is connected across the first resistor array 4 and the fully differential At the output terminal of the operational amplifier unit 1 , the second resistor array control switch 7 is connected across the second resistor array unit 5 and the output terminal of the fully differential operational amplifier unit 1 . The input end of the rectifier 8 is connected to the input end of the microphone, the output end sequence is connected to the low-pass filter 9, the digital controller 11, the digital reference meter 10, the input end of the digital comparator 12, and the other output end of the digital controller 11 is connected to the full differential operation. Putting unit 1 is electrically connected. The input terminals of the add-subtract counter 13 are electrically connected with two clocks 22, 33 and the output terminals of the digital comparator 12 respectively, and the output terminals of the add-subtract counter 13 are respectively connected with the first array control switch 6, the second array control switch 7, and the digital comparator 12. The other input end is electrically connected.

Referring to FIG. 2 , in practical applications, considering lower power supply voltage and lower power consumption, the bias current of the fully differential operational amplifier unit 1 is designed as a programmable bias current source 14, which is performed by a digital controller 11. control, and the differential input pair M1, M2MOS transistors work in the sub-threshold region 15, and a cascaded circuit topology is adopted at the same time. This circuit also has a common-mode output detection circuit, a common-mode input detection circuit and the function of reducing the power consumption of the op amp when the volume is high. In order to prevent the input common-mode level from exceeding the common-mode input range of the op amp due to the transient output of the common-mode output level being too high, in Figure 2, the input common-mode detection circuit composed of M6, M7, M8, M12, and M13 MOS transistors It will make the input common-mode level of the fully differential op amp start to track the output common-mode level. The output common mode level is determined by the output common mode feedback loop formed by M9, M10, M11, M14, and M16 MOS transistors. When the volume level is different, the digital controller reduces or increases the programmable bias current source composed of M3, M4, M5, M6, and M11 MOS tubes, so as to achieve the purpose of a fully differential operational amplifier with high efficiency. The gain bandwidth is:

ω _AGC ＝ β _i ω _OTA ＝ β _i g _m /C _comp

Among them, β _i is the full differential feedback coefficient, determined by the feedback loop, g _m , C _comp is M1, the transconductance of the M2MOS transistor and the full differential operational amplifier compensation capacitor. Since M1 and M2MOS transistors work in the subthreshold region, linear control of the transconductance can be realized to achieve constant gain bandwidth under different gains.

At the same time, the output signal of the microphone is rectified by the rectifier, refer to FIG. 3 . Considering that the rectifier works in a low power supply voltage state, the rectifier adopts the cascaded form of the current mode circuit, that is, the resistors and capacitors C and R are used to convert the voltage into a current, and at the same time, two cascaded feedback loops are used to respectively control the converted forward current and The negative current is rectified. Its tubes all work in the saturation region, and the minimum working voltage they can withstand is:

_VDD-MIN ＝ _2VGS - _VTH

Among them, V _GS is the gate-source voltage of the MOS tube, and V _TH is the threshold voltage of the MOS tube.

In Figure 3, the feedback loop for rectifying the forward current is composed of M1, M2, M3, and M4MOS transistors, which are transimpedance operational amplifier structures with voltage-current negative feedback, and the purpose is to reduce the input resistance of the transimpedance operational amplifier, so that current can flow in. The input impedance of this transimpedance op amp can be determined by the following formula:

${\mathrm{<span\; class="notranslate">\; z</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; mg</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; comp</span>}}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; mg</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; md</span>}\mathrm{<span\; class="notranslate">\; 8</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; comp</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}{\mathrm{<span\; class="notranslate">\; the\; s</span>}}^{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; g</span>}}_{\mathrm{<span\; class="notranslate">\; md</span>}\mathrm{<span\; class="notranslate">\; 8</span>}}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; comp</span>}}\mathrm{<span\; class="notranslate">\; the\; s</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; 1</span>}}$

In the formula, g _md and g _mg are the gate transconductance and drain transconductance of the tube respectively, and C _comp is the compensation capacitance.

The feedback loop for rectifying the negative current is composed of M5, M6, M7, and M8 MOS tubes. When the current is a negative current, the signal will be rectified by cascading the negative current feedback loop through the positive current feedback loop. C _comp1 is the compensation capacitance of the negative current feedback loop. In order to make the response time of positive current rectification equal to the response time of negative current rectification, the relationship between C _comp and C _comp1 can be determined by the following formula:

${\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; comp</span>}\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; I</span>}}{\mathrm{<span\; class="notranslate">\; 5</span>}{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; bias</span>}}}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; comp</span>}}$

In the formula, I _bias and I are the bias currents of M4 and M5 MOS tubes respectively. Then the rectified signal is filtered by a filter to obtain the energy signal of the input current. This filter uses the current-voltage-current filtering method to filter the rectified current signal, refer to Figure 4. M6, M7, M8, M9, M10 MOS tubes constitute the core circuit of the filter, which is similar to the traditional active op amp filter circuit, the difference is that the M8 and M9 MOS tubes of this circuit work in the sub-threshold region. In order to accurately mirror M1 and the current of M2, add buffer 1 in the circuit. The relationship between its input and output is

${\mathrm{<span\; class="notranslate">\; e</span>}}^{\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; out</span>}}}{\mathrm{<span\; class="notranslate">\; no</span>}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; I</span>}}{\mathrm{<span\; class="notranslate">\; no</span>}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}\mathrm{<span\; class="notranslate">\; C</span>}}<span\; class="notranslate">\; \&Integral;</span>{\mathrm{<span\; class="notranslate">\; e</span>}}^{\frac{{\mathrm{<span\; class="notranslate">\; V</span>}}_{\mathrm{<span\; class="notranslate">\; in</span>}}}{\mathrm{<span\; class="notranslate">\; no</span>}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; T</span>}}}}\mathrm{<span\; class="notranslate">\; dt</span>}$

Among them, I is a parameter for adjusting the charging and discharging of the capacitor C.

In Figure 4, M1 and M2MOS transistors perform the functions of current-voltage and voltage-current conversion respectively. M1, M2MOS tubes work in the sub-threshold region. Combined with the above relational expression, the -3dB cut-off frequency expression of the overall filter can be obtained as

${\mathrm{<span\; class="notranslate">\; f</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; I</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}\mathrm{<span\; class="notranslate">\; \&pi;</span>}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; t</span>}}\mathrm{<span\; class="notranslate">\; C</span>}}$

After obtaining the energy of the filter current, in order to wait for the digital control reference code, it can be obtained by comparing with the reference level through the digital controller.

Referring to Figure 5, the digital controller uses the comparison between the output of the previous stage current module and the reference current I1-8 to obtain the logic control level, and then the encoder 16 obtains the control code 17 and the fully differential operational amplifier bias for the digital circuit Control code 18 for current regulation. And the reference current is obtained by the same rectifier 8 and filter 9 . Its purpose is to prevent the influence of the process angle (PVT) on the reference flow. At the same time, in order to prevent the digital controller 11 from oscillating in the critical state, I ₀ is used as the hysteresis current of the digital controller 11 to ensure the stable operation of the digital controller 11.

The digital comparator provides the enable signal for the addition and subtraction of the up-down counter. The up-down counter provides the control code signal to control the op amp resistor array. When the volume changes from a low level to a high level, when the automatic gain control circuit compresses the signal, the counter will count down. Count; when the volume changes from a high level to a high level, the automatic gain control circuit will restore the signal or maintain it to the initial value. In a specific implementation, the digital comparator may output an enabling signal so that the counter stops counting up and the counting is completed.

As can be seen from the above description, the current mode feed-forward automatic gain control circuit of the present invention, which operates at extremely low supply voltage, connects the output end of the microphone to the input end of the closed-loop operational amplifier of automatic gain control, and simultaneously connects the output end of the microphone to the input end of the rectifier. The output voltage of the microphone is amplified by the ratio of the resistor array by the fully differential operational amplifier. At the same time, in order to obtain different digital control signals corresponding to different volume levels, the microphone output signal needs to be quantized. First, the rectifier rectifies the microphone output voltage signal, and the rectified output signal is filtered by the filter to obtain the input signal energy. . The energy signal is compared and processed by the digital controller and the reference level to obtain a digital control signal, and the digital reference code corresponding to the volume level is found on the digital reference table by the digital control signal. When the volume is at the normal volume level, the counter does not work, and the signal is amplified normally; when the transition from low volume level to high volume level, the counter counts down, and the counter counts down from the previous coding state to the current reference coding state, then all The magnification of the differential op amp changes from large to small to achieve signal compression. When transitioning from a high volume level to a low volume level, the counter counts up, and the counter counts up from the previous encoding state to the current reference encoding state, then the magnification of the fully differential op amp changes from small to large, realizing the recovery of the signal enlarge. Stops counting up and down of the counter when the counter output code is the same as the digital reference code. The gain value of the automatic gain control is determined by the digital reference code in the digital reference table, so that the volume corresponds to different digital reference codes at different levels: this realizes the function of the microphone readout circuit to realize the automatic gain control according to the volume level.

The above is the description of the embodiment of the feedforward automatic gain control circuit working on the extremely low power supply voltage and current mode of the present invention to make the microphone work on the very low power supply voltage and current mode feedforward automatic gain control. For those skilled in the art, it is already Certain advantages of obtaining this circuit are obvious. It should also be understood that various alterations, modifications and alternative embodiments can be made within the scope and spirit of the invention. The protection scope of the present invention is completely limited by the protection scope defined by the claims.

Claims (5)

1. A current-mode feed-forward automatic gain control circuit that works on an extremely low power supply voltage and is used for hearing aids; it is characterized in that it includes: a fully differential operational amplifier unit, a capacitive resistance device, a resistor array, an array control switch, a rectifier, and a filter , digital reference table, digital controller, digital comparator and up and down counter; A fully differential operational amplifier unit for amplifying the microphone output signal; The two input terminals of the fully differential operational amplifier unit are respectively connected to the microphone output terminal and the ground through two capacitive resistance devices to form a high-pass filter for the input signal; Two resistor arrays are respectively connected between the two output terminals and the two input terminals of the fully differential operational amplifier unit to determine the gain value of the fully differential operational amplifier to the signal; The two resistor array control switches are respectively connected in parallel to one side of the two resistor arrays to control the gain value of the fully differential operational amplifier unit; A rectifier is used to rectify the small output signal of the microphone; The rectifier and a filter form an envelope detection circuit, which is used to filter the rectified signal to obtain the energy of the output signal of the detection microphone; a digital controller, which compares the output value of the peak detection circuit with the reference level; A digital reference table, used to look up the corresponding digital reference code; An addition and subtraction counter, used for automatic gain control to compress and restore the signal; A digital comparator controls the addition and subtraction counter to perform addition and subtraction operations, and compares the output code of the addition and subtraction counter with the reference code to output the enabling logic of compression and restoration. 2. the current mode feedforward automatic gain control circuit as claimed in claim 1, is characterized in that, concrete circuit is as follows: the first and the second input terminal of fully differential op-amp unit are respectively through the first and the second capacitive resistance device simultaneously The ground is connected to the output terminal of the microphone, the first resistor array is connected across the first input terminal and the second output terminal of the fully differential operational amplifier unit, and the second resistor array is connected across the second input terminal of the fully differential operational amplifier unit and the first The output terminal; the first resistor array control switch is connected across the first resistor array and an output terminal of the fully differential operational amplifier unit, and the second resistor array control switch is connected across the second resistor array and the other output terminal of the fully differential operational amplifier unit; The input end of the rectifier is connected to the output end of the microphone, the order of the rectifier is electrically connected to one input end of the low-pass filter, the digital controller, the digital reference meter, and the digital comparator, and the other output end of the digital controller is electrically connected to the fully differential operational amplifier unit; The input terminal of the addition and subtraction counter is electrically connected with the two clocks and the two output terminals of the digital comparator, and the output terminal of the addition and subtraction counter is respectively electrically connected with the first array control switch, the second array control switch, and the other input end of the digital comparator. 3. The current mode feed-forward automatic gain control circuit as claimed in claim 1, wherein the fully differential op-amp unit adjusts the bias current according to the difference in the amplification factor, so that the operation is affected when the volume is louder. The power consumption is reduced; among them, M1 and M2MOS tubes work in the sub-threshold region, so that they have a constant bandwidth. 4. The current mode feed-forward automatic gain control circuit as claimed in claim 1, wherein the rectifier and the filter rectify the weak signal above 100uV output by the microphone under the extremely low power supply voltage of 0.8V And filtering, and the transmission signal of the rectifier and the filter is a current mode signal; the M8 and M9 MOS tubes of the filter circuit work in the sub-threshold region. 5. The current mode feed-forward automatic gain control circuit as claimed in claim 1, wherein the digital controller, under the extremely low power supply voltage of 0.8V, carries out logical control to the weak signal above 100uV output by the microphone output.

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