CN108563274A - A kind of continuously adjustable linearin-dB variable gain circuit structure - Google Patents

Abstract

The invention discloses a continuously adjustable decibel linear variable gain circuit structure, including: an amplifier core circuit, a common-mode output bias circuit, an exponential proportional current generating circuit, and an exponential proportional current bias circuit; the continuously adjustable gain circuit proposed by the present invention The precise decibel linear variable gain amplification structure, through the ingenious systematic design of three sets of negative feedback structures, two sets of bias replication structures and a current shunt structure differential pair, in a differential amplifier transistor pair of a current steering structure, in ensuring When the total current is constant, that is, the output common-mode voltage is constant, a transistor channel current that changes exponentially with the linear change of the applied control voltage is simply, effectively and accurately realized. This current determines the transconductance of the transistor. As the control voltage changes exponentially, a continuously adjustable variable gain amplifier structure satisfying decibel linear gain control is realized.

Description

A Continuously Adjustable Decibel Linear Variable Gain Circuit Structure

technical field

The invention relates to a gain amplifier, in particular to a novel continuously adjustable decibel linear variable gain circuit structure.

Background technique

Variable gain amplifier circuits are widely used in communication circuit systems, such as automatic gain control, etc., to improve the dynamic range of the system. Usually this kind of gain control requires that the gain of the circuit changes exponentially with the change of the linear control voltage, that is, changes in decibels linearly (dB-Linear). That is, the control voltage is linear, while the gain is linear in decibels (exponential change). There are two broad approaches to this function, one based on a discrete programmable step-by-step digital gain control mechanism, and the other a purely analog continuously adjustable gain control mechanism. In many applications, pure analog continuously adjustable decibel linear control is required to avoid the glitch phenomenon that occurs in digital gain control.

The pure analog continuously adjustable decibel linear continuous gain circuit mainly has the following types of implementation methods. The first is to use a pseudo-exponential control circuit structure based on Taylor series approximation. The disadvantage of this structure lies in the gain and The accuracy of the exponential relationship between the voltages is not high; the second is an accurate decibel linear control circuit, usually with a three-stage tube as the core, through a series of conversion circuits to obtain a wide range of accurate decibel linear gain control relationships, this Such circuits have a wide range of applications, but the difficulty lies in the design of large-scale accurate decibel linear gain control circuits. Usually, the decibel linear control structure of such circuits is extremely complex.

Contents of the invention

For this reason, the present invention proposes a continuously adjustable decibel linear control circuit, and achieves the purpose of simplifying the circuit structure by proposing a novel exponential current generation and replication structure.

The purpose of the present invention is to provide a novel continuously adjustable decibel linear variable gain circuit structure, the circuit structure is simple, and the realized amplifying circuit also has the advantage of a controllable constant common-mode output voltage.

To achieve the above object, the present invention adopts the following technical solutions:

A continuously adjustable decibel linear variable gain circuit structure, comprising: an amplifier core circuit, a common-mode output bias circuit, an exponential proportional current generating circuit, and an exponential proportional current bias circuit;

The core circuit of the amplifier is a differential pair current steering structure, including a differential pair M ₁ /M 2 composed of field effect transistors M1 and _M2 , a differential pair M ₃ /M ₄ composed of field effect transistors M3 and M4, and a pair of differential load resistors R ₁ /R ₂ , and the tail current bias transistor M ₁₃ , the input differential signal is V _IN+ /V _IN- , connected to the gates of the differential pair M ₁ /M ₂ respectively, and the output differential signal is V _OUT+ /V _OUT -, V _OUT+ is connected to the drain of M ₂ /M ₄ , V _OUT- is connected to the drain of M ₁ /M ₃ , and the drain of the differential pair M ₁ /M ₂ is respectively connected to the high-level VDD through the resistor R ₁ /R ₂ . The sources of the differential pairs M ₁ /M ₂ and M ₃ /M ₄ are respectively connected to the drain of the transistor M ₁₃ , and the source of M ₁₃ is grounded;

The common-mode output bias circuit generates a voltage V _B1 and loads it on the gate of _M13 ;

The exponential proportional current generation circuit and the exponential proportional current bias circuit are sequentially connected to generate a voltage VB2 to be loaded on the gate of _M3 / _M4 ; through the joint action of the voltage _VB1 and the voltage _VB2 , the tail current transistor _M13 The bias current I ₀ is constant and controllable, and the channel current I ₁ flowing through M ₁ has an exponential relationship with I ₀ , so that the gain changes linearly in decibels.

Preferably, the common mode output bias circuit includes a differential pair M ₅ /M ₆ composed of field effect transistors M5 and M6, a differential pair M ₇ /M 8 composed of field effect transistors M7 and _M8 , and a pair of differential load resistors R ₃ /R ₄ , and tail current bias transistor M ₁₄ ; the gate of the differential pair M ₅ /M ₆ is connected to an external common-mode voltage V _CM , and the voltage added to the gate of M ₇ /M ₈ comes from an exponential proportional current bias The bias voltage V _B2 generated by the circuit, the output terminals of the differential load resistors R ₃ /R ₄ are shorted together and connected to the positive input terminal of the operational amplifier OP1, and the negative input terminal of OP1 is connected to the common-mode reference voltage V _CM , OP1 The output terminal voltage VB1 is connected to the gate of M ₁₄ to form a negative feedback loop.

Preferably, the negative feedback loop formed by the common-mode output bias circuit and the amplifier core circuit forces the output terminal voltage V _X1 of the load resistor R ₃ /R ₄ in the common-mode output bias circuit to be equal to V _CM , then V _DD -1/2*I ₀ *R=V _CM , I ₀ is determined by this formula, that is, I ₀ =2*(V _DD −V _CM )/R.

Preferably, the exponential proportional current generating circuit includes a differential structure transistor Q ₁ /Q ₂ composed of transistors Q ₁ and Q ₂ , a pair of differential resistors R ₅ /R ₆ , a tail current transistor M ₁₆ and an operational amplifier OP2; The positive input terminal of OP2 is connected to the output terminal V _X3 of the load resistance R of the Q ₂ branch, the negative input terminal is connected to the external common mode voltage VCM, the output terminal of OP2 is connected to the gate of M ₁₆ , and forms a negative feedback loop with M ₁₆ and Q ₂ .

Preferably, the exponential proportional current bias circuit includes a differential pair M ₉ /M ₁₀ composed of field effect transistors M9 and M10, a differential pair M ₁₁ /M ₁₂ composed of field effect transistors M11 and M12, and a pair of differential resistor pairs R ₇ /R ₈ , the tail current transistor M ₁₅ , and the operational amplifier OP3, the differential pair M ₉ /M ₁₀ is connected to the differential resistance pair R ₇ /R ₈ , and M ₁₁ /M ₁₂ is directly connected to the supply voltage; the gate of the tail current transistor M ₁₅ Load the bias voltage V _B1 generated by the common-mode output bias circuit, the output of the resistor pair R ₇ /R ₈ is shorted and connected to the negative input of the operational amplifier OP3, and the positive input of OP3 is connected to an exponentially proportional current The voltage V _Y1 generated by the generation circuit, the output terminal of OP3 generates a bias voltage V _B2 , which is loaded to the gate of the differential pair M ₁₁ /M ₁₂ to form a negative feedback loop.

Preferably, the common-mode voltage of the differential signal V _IN+ /V _IN- loaded on the gate of the differential pair M ₁ /M ₂ is also set to V _CM during operation. Due to the relationship of the replica bias V _B1 , the gate of M ₁₄ Both the polar bias voltage and the gate bias voltage of M ₁₃ are equal to V _B1 , the tail current of M ₁₃ is equal to the tail current I ₀ of M ₁₄ , that is, I ₀ =2*(V _DD -V _CM )/R, and V _X2 = V _X1 = V _CM ; the current of the differential pair M ₁ /M ₂ is set to I ₁ , the current of the differential pair M ₃ /M ₄ is set to I ₂ , in the distribution relationship between I ₁ and I ₂ , I ₁ is at a constant The proportion of I ₀ in the total current is determined by the bias voltage V _B2 of the gate of M ₃ .

Preferably, the negative feedback loop formed by the exponential proportional current generation circuit makes V _X3 =V _X2 =V _CM , and the current I _B2 of the Q ₂ branch is equal to the current of the M ₁ branch in the amplifier core circuit, that is, I _B2 = I ₀ /2=(V _DD -V _CM )/R, which is a constant and controllable value; at the same time, the voltage _VR loaded on the base of Q ₂ is a reference voltage, and the voltage loaded on the base of Q ₁ is a Linear control voltage V _C .

Preferably, the currents I _B1 and I _B2 of the two branches of Q ₁ and Q ₂ have the following relationship established:

In the formula (1), V _T =kT/q, k is Boltzmann's constant, q is the electron charge, T is the absolute temperature, that is to say, I _B1 and I _B2 are exponentially related, or I _B1 and I ₀ are in the form of Exponential relationship; I _B1 This exponential current produces a voltage V _Y1 on the resistor R of the Q ₁ branch, which is connected to the positive input of the operational amplifier OP3 of the exponential proportional current bias circuit, and is biased by negative feedback Voltage V _B2 .

Preferably, the exponential proportional current bias circuit forms a negative feedback loop, so that the voltages V _Y2 and V _Y1 are equal, and the current I ₁ /2 flowing through the resistor R on the M ₉ /M ₁₀ branch is equal to I _B1 :

Therefore, I ₁ and I ₀ are exponentially related, where I ₀ is constant and controllable.

Preferably, the gain expression of the continuously adjustable decibel linear variable gain circuit structure is:

Where C _ox is the capacitance per unit area of the gate oxide layer of the transistor, is the width-to-length ratio of the M ₁ transistor, μ _n is the mobility of electrons, R is the load resistance, V _DD is the power supply voltage, V _CM is the external common-mode voltage, _VR is the control reference voltage, and V _C is the control of linear change Voltage, V _T =kT/q, k is Boltzmann's constant, q is electron quantity, T is absolute temperature, continuously adjustable in decibels The gain of the linear variable gain circuit structure is in decibels as the control voltage V _C changes linearly linear change.

Advantages of the present invention:

The continuously adjustable and accurate decibel linear variable gain amplification structure proposed by the present invention, through the ingenious systematic design of three sets of negative feedback structures, two sets of bias replication structures and a current shunt structure differential pair, in the differential pair of a current steering structure In the middle of the amplifying transistor pair, under the condition that the total current remains unchanged, that is, the output common-mode voltage remains unchanged, a simple, effective and accurate realization of a transistor channel current that changes exponentially with the linear change of the applied control voltage is determined by the current. The transconductance of the transistor also changes exponentially with the linear change of the control voltage, thereby realizing a continuously adjustable variable gain amplifier structure satisfying decibel linear gain control.

Description of drawings

The present invention will be further described below in conjunction with accompanying drawing and embodiment:

Fig. 1 is the schematic diagram of the continuously adjustable decibel linear variable gain circuit structure of the present invention;

The relationship between the gain and the control voltage of the decibel linear variable gain circuit realized by the parameters in Table 1 in Fig. 2.

Detailed ways

As shown in Figure 1, the present invention comprises four parts, upper left corner circuit module 1 amplifying circuit core part, right upper corner circuit module 2 common mode output bias circuit, left lower corner circuit module 3 exponential proportional current generation circuit and right lower corner circuit module 4 exponentially proportional current bias circuit. The whole invention will be described in detail below based on these four parts.

The input terminal of the whole circuit is V _IN+ /V _IN- , the output is V _OUT+ /V _OUT- , the external common mode voltage is V _CM , the reference voltage V _R is external, and the linear gain control voltage V _C is external. The gain of the amplifier, that is, the ratio of the output signal to the input signal, changes linearly in decibels (exponential change) with the linear change of the control voltage V _C .

The core circuit of the amplifier is a traditional differential pair current steering structure, as shown in the upper left corner of Figure 1, which consists of two pairs of differential pairs M ₁ /M ₂ , M ₃ /M ₄ , and a pair of differential load resistors R ₁ /R ₂ ( The value is R), and the tail current bias transistor M ₁₃ is composed, the input differential signal is V _IN+ /V _IN- , the output differential signal is V _OUT+ /V _OUT- , and the bias current in the tail current transistor M ₁₃ is I ₀ , the common-mode voltage of the output differential signal V _OUT+ /V _OUT- is the power supply voltage V _DD minus the voltage drop on the resistor R 1/2*I ₀ *R, and the gain of the amplifier is A _V ＝g _m1 *R , where g _m1 is the transconductance of transistor _M1 , to make the gain of the amplifier show a linear change in decibels (exponential change) with the linear change of V _C , it is necessary to make the transconductance of transistor _M1 change exponentially, or The channel current _I1 of the transistor _M1 changes exponentially. In this design, we keep the tail current I ₀ of M ₁₃ constant by replicating the bias technique, and at the same time make the channel current I ₁ of M ₁ change linearly with the control voltage V _C through the ingenious exponential current replication technique showed an exponential change. Although the circuit structure of the amplifier core circuit in the upper left corner of the present invention is a traditional current steering differential pair structure on the surface, the core of the invention is the voltage V _B1 loaded on the M ₁₃ gate, and the voltage V B1 loaded on the M ₃ /M ₄ gate The voltage V _B2 of the present invention is generated by the second part of the common mode output bias circuit and the third and fourth part of the exponential proportional current generation and bias circuit in Fig. 1 of the present invention respectively, through the combination of V _B1 and V _B2 On the one hand, it makes the output common-mode voltage V _X2 of the amplifier equal to the external common-mode bias voltage V _CM , which means that the current I ₀ flowing through the load resistance of the amplifier is constant and controllable; on the other hand, it makes the channel flowing through M ₁ The channel current I ₁ has an exponential relationship with the total current I ₀ , so as to achieve the design purpose that the gain changes linearly in decibels.

The method to realize the constant tail current I ₀ of M ₁₃ is as follows, as shown in the common mode output bias circuit in the upper right corner of Fig. 1, two pairs of differential pairs M ₅ /M ₆ , M ₇ /M ₈ in this part of the circuit, Differential Load Resistor R ₃ /R ₄ , and Tail Current Bias Transistor M ₁₄ and Top Left Circuit in Figure 1 Amplifier Core Circuit M ₁ /M ₂ , M ₃ /M ₄ , Differential Load Resistor R ₁ /R ₂ , The tail current bias transistor M ₁₃ is exactly the same, the gate of the differential pair M ₅ /M ₆ in circuit 2 in the upper right corner of Figure 1 is connected to the external common mode voltage V _CM , and the voltage applied to the gate of M ₇ /M ₈ comes from Figure 1 The bias voltage V _B2 generated by circuit 4 in the lower right corner, the output terminals of a pair of differential load resistors in circuit 2 in the upper right corner of Figure 1 are shorted together and connected to the positive input terminal of the operational amplifier OP1, and the negative input terminal of OP1 is connected to The common-mode reference voltage V _CM , the output terminal of OP1 is connected to the gate of M ₁₄ , forming a negative feedback loop, which forces the output terminal voltage V _X1 of the load resistor R ₃ /R ₄ in the circuit 2 in the upper right corner of Figure 1 to be equal to V _CM , which means that the following formula holds: V _DD −1/2*I ₀ *R=V _CM , and I ₀ can be determined by this formula, that is, I ₀ =2*(V _DD −V _CM )/R. The common-mode voltage of the differential signal V _IN+ /V _IN- loaded on the gate of the differential input pair M ₁ /M ₂ in circuit 1 in the upper left corner of Figure 1 is also set to V _CM during operation, due to the relationship of the replica bias V _B1 , That is, the main structure of circuit 1 and circuit 2 in Fig. 1 is the same, the gate bias voltage of M ₁₄ and the gate bias voltage of M ₁₃ are both equal to V _B1 , the current distribution relationship in circuit 1 in the upper left corner of Fig. 1 and the circuit 2 in the upper right corner It is exactly the same, that is, the tail current of M ₁₃ is equal to the tail current I ₀ of M ₁₄ , that is, I ₀ =2*(V _DD -V _CM )/R, and V _X2 =V _X1 =V _CM , while I ₁ and I ₂ The distribution relationship or the proportion of _I1 in the constant _I0 total current is determined by the bias voltage V _B2 of the gate of _M3 . In Figure 1, the circuit 3 in the lower left corner and the circuit 4 in the lower right corner cooperate to generate the bias voltage V _B2 , so that the value of I ₁ is taken as a part of I ₀ and changes exponentially, and the specific implementation method of the invention is described as follows.

Circuit 3 in the lower left corner of Figure 1 is composed of a pair of differential transistors Q ₁ and Q ₂ , a pair of differential resistors R ₅ /R ₆ , tail current transistor M ₁₆ and operational amplifier OP2. The positive input terminal of operational amplifier OP2 is connected to Q ₂ The output terminal V _X3 of the road load resistor R ₅ /R ₆ , the negative input terminal is connected to the external common mode voltage VCM, the output terminal of OP2 is connected to the gate of M ₁₆ , and forms a negative feedback loop with M ₁₆ and Q ₂ , so that the following relationship is established V _X3 =V _X2 =V _CM . This means that the current I _B2 of the Q ₂ branch is equal to the current of the M ₁ branch in the circuit 1 in the upper left corner of Figure 1, that is, I _B2 =I ₀ /2=(V _DD -V _CM )/R, which is a constant and controllable value. At the same time, the voltage V _R loaded on the base of Q ₂ is a reference voltage, and the voltage loaded on the base of Q ₁ is a linear control voltage V _C . Due to the characteristics of the triode, the current I _B1 of the two branches of Q ₁ and Q ₂ and I _B2 have the following relationship established:

In the formula (1), V _T =kT/q, k is Boltzmann's constant, q is the electron charge, T is the absolute temperature, that is to say, I _B1 and I _B2 are exponentially related, or I _B1 and I ₀ are in the form of Exponential relationship. The exponential current of I _B1 produces a voltage V _Y1 on the resistor _R5 / _R6 of the _Q1 branch, which is connected to the positive input of the operational amplifier OP3 of the circuit 4 in the lower right corner of Figure 1, and is biased by negative feedback. Set the voltage V _B2 , the specific design details are as follows. Circuit 4 in the lower right corner of Figure 1 is composed of differential pairs M ₉ /M ₁₀ , M ₁₁ /M ₁₂ and tail current transistor M ₁₅ , a pair of differential resistors R ₇ /R ₈ , and an operational amplifier OP3. Its transistors and load resistor components The value of is exactly the same as the components in circuit 2 in the upper right corner of Figure 1, but the connection method is slightly different. In circuit 2 in the upper right corner, the differential pair M ₅ /M ₆ and M ₇ /M ₈ are connected in parallel, and the two branches The current summed to the differential resistor. In circuit 4 in the lower right corner, the differential pair M ₉ /M ₁₀ is connected to the differential resistor pair R ₇ /R ₈ , and M ₁₁ /M ₁₂ is directly connected to the power supply voltage. The purpose of this is to form a current shunt structure, and M ₉ /M ₁₀ , the currents of the two branches of M ₁₁ /M ₁₂ are separated, and the gate of the tail current transistor M ₁₅ in the circuit 4 in the lower right corner of Figure 1 is loaded with the bias voltage V _B1 generated by the circuit 2 in the upper right corner of Figure 1, a pair The output terminal of the differential resistor R ₇ /R ₈ is shorted and connected to the negative input terminal of the operational amplifier OP3, the positive input terminal of OP3 is connected to the voltage V _Y1 generated by circuit 3 in the lower left corner of Figure 1, and the output terminal of OP3 produces the key bias The voltage V _B2 is loaded to the gate of the differential pair M ₁₁ /M ₁₂ to form a negative feedback loop, so that the voltage V _Y2 and V _Y1 are equal, which means that the current I ₁ flowing through the resistor R on the M ₉ /M ₁₀ branch /2 is equal to I _B1 , that is to say, the following equation holds:

Therefore, I ₁ and I ₀ have an exponential relationship and I ₀ is a constant and controllable value. Due to the replication bias, the M ₁ /M ₂ branch current I ₁ and the total tail of the circuit 1 in the upper left corner of Fig. 1 The current I ₀ is exactly the same as the corresponding current in circuit 2 and circuit 3, that is, I ₁ and I ₀ in the main amplifier also satisfy the relationship of formula (2), and I ₀ =2*(V _DD -V _CM )/R , is a current that can be controlled by controlling the external common-mode voltage V _CM . Since I ₀ is a fixed value when V _CM is fixed, then I ₁ is a value that changes exponentially with the linear change of control voltage V _{C -VR} , because the transconductance g _m1 of M ₁ transistor is proportional to the current The half power of I ₁ is also a value that changes exponentially with the linear change of the control voltage V _{C -VR} , so the gain of the amplifier core circuit in Figure 1 is also linear with the control voltage V _{C -VR} It changes exponentially, that is to say, it is a variable gain amplification structure that satisfies the decibel linear control relationship. The gain expression for this amplifier is:

Where C _ox is the capacitance per unit area of the gate oxide layer of the transistor, is the width-to-length ratio of the M ₁ transistor, μ _n is the mobility of electrons, R is the load resistance, V _DD is the power supply voltage, V _CM is the external common-mode voltage, _VR is the control reference voltage, and V _C is the control of linear change Voltage, V _T =kT/q, k is Boltzmann's constant, q is electron charge, and T is absolute temperature. It can be seen from formula (3) that the gain of the circuit module changes exponentially, or in other words, changes linearly in decibels as the control voltage V _C changes linearly.

A specific implementation of a new decibel linear variable gain circuit structure based on replica bias technology proposed by the present invention is given below, using a 0.13um CMOS process. The parameters of each key transistor in the example circuit are shown in Table 1. The operational amplifier in the circuit is a common circuit element, so no special description is given in the embodiment. The circuit operates on a supply voltage of 1.2V. The circuit is implemented and simulated using the parameters shown in Table 1. The simulation results are shown in Figure 2. The results show that, under the power supply voltage of 1.2V, the common mode voltage V _CM of 0.8V and the reference voltage _VR of 0.8V are used, and the control voltage V _C changes linearly from 0.8V to 0.9V. The circuit The gain changes from -8dB to 10dB, and the change of its decibel value is linear, which realizes the precise continuous adjustable decibel linear gain control. Cascade connection of a plurality of identical amplifying units can realize a wider gain control range. For example, cascade connection of four identical amplifying circuits proposed by the present invention can realize gain control from -32dB to 40dB.

Table 1 The structure parameter table of the novel continuously adjustable decibel linear variable gain circuit proposed by the present invention

The above-mentioned embodiments are only to illustrate the technical conception and characteristics of the present invention, and its purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and not to limit the protection scope of the present invention. All modifications made according to the spirit of the main technical solutions of the present invention shall fall within the protection scope of the present invention.

Claims (10)

1. A continuously adjustable decibel linear variable gain circuit structure is characterized in that it comprises: amplifier core circuit, common mode output bias circuit, exponential proportional current generation circuit, exponential proportional current bias circuit; The core circuit of the amplifier is a differential pair current steering structure, including a differential pair M ₁ /M 2 composed of field effect transistors M1 and _M2 , a differential pair M ₃ /M ₄ composed of field effect transistors M3 and M4, and a pair of differential load resistors R ₁ /R ₂ , and the tail current bias transistor M ₁₃ , the input differential signal is V _IN+ /V _IN- , connected to the gates of the differential pair M ₁ /M ₂ respectively, and the output differential signal is V _OUT+ /V _OUT- , V _OUT+ is connected to the drain of M ₂ /M ₄ , V _OUT- is connected to the drain of M ₁ /M ₃ , and the drain of the differential pair M ₁ /M ₂ is respectively connected to the high-level VDD through the resistor R ₁ /R ₂ . The sources of the differential pairs M ₁ /M ₂ and M ₃ /M ₄ are respectively connected to the drain of the transistor M ₁₃ , and the source of M ₁₃ is grounded; The common-mode output bias circuit generates a voltage V _B1 and loads it on the gate of _M13 ; The exponential proportional current generation circuit and the exponential proportional current bias circuit are sequentially connected to generate a voltage VB2 to be loaded on the gate of _M3 / _M4 ; through the joint action of the voltage _VB1 and the voltage _VB2 , the tail current transistor _M13 The bias current I ₀ is constant and controllable, and the channel current I ₁ flowing through M ₁ has an exponential relationship with I ₀ , so that the gain changes linearly in decibels. 2. The novel continuously adjustable decibel linear variable gain circuit structure according to claim 1, characterized in that the common mode output bias circuit comprises a differential pair M ₅ /M ₆ composed of field effect transistors M5 and M6 , the differential pair M ₇ /M ₈ composed of field effect transistors M7 and M8, a pair of differential load resistors R ₃ /R ₄ , and the tail current bias transistor M ₁₄ ; the gate of the differential pair M ₅ /M ₆ is connected to an external common mode The voltage V _CM , the voltage added to the gate of M ₇ /M ₈ comes from the bias voltage V _B2 generated by the exponential proportional current bias circuit, and the output ends of the differential load resistors R ₃ /R ₄ are shorted together and connected to The positive input terminal of the operational amplifier OP1, the negative input terminal of OP1 are connected to the common-mode reference voltage V _CM , the output terminal voltage VB1 of OP1 is connected to the gate of M ₁₄ , forming a negative feedback loop. 3. The novel continuously adjustable decibel linear variable gain circuit structure according to claim 2, characterized in that, the negative feedback loop formed by the common-mode output bias circuit and the amplifier core circuit forces the common-mode output bias circuit to The output terminal voltage V _X1 of the load resistor R ₃ /R ₄ is equal to V _CM , then V _DD -1/2*I ₀ *R=V _CM , and I ₀ is determined by this formula, that is, I ₀ =2*(V _DD -V _CM )/R. 4. The novel continuously adjustable decibel linear variable gain circuit structure according to claim 3, characterized in that, the exponential proportional current generating circuit comprises a differential structure triode Q ₁ /Q ₂ composed of triodes Q ₁ and Q ₂ , A pair of differential resistors R ₅ /R ₆ , tail current transistor M ₁₆ and operational amplifier OP2; the positive input terminal of the operational amplifier OP2 is connected to the output terminal V _X3 of the load resistor R of the Q ₂ branch, and the negative input terminal is connected to the external common mode voltage VCM , the output terminal of OP2 is connected to the gate of M ₁₆ , forming a negative feedback loop with M ₁₆ and Q ₂ . 5. The novel continuously adjustable decibel linear variable gain circuit structure according to claim 4, wherein the exponential proportional current bias circuit comprises a differential pair M ₉ /M ₁₀ composed of field effect transistors M9 and M10, The differential pair M ₁₁ /M ₁₂ composed of field effect transistors M11 and M12, a pair of differential resistor pairs R ₇ /R ₈ , the tail current transistor M ₁₅ , and the operational amplifier OP3, and the differential pair M ₉ /M ₁₀ are connected to the differential resistor pair R ₇ /R ₈ , M ₁₁ /M ₁₂ are directly connected to the supply voltage; the gate of the tail current transistor M ₁₅ is loaded with the bias voltage V _B1 generated in the common-mode output bias circuit, and the output terminal of the resistor pair R ₇ /R ₈ Short circuit and connect to the negative input terminal of the operational amplifier OP3, the positive input terminal of OP3 is connected to the voltage V _Y1 generated by the exponential proportional current generation circuit, the output terminal of OP3 generates the bias voltage V _B2 , and loads it to the differential pair M ₁₁ /M The gate of ₁₂ constitutes a negative feedback loop. 6. The novel continuously adjustable decibel linear variable gain circuit structure according to claim 3, wherein the common-mode voltage of the differential signal V _IN+ /V _IN- loaded on the gate of the differential pair M ₁ /M ₂ It is also set to V _CM during operation. Due to the relationship of replica bias V _B1 , the gate bias voltage of M ₁₄ and the gate bias voltage of M ₁₃ are both equal to V _B1 , and the tail current of M ₁₃ is equal to the tail current I of M _{14 0} , that is, I ₀ =2*(V _DD -V _CM )/R, and V _X2 =V _X1 =V _CM ; the current of the differential pair M ₁ /M ₂ is set to I ₁ , and the current of the differential pair M ₃ /M ₄ The current is set as I ₂ , and in the distribution relationship between I ₁ and I ₂ , the proportion of I ₁ in the constant I ₀ total current is determined by the bias voltage V _B2 of the gate of M ₃ . 7. The novel continuously adjustable decibel linear variable gain circuit structure according to claim 5, characterized in that, the negative feedback loop formed by the exponential proportional current generating circuit makes V _X3 =V _X2 =V _CM , Q ₂ The current I _B2 of the branch is equal to the current of the M ₁ branch in the core circuit of the amplifier, that is, I _B2 =I ₀ /2=(V _DD -V _CM )/R, which is a constant and controllable value; at the same time, the Q ₂ base The voltage _VR loaded on the pole is a reference voltage, and the voltage loaded on the base of Q ₁ is a linear control voltage V _C . 8. novel continuously adjustable decibel linear variable gain circuit structure according to claim 7, is characterized in that, the current I _B1 and I _B2 of Q ₁ and Q ₂ two branches have following relational expression to set up: In the formula (1), V _T =kT/q, k is Boltzmann's constant, q is the electron charge, T is the absolute temperature, that is to say, I _B1 and I _B2 are exponentially related, or I _B1 and I ₀ are in the form of Exponential relationship; I _B1 This exponential current produces a voltage V _Y1 on the resistor R of the Q ₁ branch, which is connected to the positive input of the operational amplifier OP3 of the exponential proportional current bias circuit, and is biased by negative feedback Voltage V _B2 . 9. The novel continuously adjustable decibel linear variable gain circuit structure according to claim 8, characterized in that, the exponential proportional current bias circuit forms a negative feedback loop, so that the voltages V _Y2 and V _Y1 are equal, M ₉ / The current I ₁ /2 flowing through the resistor R on the branch M ₁₀ is equal to I _B1 : Therefore, I ₁ and I ₀ are exponentially related, where I ₀ is constant and controllable. 10. The novel continuously adjustable decibel linear variable gain circuit structure according to claim 9, wherein the gain expression of the continuously adjustable decibel linear variable gain circuit structure is: Where C _ox is the capacitance per unit area of the gate oxide layer of the transistor, is the width-to-length ratio of the M ₁ transistor, μ _n is the mobility of electrons, R is the load resistance, V _DD is the power supply voltage, V _CM is the external common-mode voltage, _VR is the control reference voltage, and V _C is the control of linear change Voltage, V _T =kT/q, k is Boltzmann's constant, q is electron quantity, T is absolute temperature, continuously adjustable in decibels The gain of the linear variable gain circuit structure is in decibels as the control voltage V _C changes linearly linear change.