CN106603018A - Low-noise preamplifier and noise optimization method - Google Patents

Abstract

The invention relates to a low-noise preamplifier and a noise optimization method, belonging to the technical field of signal processing. A low-noise preamplifier provided by the present invention includes a signal amplifying circuit, a power supply circuit, and a filter circuit; it also includes an offset zeroing circuit and a temperature compensation circuit connected to the input end of the amplifier; the offset zeroing circuit includes a zeroing potential and a reference voltage source for compensating the offset voltage of the amplifier; the temperature compensation circuit includes three operational amplifiers, two of which have the same temperature drift, and are used to suppress noise caused by temperature drift, and also provide Noise optimization methods for preamplifiers. A low-noise preamplifier and noise optimization method provided by the present invention effectively solve the problems of noise interference and temperature drift in the small signal amplification process, and improve the gain accuracy and system stability at the same time, and the preamplifier The gain is adjustable, which can be applied to the needs of different signal gains.

Description

A low-noise preamplifier and noise optimization method

technical field

The invention relates to the technical field of signal processing, in particular to a low-noise preamplifier and a noise optimization method.

Background technique

The preamplifier is mainly used in the field of weak signal detection. From the amplifying circuit composed of discrete components at the beginning, to the current integrated operational amplifier as the main component of the preamplifier, the preamplifier is moving towards low noise, high speed, and high resolution. , wide bandwidth, and high integration.

Most of the existing preamplifiers are designed with discrete components or integrated operational amplifiers. Among them, the discrete components are generally triodes or MOS tubes, which are composed of a bias circuit, an input stage, an intermediate stage and an output stage. The preamplifier circuit composed of discrete components has a complex structure and is difficult to debug. The preamplifier designed with integrated op amp generally adopts a multi-stage op amp cascade circuit structure, and sometimes uses two op amps at the input to form a differential input. The preamplifier composed of integrated op amps has a simple structure and is easy to debug , but the noise is large, the temperature is greatly affected, and the system is not stable enough. Especially for the amplification of weak signals, which are difficult to detect and susceptible to interference, the preamplifier must have the characteristics of low noise, low temperature drift, high precision and high gain. The existing amplifiers have small gain and weak anti-interference ability. The temperature drift is large and cannot meet the demand.

Contents of the invention

In view of this, the object of the present invention is to provide a low-noise preamplifier and a noise optimization method, which are used to solve the problems of noise interference and temperature drift in the process of amplifying small signals.

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

A low-noise preamplifier, including a signal amplifying circuit, a power supply circuit, and a filter circuit; it also includes an offset zeroing circuit and a temperature compensation circuit connected to the input of the amplifier; the offset zeroing circuit includes a zeroing potentiometer and a reference The voltage source is used to compensate the offset voltage of the amplifier; the temperature compensation circuit includes three operational amplifiers, two of which have the same temperature drift.

Further, the signal amplifying circuit includes multi-stage cascaded operational amplifiers, and the operational amplifiers of each stage adopt a non-inverting negative feedback amplification structure.

Further, the preamplifier also includes a parameter control module for adjusting the gain accuracy of the amplifier.

Further, the parameter control module includes a gain parameter control circuit and a gain precision control circuit; the gain parameter control circuit includes a relay, an operational amplifier unit, a manual switch, and a digital I/O control interface; the gain precision control circuit includes a relay, Potentiometers, feedback loops, voltage followers.

Further, the preamplifier also includes a filter circuit, and the filter circuit includes an RC first-order filter circuit and an RC second-order filter circuit.

Further, the power supply circuit includes a digital power supply circuit and an analog power supply circuit, the digital power supply circuit is used to provide power to the gain control circuit, and the analog power supply circuit is used to provide power to the preamplifier.

A noise optimization method of a preamplifier, an offset zeroing circuit and a temperature compensation circuit are set at the input end of the amplifier; the offset zeroing circuit includes a zeroing potentiometer and a reference voltage source, by adjusting the reference voltage value and the potentiometer value , to ensure that when the input signal is zero, the output of the amplifier is also zero, and the offset voltage of the compensation amplifier is compensated; the temperature compensation circuit includes three operational amplifiers, two of which have the same temperature drift. The op amp generates voltage noise that is optimized with another op amp that has the same temperature drift.

The method also includes a gain parameter control method. The gain parameter control circuit is connected to the signal amplification circuit by setting the gain parameter control circuit. The gain parameter control circuit includes a relay, an operational amplifier unit, a manual switch, and a digital I/O control interface. Through the manual switch or I The /O control interface controls the relay to adjust the gain of the signal amplifying circuit.

The method also includes a gain precision control method. The gain precision control circuit is connected to the signal amplification circuit by setting the gain precision control circuit. The gain precision control circuit includes a relay, a potentiometer, a feedback loop, and a voltage follower. By adjusting the value of the potentiometer in the feedback loop Controls gain accuracy.

The beneficial effect of the present invention is: a kind of low-noise preamplifier and noise optimization method provided by the present invention have the following advantages:

1. By setting an offset zeroing circuit at the input end of the amplifier, the noise generated by the op amp is compensated. The compensated noise mainly includes the bias voltage and offset voltage of the op amp.

2. By setting the temperature compensation circuit, it is used to offset the voltage drift of the signal amplification module due to temperature changes, avoiding the distortion of the signal due to temperature changes, so that the preamplifier can maintain good performance under different operating temperatures.

3. By setting the gain parameter control circuit, it is used to adjust the variation range of the preamplifier gain. The amplifier gain has a wide range of variation and can be applied to different signal gain requirements. The introduction of the I/O interface allows the preamplifier to be remotely controlled by the host computer .

4. By setting the gain precision control circuit and adjusting the value of the potentiometer in the feedback loop of the precision control circuit, the precision of each gain can be precisely controlled. The feedback loop increases the stability and anti-noise ability of the system.

Description of drawings

In order to make the purpose, technical scheme and beneficial effect of the present invention clearer, the present invention provides the following drawings for illustration:

Fig. 1 is the principle block diagram of low noise preamplifier of the present invention;

Fig. 2 is the gain control circuit diagram of preamplifier described in the present invention;

Fig. 3 is the principle schematic diagram of preamplifier negative feedback circuit of the present invention;

Fig. 4 is the temperature compensation circuit diagram of the present invention;

Fig. 5 is a power circuit diagram of the present invention;

Fig. 6 is a diagram of frequency characteristics of different gains in the embodiment.

detailed description

The preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

A low-noise preamplifier provided by the invention includes a signal amplifying circuit, a power supply circuit, a filter circuit, an offset zeroing circuit, a temperature compensation circuit, and a parameter control module.

Among them, the signal amplification circuit adopts a cascaded amplification structure, including the first operational amplifier stage, the second operational amplifier stage...the Nth operational amplifier stage, each stage uses the same operational amplifier, has the same unit circuit structure, and It is the same direction negative feedback amplifier circuit.

The power supply circuit includes a digital power supply circuit and an analog power supply circuit. The digital power supply circuit generates a ±12V power rail to provide power for the gain control circuit. The analog power supply circuit generates a ±5V power rail to provide power for the operational amplifier.

The filter circuit includes an RC first-order filter circuit and an RC second-order filter circuit. The signal passes through the filter circuit to filter out high-frequency noise outside the signal bandwidth.

The offset zeroing circuit includes a zeroing potentiometer, a reference voltage source, and by adjusting the reference voltage value and the potentiometer value, the offset voltage of the operational amplifier is compensated to achieve the purpose of reducing the noise figure of the amplifier.

The temperature compensation circuit includes three operational amplifiers, two of which are identical and have the same temperature drift, and the other operational amplifier forms an active low-pass filter.

The parameter control module includes a gain parameter control circuit and a gain precision control circuit. The gain parameter control circuit includes a relay, an op amp unit, a manual switch, and a digital I/O control interface. The relay is controlled through a manual or digital port to control the circuit path and achieve the purpose of gain control. The gain accuracy control circuit includes relays, potentiometers, feedback loops, and voltage followers. By adjusting the value of the potentiometer in the feedback loop, the purpose of precisely controlling the gain of each gear is achieved.

In the circuit of the preamplifier, the noise inside the circuit mainly includes the offset voltage noise generated by the op amp itself and the noise caused by the temperature drift. Aiming at the respective characteristics of these two noises, the present invention provides a preamplifier noise The optimization method adopts two different noise suppression techniques, and then adds a negative feedback loop to the circuit to further reduce the noise of the preamplifier circuit.

A noise optimization method for a preamplifier, specifically setting an offset zeroing circuit and a temperature compensation circuit at the input end of the amplifier. The offset zeroing circuit includes a zeroing potentiometer and a reference voltage source. When the input signal is zero, the op amp has an output voltage, which is the noise voltage generated by the op amp. The noise of the operational amplifier is mainly caused by the input bias current and input offset voltage. According to the noise characteristics of the operational amplifier, the offset zeroing circuit adopts the method of zeroing the output voltage through the potentiometer to suppress the noise of the operational amplifier. A potentiometer is added to the same input terminal of the op amp and the bias voltage is provided by the power supply, and the value of the potentiometer is adjusted so that when the input signal is zero, the output of the op amp is also zero.

When the temperature changes, the output voltage of the op amp will change with the temperature, causing the output signal to shift and making the output signal unstable. According to the temperature drift characteristics of the operational amplifier, temperature compensation technology is adopted. According to the same temperature drift characteristics of the same operational amplifier, the drift caused by temperature is also the same. By using an additional identical operational amplifier, the drift amount of the operational amplifier due to temperature changes Compensate the temperature drift of the operational amplifier in the amplifier circuit.

The method also includes a gain parameter control method. The gain parameter control circuit is connected to the signal amplification circuit by setting the gain parameter control circuit. The gain parameter control circuit includes a relay, an operational amplifier unit, a manual switch, and a digital I/O control interface. Through the manual switch or I The /O control interface controls the relay to adjust the gain of the signal amplifying circuit.

The method also includes a gain precision control method. The gain precision control circuit is connected to the signal amplification circuit by setting the gain precision control circuit. The gain precision control circuit includes a relay, a potentiometer, a feedback loop, and a voltage follower. By adjusting the value of the potentiometer in the feedback loop Controls gain accuracy.

There will be gain, signal bandwidth and other changing factors in the signal amplification process, so that the preamplifier has gain fluctuations, which affect the gain accuracy. In order to reduce the impact of signal fluctuations, the method of the present invention uses negative feedback technology. A negative feedback loop is added to the circuit. After the introduction of negative feedback, it can not only accurately control the gain accuracy of the amplifier, but also effectively suppress the circuit noise.

Example

The structure of the preamplifier is shown in Figure 1, including a noise optimization circuit 1, specifically a temperature compensation circuit and an offset zeroing circuit, a signal amplification circuit 2, and a parameter control module 3.

Among them, signal amplifying circuit 2: the signal amplifying circuit adopts a cascaded amplifying structure, which is divided into six stages, and the gain of each stage is 10dB. Each stage of operational amplifier adopts the same direction negative feedback amplification structure to improve the anti-noise ability and signal amplification ability of the circuit. The peripheral resistance should not be too large, and the value is generally tens of ohms to hundreds of ohms.

Gain control circuit 3: The traditional preamplifier generally has a fixed gain and is applicable to a narrow range. The present invention adopts the gain control technology, and the gain of each operational amplifier is fixed at 10dB. The signal amplification path is controlled by the relay. When the signal passes through the first path, the gain is 10dB. When the signal passes through the second path, the signal is divided. The divided signal is then amplified by the op amp, and the overall gain of the signal is 0dB.

The parameter control module includes a gain parameter control circuit and a gain precision control circuit. The relay is controlled by a manual switch or an I/O control port, and the relay selects a 0dB or 10dB signal path to achieve the purpose of controlling the gain. Adjust the potentiometer through the parameter control unit The value of each gain can make the accuracy of each gain within 0.3dB, and adding a feedback loop makes the system more stable and the noise suppression ability stronger.

Figure 2 is the gain control circuit diagram. The preamplifier is divided into 6 amplification stages. Except for the first stage, each stage of amplification is composed of operational amplifiers, voltage divider resistors and corresponding relays. The signal path is changed by controlling the relays. When 2-3 is connected, Vin is divided by resistors R2 and R4, and the gain of the amplifying unit is 0dB. When 4-3 is connected, the gain of the amplifying unit is 10dB. The gain of the preamplifier is controlled by a manual switch or an I/O port, and the manual control switch and the mode selection switch together form a single-pole multi-throw gain switch. When the gain switch is turned to 1, the 3-8 decoder is enabled. At this time, the amplifier gain can be controlled through the I/O port. At this time, the gain adjustment range is 10-60dB. When the gain switch is turned to 2-6, the gains are respectively 20~60dB.

In the design of the preamplifier, in order to ensure the accuracy of the gain, the method of adding a negative feedback loop is adopted. As shown in Figure 3, the negative feedback is added to the circuit, which not only improves the gain accuracy, but also effectively suppresses the circuit noise and improves the system performance. stability.

The total gain of the closed-loop system is:

Among them, α is the system open-loop gain, β is the feedback coefficient, and β is precisely adjusted by the potentiometer. Due to device errors and channel changes, α is difficult to adjust. At this time, the amplifier gain can be accurately adjusted by adjusting the β value. When the gain switch is turned to gear 1 and the output of the decoder is all zero at the same time, the amplification factor α of the first-stage amplifier in Figure 3 is a fixed value, and the potentiometer W6 is properly adjusted to adjust the feedback coefficient β Adjust the total gain A to 10dB precisely. Similarly, when the gain switch is set to level 2, adjust the potentiometer W1 to adjust the gain to 20dB, then adjust the potentiometers W2, W3, W4 and W5 in turn to ensure that the gain of each level is 10dB, 20dB, 30dB, 40dB, 50dB, 60dB.

After adding negative feedback, the output signal of the circuit is:

In the formula, x _n is the noise introduced by the nth amplification node, and α _n is the open-loop gain of the nth amplification node of the system. During the transmission process, x _n will be amplified together with the signal transmitted from the previous stage. If there is no negative feedback, the value of the noise in the output signal is x _n α _n ... α ₆ . After the introduction of negative feedback, x ₂ ~ x ₆ are subject to the positive force between the input and the entry point of the interference itself. The attenuation of the gain, the value of x _n in the output signal is However, the value of 1+α ₁ α ₂ ... α ₆ β is greater than 1, and the noise x ₁ is already very small after being suppressed by the offset and temperature compensation of the first-stage amplifier and can be ignored. Therefore, the entire preamplifier can be ignored by introducing negative feedback loop, circuit noise is effectively suppressed.

Figure 4 is the temperature compensation circuit of the preamplifier. U1 and U3 in Figure 4 are the same operational amplifier with the same temperature drift coefficient. U1 is a voltage follower. When the temperature changes, the voltage noise generated by U1 due to temperature changes is reversed. After inverse amplification to the amplifier U2, it is connected to the same direction end of U3 to compensate the error caused by temperature drift of U3 and subsequent circuits. The AD706 unit itself is a reverse low-pass filter, which will filter out high-frequency noise outside the signal bandwidth. , to ensure that only the error signal caused by temperature drift is connected to U3, and U2 has extremely low temperature drift, which avoids the introduction of additional noise by U2.

Figure 5 shows the preamplifier power supply module, which provides ±15V power rails for the preamplifier through the regulated power supply, and the power supply provides power for the digital circuit module and analog circuit module after passing through the TA78/9L12F and L78/905CT voltage regulator chips .

Finally, it should be noted that the above preferred embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail through the above preferred embodiments, those skilled in the art should understand that it can be described in terms of form and Various changes may be made in the details without departing from the scope of the invention defined by the claims.

Claims (9)

1. A low-noise preamplifier is characterized in that: comprise signal amplifying circuit, power supply circuit, filtering circuit; Also comprise offset zeroing circuit and temperature compensation circuit, be connected to the input end of amplifier; Described offset zeroing circuit comprises The zero adjustment potentiometer and the reference voltage source are used to compensate the offset voltage of the amplifier; the temperature compensation circuit includes three operational amplifiers, two of which have the same temperature drift. 2. A kind of low-noise preamplifier according to claim 1, characterized in that: said signal amplifying circuit comprises multi-stage cascaded operational amplifiers, and the operational amplifiers of each stage all adopt the same direction negative feedback amplification structure. 3. A low-noise preamplifier according to claim 1, characterized in that: the preamplifier further comprises a parameter control module for adjusting the gain accuracy of the amplifier. 4. a kind of low-noise preamplifier according to claim 3, is characterized in that: described parameter control module comprises gain parameter control circuit and gain precision control circuit; Described gain parameter control circuit comprises relay, operational amplifier unit, Manual switch, digital I/O control interface; the gain precision control circuit includes a relay, a potentiometer, a feedback loop, and a voltage follower. 5. A low-noise preamplifier according to claim 1, characterized in that: the preamplifier further comprises a filter circuit, and the filter circuit includes an RC first-order filter circuit and an RC second-order filter circuit. 6. a kind of low-noise preamplifier according to claim 1, is characterized in that: described power supply circuit comprises digital power supply circuit and analog power supply circuit, and described digital power supply circuit is used for providing power for gain control circuit, and described The analog power supply circuit is used to provide power to the preamplifier. 7. A noise optimization method of a preamplifier, characterized in that: an offset zeroing circuit and a temperature compensation circuit are set at the input of the amplifier; the offset zeroing circuit includes a zeroing potentiometer and a reference voltage source, by adjusting the reference The voltage value and the potentiometer value ensure that when the input signal is zero, the output of the amplifier is also zero, and the offset voltage of the compensation amplifier is compensated; the temperature compensation circuit includes three operational amplifiers, two of which have the same temperature drift , the op amp generates voltage noise after temperature change, which is optimized by another op amp with the same temperature drift. 8. the noise optimization method of a kind of preamplifier according to claim 7, it is characterized in that: described method also comprises gain parameter control method, is connected with signal amplifying circuit by setting gain parameter control circuit, described gain parameter control The circuit includes a relay, an operational amplifier unit, a manual switch, and a digital I/O control interface, and the relay is controlled by the manual switch or the I/O control interface to adjust the gain of the signal amplification circuit. 9. the noise optimization method of a kind of preamplifier according to claim 7, it is characterized in that: described method also comprises gain accuracy control method, by setting gain accuracy control circuit and signal amplifying circuit connection, described gain accuracy control The circuit includes a relay, a potentiometer, a feedback loop, and a voltage follower, and the gain accuracy is controlled by adjusting the value of the potentiometer in the feedback loop.