Nonlinear distortion analysis and circuit application of integrated operational amplifier

0 Introduction

Operational amplifiers are widely used in various circuits. They can not only realize linear operation circuit functions such as addition and multiplication, but also form nonlinear circuits such as limiting circuits and function generation circuits. Different connection methods can realize different circuit functions. Integrated operational amplifiers integrate operational amplifiers and some peripheral circuits on a silicon chip to form electronic circuits with specific functions. Integrated operational amplifiers are small in size, easy to use and flexible, and are suitable for use in portable devices such as mobile communications and digital products.

Linear characteristics are an important parameter for examining integrated operational amplifiers with amplification functions and receiving RF front-end circuits, and the linear range also has a great influence on the connection method of the integrated operational amplifier. If the linear range of the integrated operational amplifier is too small, the output signal will generate multiple harmonics and large harmonic power, which will seriously affect the function of the entire circuit. Based on the nonlinear analysis of the integrated operational amplifier, the cause of the nonlinear distortion of the circuit can be found, and the normal operation of the integrated operational amplifier can be achieved by changing the connection method of the integrated operational amplifier without changing the circuit design. The integrated operational amplifier circuit designed and optimized in this paper is applied to the RF front-end circuit of the positioning system to complete the amplification and output of the baseband sweep signal, which can effectively suppress the generation of harmonics of the integrated operational amplifier, achieve high gain of the RF receiving front-end circuit, and improve the driving ability of the back-end circuit design part.

l Differential circuit access method and nonlinear parameters of integrated operational amplifiers

The general integrated operational amplifier circuit consists of bias circuit, input stage, intermediate stage and output stage. The input stage is composed of differential circuit. The differential circuit has two signal input methods: dual-ended input and single-ended input; the bias circuit can be powered by single power supply and dual power supply. In mobile communications or portable devices, a single power supply is generally used. The integrated operational amplifier with a single power supply requires the input signal to be in a unipolar form, that is, the input signal is always positive or negative. The differential input stage can be used to ensure the signal polarity of the input intermediate stage circuit. At the same time, the differential input stage amplifier circuit can effectively suppress the common mode signal and enhance the common mode rejection ratio of the integrated operational amplifier. However, when the common mode input signal is large, the differential pair tube will enter a nonlinear working state, and the amplifier will lose its common mode rejection capability, which seriously affects the common mode rejection ratio of the integrated operational amplifier.

In addition to the maximum common-mode input voltage, the nonlinear characteristic parameters of the integrated operational amplifier include output voltage, output current, maximum output slew rate, and maximum input slew rate, which are also important indicators related to its nonlinear characteristics. Since the output stage transistor of the integrated operational amplifier has a certain saturation voltage drop, its maximum output voltage is 1 to 2 V lower than the power supply voltage, or even lower. The slew rate is the conversion rate of voltage, which is an important indicator of the integrated operational amplifier when working with large signals and high-frequency signals. The higher the slew rate, the faster the voltage response speed of the integrated operational amplifier, and the more it can ensure that the operational amplifier works at a higher frequency; on the contrary, if the slew rate is too low, the output voltage of the operational amplifier cannot immediately follow the change of the step input voltage, and the output signal will be distorted, showing the nonlinear characteristics of the integrated operational amplifier. Regardless of whether it is powered by a single power supply or a positive or negative power supply, the maximum common-mode voltage of the integrated operational amplifier is generally about 2 V lower than the positive and negative voltages of the power supply; for the integrated operational amplifier powered by a single 3.3 V power supply, its maximum common-mode input voltage range is very small, so when the integrated operational amplifier is powered by a low power supply, the impact of the input common-mode signal must be considered.

2 Circuit application of dual-end input integrated amplifier AD8062

In the receiver RF front-end circuit, the modern mixer integrates the function of an operational amplifier, but the output impedance of the internal integrated operational amplifier is high, which makes the driving ability of the mixer back-end load weak, and the voltage gain is also easily affected by the change of load impedance. In order to enable the receiver to receive signals with a large dynamic range, the RF front-end adopts a gain control amplifier circuit; at the same time, considering the high sensitivity requirement of the receiver, a fixed gain integrated operational amplifier is used to re-amplify the weak signal to ensure that the received signal reaches the minimum threshold of A/D sampling. The integrated operational amplifier AD8062 has a very high input impedance and a low output impedance, which can ensure the efficient transmission of the down-converted signal and can effectively improve the load driving ability. In order to meet the gain design requirements of the entire circuit system, it is particularly necessary for the receiving front end to amplify the baseband signal after down-conversion.

2.1 Introduction to the main performance of AD8062

The output voltage swing is 6 mV; the 3 dB bandwidth is 500 MHz; the voltage slew rate is 800 V/μs; the differential amplification phase error is 0.04°; and the power supply voltage is 2.7 to 8 V. AD8062 is a dual integrated operational amplifier that can amplify and output two signals at the same time. The common-mode input voltage range is also large, and it can be used in low-power supply voltage circuits; the output end adopts a rail-to-rail feedback amplifier method, which expands the output voltage range and makes it more convenient to use AD8062. Compared with the same type of current feedback amplifier, AD8062 has a wider signal input bandwidth and a higher slew rate characteristic, which is suitable for application in spread spectrum communication circuits.

2.2 AD8062 circuit implementation and application

In the following circuit design, AD8062 adopts a single power supply of 3.3 V, and the input end uses I/Q two-way orthogonal signals. The input signal used for testing is a single-frequency signal, the input signal power range is -28 to -12 dBm, and the channel bandwidth is 5 MHz. The circuit design of the integrated operational amplifier is applied to the receiving RF front-end circuit of the ISM band positioning system. After the AD8347 down-converts the I/Q orthogonal RF signal, the baseband signal is amplified and output. The AD8062 differential amplifier adopts a double-ended input and single-ended output mode. The I/Q two-way bipolar signals are combined by the integrated operational amplifier and transmitted to the A/D converter for sampling. When the received signal is a frequency hopping signal, the voltage change rate of AD8062 is very fast and can instantly follow the change of the step voltage. The initial design adopts the circuit connection method shown in Figure 1 (a), in which the designed MIMO system receiving RF front-end has been applied. In the MIMO system, a single power supply of 5 V is used, the input signal frequency range is 0 to 5 MHz, and the power is -12 dBm. Under the above conditions, the amplifier power gain is 11 dB, and the harmonic distortion of the signal output is less than -40 dBc, which meets the requirements of receiver sensitivity and back-end A/D sampling. However, the connection method of AD8062 is directly used for a single 3.3 V power supply. When the input signal frequency is 1 MHz, the output characteristics of the circuit are shown in Figures 2 and 3.

The differential amplifier circuit connection mode of AD8062 is 1. The voltage signals of the two bipolar inputs of the integrated operational amplifier I/Q are equal in magnitude and opposite in sign. Theoretically, the gain (dB) of the bipolar signal output by the down-converter to AD8062 is calculated by the following formula:

When the downconverter output signal is -28 dBm, it can be seen from Figure 2 that the gain of AD8062 is about 11 dB and the signal-to-noise ratio is about 20 dB, which basically meets the theoretical calculation value. However, when the downconverter output signal is -12 dBm, the power of each harmonic of the signal input to AD8062 is greater than -50 dBc. It can be found from Figure 3 that when a large signal is input to AD8062, the nonlinear characteristics of AD8062 cause the output signal to be severely distorted. The second harmonic within the 5 MHz signal bandwidth is -10 dBc, and the signal output power cannot meet the theoretical calculation value. The severe in-band harmonic distortion of AD8062 makes it impossible for the back end to detect useful signals, making this circuit unusable.

2.3 Nonlinear Analysis and Circuit Optimization of AD8062

Among the nonlinear parameters of integrated op amps , the rate of change of the voltage of a single-frequency input signal is very low, and it is basically unnecessary to consider that the slew rate characteristic of AD8062 is the cause of harmonic distortion. Considering that the maximum common-mode input voltage range of a single-power integrated op amp will also decrease when the power supply voltage decreases, the harmonic distortion generated by circuit connection method 1 has a great relationship with the maximum input common-mode voltage. The maximum common-mode voltage range of the input signal allowed by A-D8062 is (-Vs—O.2 V) to (+Vs—1.8 V). The smaller the power supply voltage, the smaller the maximum common-mode input voltage range of AD8062. If it exceeds this maximum range, the chip may be burned. In addition, the common-mode input voltage range related to the nonlinear parameters of the integrated op amp is smaller than the above range. In a circuit powered by a single power supply of 5 V, the maximum common-mode voltage range of the input signal allowed is -5.2 to +3.2 V, while in a circuit powered by a single power supply of 3.3 V, the range is -3.5 to +1.5 V. The reduction of power supply voltage reduces the maximum input common-mode voltage range of AD8062, which may be the cause of the nonlinear distortion of the integrated operational amplifier. The signal input end of connection mode 1 does not have a DC blocking capacitor, which will inevitably cause the integrated operational amplifier to amplify the DC signal, which will cause harmonic distortion of the output signal. This is because when the DC voltage exceeds the maximum input common-mode voltage range, the static operating point of the integrated operational amplifier is greatly offset, the output current of one tube in the differential pair tube tends to saturate, and the output current of the other tube tends to be cut off. The difference between the output currents of the two tubes no longer changes with the input signal, but shows the limiting circuit characteristics of the integrated operational amplifier, causing the nonlinear distortion of the output signal of the integrated operational amplifier when a large signal is input. It can be judged from this that in the case of large signal input, the peak value of AD-8062 exceeds the input common-mode voltage range. In order to solve the problem of excessive common-mode voltage, the above circuit is optimized, that is, a DC blocking capacitor is added to the negative input end of the integrated operational amplifier of connection mode 2, and its connection is shown in Figure 1 (b).

In the differential amplifier circuit connection mode 2 of AD8062, the DC blocking capacitor can effectively reduce the DC voltage input to the inverting terminal, and at the same time pass the AC useful signal, thus reducing the input common mode voltage of the integrated operational amplifier, maintaining the larger linear range of the differential circuit at the static operating point, and the output current of the differential pair tube can linearly follow the input signal change. In addition to some influence on the low-frequency signal, the addition of the DC blocking capacitor can efficiently transmit the differential mode voltage of the input integrated operational amplifier to the A/D converter. When designing the ISM band positioning system, the minimum frequency of the baseband sweep signal is 100 kHz, so that the influence of the DC blocking capacitor on the low-frequency signal can be minimized. The characteristic spectrum characteristics of the optimized circuit are shown in Figures 4 and 5.

The frequency input conditions of connection mode 2 are the same as those of mode 1. When the input single-frequency signal power is -28 dBm, the integrated operational amplifier gain of the optimized circuit is greater than 11 dB, and the out-of-band noise is more suppressed. The signal-to-noise ratio is also improved by 1 dB compared with connection mode 1. When the input single-frequency signal power is -12 dBm, due to the reduction of the common-mode input voltage, the AD8062 can work in the linear range, the power of each harmonic is less than -45 dBc, and the output signal power is also improved by 4 dB. The optimized circuit successfully completed the test at the front end of the RF receiver and can provide a larger signal power for the back-end circuit detection signal. And meet the A/D sampling threshold requirements.

At the same time, in the overall test of the ISM band positioning system, the use of AD8062 can make the receiving RF front end meet the receiver sensitivity requirements, and the signal processing part can correctly capture the positioning data. In the test debugging, it was found that the I/Q two-way input signal power of the dual integrated operational amplifier AD8062 cannot have too much deviation, otherwise the circuit cannot work properly. In the designed positioning system RF front-end circuit, the I/Q two-way signal output power deviation of the orthogonal down converter AD8347 is small, and AD8062 can meet the design requirements.

3 Conclusion

Through the nonlinear analysis of the integrated operational amplifier connection circuit, the cause of the harmonic distortion of AD8062 was found, and a new circuit connection method was optimized and designed. This optimized circuit design improves the sensitivity of the receiving RF front end by 1 dB without changing the integrated operational amplifier gain, greatly improves the dynamic range of the entire system, and can ensure that the harmonic distortion of the receiver is less than -50 dBc when a large signal is input. In the design of the ISM band positioning system, the signal-to-noise ratio and power of the optimized circuit output signal can meet the sampling requirements of A/D, and can provide relevant data for the back-end positioning detection algorithm. In the design system with different conditions and different circuits. The application of integrated operational amplifiers will also have nonlinear distortion problems. The above-mentioned nonlinear analysis method of integrated operational amplifiers has certain reference value.