How Do I Choose the Best Computer Amplifier?

Op amp is short for operational amplifier. In the actual circuit, a function module is usually combined with the feedback network. Because it was early used in analog computers to implement mathematical operations, it was named "Operational Amplifier" and this name has continued to this day. An op amp is a circuit unit named from the perspective of function. It can be implemented by discrete devices or in semiconductor chips. With the development of semiconductor technology, most op amps now exist in the form of a single chip. There are many types of op amps today, which are widely used in almost all industries.

Chinese name: Op amp
Foreign name: operational amplifier

Short name: Operational Amplifier
Usually combined: The feedback network together forms some kind of functional module

Op Amp Introduction

An operational amplifier (OPA) is an amplifier circuit capable of performing mathematical operations on signals. It used to be a basic component of analog computers, so

Principle of icl7650 chopper-stabilized operational amplifier Get the name. In addition to maintaining the original high gain and input impedance, the operational amplifier made by integrated circuit technology also has the advantages of exquisiteness, cheapness, and flexible use. Therefore, it is used in active filters and switched capacitor circuits. , Digital-to-analog and analog-to-digital converters, DC signal amplification, waveform generation and transformation, and signal processing are widely used.

The DC amplifier circuit is widely used in industrial technology, especially in some measuring instruments and automation control systems. For example, in some automatic control systems, the non-electricity to be controlled (such as temperature, speed, pressure, flow rate, illuminance, etc.) must be converted into an electrical signal by a sensor, and then compared with a given quantity to obtain a weak deviation signal. Because the amplitude and power of this weak deviation signal are not enough to promote the display or the actuator, the deviation signal needs to be amplified to the required level, and then the actuator or the instrument is sent to the instrument for display, so as to achieve automatic control and measurement. the goal of. Because the amplified signals are mostly DC signals that change relatively slowly, amplifiers that analyze AC signal amplification can not effectively couple such signals due to the presence of components such as capacitors, so they cannot achieve amplification of such signals. The most common device that can effectively amplify a slowly changing DC signal is an operational amplifier. The operational amplifier was first invented as an analog signal operation (realization of addition, subtraction, multiplication, division, proportional differential integration, etc.) unit. It is a basic component of an analog electronic computer and consists of a vacuum tube. The operational amplifier used is a directly coupled circuit with high magnification composed of multiple transistors, integrated on a tiny silicon chip.

Basic classification of op amps

Classification of integrated operational amplifiers

According to the parameters of the integrated operational amplifier, the integrated operational amplifier

Principle of icl7650 chopper-stabilized operational amplifier Can be divided into the following categories.

Op amp universal

General purpose operational amplifiers are designed for general purpose. The main characteristics of this type of device are low price and large product volume, and its performance indicators can be suitable for general use. For example, mA741 (single op amp), LM358 (dual op amp), LM324 (quad op amp) and LF356 with FET as input stage belong to this category. They are currently the most widely used integrated operational amplifiers.

Op amp precision op amp

Precision operational amplifiers generally refer to op amps with offset voltages below 1 mV, and at the same time emphasize that the offset value of the offset voltage with temperature changes must be less than 100V. For DC input signals, VOS and its temperature drift are small enough, but for AC input signals, we must also consider the input voltage noise and input current noise of the op amp. In many applications, the input voltage noise ^[1] sound and Input current noise is even more important. At the same time, many application designs require the use of a programmable high-precision operational amplifier (PVGA) to dynamically adjust the amplification in the signal chain.

When it comes to implementing the input processing design of many high-end sensors, there are some challenges in choosing the best precision operational amplifier.

When the sensor type and / or its use environment bring many special requirements, such as ultra-low power consumption, low noise, zero drift, rail-to-rail input and output, reliable thermal stability, and thousands of readings and ( Or) Provide consistent performance reproducibility under harsh operating conditions, making op amp selection particularly difficult.

In complex sensor-based applications, designers need to consider multiple aspects in order to obtain a precision op amp with the best combination of specifications and performance, while also considering cost. Specifically, chopper-stabilized operational amplifiers (zero-drift amplifiers) are well-suited for applications requiring ultra-low offset voltages and zero-drift. The chopper operational amplifier achieves high DC accuracy through a calibration mechanism implemented continuously on the chip.

The difference between precision operational amplifier circuits and ordinary operational amplifier circuits:

The structure of ordinary operational amplifier circuits is generally similar, and precision amplifier circuits will have more special designed circuits such as power supply decoupling and filtering. The main difference is that the performance of precision operational amplifiers is much better than that of general op amps. For example, the open-loop amplification is larger, the CMRR is larger, the speed is slower, and the GBW and SR are generally smaller. Offset voltage or offset current is relatively small, temperature drift is small, noise is low, and so on. The performance of a good precision op amp is far from comparable to that of an ordinary op amp. The offset of a general op amp is often a few mV, while the precision op amp can be as small as 1 uV. To amplify tiny signals, precision op amps must be used. Normal op amps are used, and they will introduce a lot of interference. To improve through peripheral circuits, small or fine adjustments are possible, but they cannot be changed drastically or completely.

With the introduction of various new sensors in the future, people have higher and higher requirements for the performance of electronic equipment, and a large number of automation equipment are put into use. High-precision amplifiers with low offset and low noise will be used in medical electronics, measuring instruments, automotive electronics, industrial automation Equipment and other fields. The performance indicators of high-precision operational amplifiers will keep pace with the times, and continue to innovate in the direction of lower voltage and current noise, lower offset voltage, lower offset voltage temperature drift, larger bandwidth, smaller power consumption, and higher voltage. Continuously introduce new products to meet the increasing design needs of customers.

The most commonly used precision op amps are OP07, and its families, OP27, OP37, OP177, OPA2333. There are many others, such as the products of American AD company, many of which are led by OPA.

Op amp high resistance

The characteristics of this type of integrated operational amplifier are that the differential mode input impedance is very high and the input bias current is very small. Generally, rid> (109 ~ 1012) W, IIB is several picoamperes to tens of picoamps. The main measure to achieve these indicators is to take advantage of the high input impedance of the FET, and use the FET to form the differential input stage of the operational amplifier. Using FET as the input stage not only has high input impedance, low input bias current, but also has the advantages of high speed, wideband, and low noise, but the input offset voltage is large. Common integrated devices include LF356, LF355, LF347 (quad op amp) and higher input impedance CA3130, CA3140, etc.

Op amp low temperature drift

In automatic instruments such as precision instruments and weak signal detection, it is always desirable that the offset voltage of the operational amplifier should be small and not change with temperature. The low temperature drift type operational amplifier is designed for this purpose. Commonly used high-precision, low-temperature drift operational amplifiers are OP-07, OP-27, AD508, and chopper-stabilized low-drift low-drift device ICL7650 composed of MOSFETs.

Op amp high speed

In fast A / D and D / A converters and video amplifiers, the slew rate SR of the integrated operational amplifier must be high, and the unit gain bandwidth BWG must be large enough. General-purpose integrated op amps are not suitable for high-speed applications. The occasion. The main characteristics of high-speed operational amplifiers are high slew rate and wide frequency response. Common op amps are LM318, mA715, etc., whose SR = 50 ~ 70V / us, BWG> 20MHz.

Op Amps Low Power

Because the biggest advantage of the integration of electronic circuits is that they can make complex circuits small and light,

Operational Amplifier The expansion of the application scope of the portable instrument must be compatible with the operational amplifier with low power supply voltage and low power consumption. Commonly used operational amplifiers are TL-022C, TL-060C, etc., whose operating voltage is ± 2V ~ ± 18V, and the current consumption is 50 ~ 250mA. At present, the power consumption of some products has reached the microwatt level. For example, the power supply of the ICL7600 is 1.5V and the power consumption is 10mW. It can be powered by a single battery.

Op amp high voltage high power type

The output voltage of an operational amplifier is mainly limited by the power supply. In ordinary operational amplifiers, the maximum output voltage is generally only tens of volts, and the output current is only tens of milliamps. To increase the output voltage or output current, an auxiliary circuit must be added externally to the integrated op amp. High voltage and high current integrated operational amplifier can output high voltage and high current without any additional circuit. For example, the power supply voltage of D41 integrated operational amplifier can reach ± 150V, and the output current of mA791 integrated operational amplifier can reach 1A.

Op amp instructions

Op Amp Selects Integrated Operational Amplifier Correctly

Integrated operational amplifiers are one of the most widely used devices in analog integrated circuits. In various systems composed of operational amplifiers, because of different application requirements, the performance requirements of operational amplifiers are also different.

Where there are no special requirements, try to use a universal integrated op amp as much as possible, which can reduce costs and easily ensure supply. When using multiple op amps in a system, use as many op amp integrated circuits as possible. For example, LM324 and LF347 are integrated circuits that package four op amps together.

The evaluation of the performance of integrated op amps depends on their overall performance. The figure of merit coefficient K is generally used to measure the goodness of integrated op amps, which is defined as: where SR is the conversion rate and the unit is V / ms. The larger the value, the better the AC characteristics of the op amp; Iib The input bias current of the amplifier, the unit is nA; VOS is the input offset voltage, the unit is mV. The smaller the Iib and VOS values, the better the DC characteristics of the op amp. Therefore, for circuits that amplify AC signals such as audio and video, op amps with large SR (slewing rate) are more suitable; for circuits that handle weak DC signals, op amps with high accuracy are more suitable (both offset current, Offset voltage and temperature drift are relatively small).

When choosing an integrated op amp, other factors should be considered in addition to the figure of merit coefficient. For example, the nature of the signal source is a voltage source or a current source; the nature of the load, whether the output voltage and current of the integrated op amp meet the requirements; whether the environmental conditions, the allowable operating range, operating voltage range, power consumption, and volume of the integrated op amp fulfil requirements.

Points of Operational Amplifier

1. Power supply method for integrated op amp

The integrated op amp has two power supply terminals + VCC and -VEE, but has different power supply methods. For different power supply modes, the requirements for input signals are different.

(1) Symmetric dual power supply mode

Operational amplifiers are mostly powered in this way. The positive power (+ E) and negative power (-E) relative to the common terminal (ground) are connected to the + VCC and -VEE pins of the op amp, respectively. In this way, the signal source can be directly connected to the input pin of the op amp, and the amplitude of the output voltage can reach positive and negative symmetrical power supply voltages.

(2) Single power supply mode

Single-supply operation connects the -VEE pin of the op amp to ground. At this time, in order to ensure that the internal unit circuit of the op amp has a suitable static operating point, a DC potential must be added to the input end of the op amp, as shown in Figure 3.2.1. At this time, the output of the op amp changes with the input signal based on a certain DC potential. For Figure 3.2.1 AC amplifier, the output voltage of the operational amplifier is approximately VCC / 2 when it is static. In order to isolate the DC component in the output, the capacitor C3 is connected.

Figure 3.2.1 Single-Supply Circuit for Operational Amplifier

2. Zeroing of integrated op amps

Due to the influence of the input offset voltage and input offset current of the integrated op amp, when the input signal of a linear circuit composed of an operational amplifier is zero, the output is often not equal to zero. In order to improve the operation accuracy of the circuit, it is required to compensate the error caused by the offset voltage and the offset current. This is the zero adjustment of the operational amplifier. Commonly used zeroing methods include internal zeroing and external zeroing. For integrated op amps without internal zeroing terminals, external zeroing methods should be used. The following uses mA741 as an example. Figure 3.2.2 shows the common zero-adjustment circuit. Figure 3.2.2 (a) shows the internal zeroing circuit; Figure (b) shows the external zeroing circuit.

3 Self-Excited Oscillation of Integrated Operational Amplifier

The operational amplifier is a multi-stage amplifier with high magnification. Under the condition of deep negative feedback, it is easy to generate self-excited oscillation. In order to make the amplifier work stably, a certain frequency compensation network needs to be added to eliminate self-excited oscillation. Figure 3.2.3 shows the circuit used for phase compensation.

Figure 3.2.2 Common zeroing circuit of operational amplifier Figure 3.2.3 Self-excitation cancellation of operational amplifier

In addition, to prevent low-frequency oscillation or high-frequency oscillation caused by the internal resistance of the power supply, an electrolytic capacitor (10mF) and a high-frequency filter capacitor (0.01 mF ~ 0.1mF). As shown in Figure 3.2.3.

4 Protection issues for integrated op amps

There are three aspects to the security protection of the integrated op amp: power protection, input protection and output protection.

(1) Power protection. Common faults of power supply are reverse polarity of power supply and voltage jump. Circuits for reverse power protection and sudden change in power supply voltage are shown in Figure 3.2.4 (a) and (b). For a power supply with poor performance, voltage overshoot often occurs at the moment when the power is turned on and off. Figure (b) uses FET current source and zener clamp protection. The zener voltage is greater than the normal operating voltage of the integrated op amp and less than the maximum allowable operating voltage of the integrated op amp. The current of the FET tube should be greater than the normal operating current of the integrated op amp.

(2) Input protection. If the input differential mode voltage of the integrated op amp is too high or the input common mode voltage is too high (beyond the limit parameter range of the integrated op amp), the integrated op amp will also be damaged. Figure 3.2.5 shows a typical input protection circuit.

Figure 3.2.4 Integrated op amp power protection circuit Figure 3.2.5 Integrated op amp input protection circuit

(3) Output protection. When the integrated op amp is overloaded or the output is shorted, the op amp will be damaged if there is no protection circuit. However, some integrated op amps have internal current limit protection or short circuit protection, and no additional output protection is required to use these devices. For integrated op amps without internal current limit or short circuit protection, the output protection circuit shown in Figure 3.2.6 can be used. In the circuit of Figure 3.2.6, when the output is protected, the resistor R plays the role of current limiting protection.

Figure 3.2.6 Integrated op amp output protection circuit

Main parameters of op amp

1. Common mode input resistance (RINCM) This parameter indicates the ratio of the input common mode voltage range to the change amount of the bias current in the range when the operational amplifier is operating in the linear region.

2. DC Common Mode Rejection (CMRDC) This parameter is used to measure the ability of an operational amplifier to suppress the same DC signal acting on two inputs.

3. AC Common Mode Rejection (CMRAC) CMRAC is a measure of the ability of an op amp to suppress the same AC signal acting on two inputs.

4. Gain Bandwidth Product (GBW) Gain Bandwidth Product AOL is a constant that defines the area that rolls off at -20dB / decade in the characteristic curve of the open-loop gain versus frequency.

5. Input bias current (IB) This parameter refers to the average current flowing into the input when the operational amplifier is operating in the linear region.

6. Input bias current temperature drift (TCIB) This parameter represents the amount of change in the input bias current when the temperature changes. TCIB is usually expressed in pA / ° C.

7. Input offset current (IOS) This parameter refers to the difference between the currents flowing into the two input terminals.

8. Input offset current temperature drift (TCIOS) This parameter represents the amount of change in the input offset current when the temperature changes. TCIOS is usually expressed in pA / ° C.

9. Differential mode input resistance (RIN) This parameter indicates the ratio of the change in the input voltage to the corresponding change in the input current. The change in voltage causes the change in current. When measuring at one input, the other input is connected to a fixed common-mode voltage.

10. Output impedance (ZO) This parameter refers to the internal equivalent small signal impedance of the output terminal when the operational amplifier is operating in the linear region.

11. Output voltage swing (VO) This parameter refers to the peak-to-peak value of the maximum voltage swing that can be achieved without the output signal being clamped. VO is generally defined under a specific load resistance and power supply voltage.

Principle of icl7650 chopper-stabilized operational amplifier 12. Power consumption (Pd) represents the static power consumed by a device at a given supply voltage. Pd is usually defined under no-load conditions.

13. Power supply rejection ratio (PSRR) This parameter is used to measure the ability of the operational amplifier to maintain its output unchanged when the power supply voltage changes. PSRR is usually expressed by the change in the input offset voltage caused by the power supply voltage change.

14. Slew Rate / Slew Rate (SR) This parameter refers to the maximum value of the ratio of the amount of change in output voltage to the time required for this change to occur. SR is usually expressed in units of V / & micro; s, and sometimes expressed as positive change and negative change, respectively.

15. Supply current (ICC, IDD) This parameter is the quiescent current consumed by the device at the specified supply voltage. These parameters are usually defined under no-load conditions.

16. Unit gain bandwidth (BW) This parameter refers to the maximum operating frequency of the operational amplifier when the open-loop gain is greater than 1.

17. Input Offset Voltage (VOS) This parameter indicates the voltage difference that needs to be applied to the input when the output voltage is zero.

18. Input Offset Voltage Temperature Drift (TCVOS) This parameter refers to the change in input offset voltage caused by temperature changes, usually expressed in units of & micro; V / ° C.

19. Input capacitance (CIN) CIN represents the equivalent capacitance of any one input terminal (the other input terminal is grounded) when the operational amplifier is operating in the linear region.

20. Input voltage range (VIN) This parameter refers to the range of input voltage allowed when the operational amplifier is working normally (the expected results can be obtained). VIN is usually defined under the specified power supply voltage.

21. Input voltage noise density (eN) For operational amplifiers, input voltage noise can be viewed as a series noise voltage source connected to any input terminal. EN is usually expressed in units of nV / root Hz, and is defined at a specified frequency.

22. Input current noise density (iN) For operational amplifiers, the input current noise can be regarded as two noise current sources, connected to each input terminal and the common terminal, usually expressed in units of pA / root number Hz, defined in the specified frequency.