Operational Amplifiers: Characteristics, Functions, Types and Selection

Ⅰ. Operational amplifier characteristics

1. The open-loop magnification of an ideal op amp is infinite.

2. The output voltage of the op amp cannot exceed the power supply voltage of the op amp (generally more than ten volts).

3. The input impedance of an ideal op amp is infinite.

4. The output impedance of an ideal op amp is zero.

Ⅱ. Functions of operational amplifiers

Operational amplifiers are not used alone, but are designed to be connected to other circuits to perform a variety of operations, such as:

1. It can greatly amplify the input signal.

When an operational amplifier is combined with an amplification circuit, a weak signal can be amplified into a strong signal. It acts like a loudspeaker, the input signal is the human voice, and the loudspeaker is the operational amplifier circuit. For example, such circuits can be used to amplify tiny sensor signals. Sensor signal processing can be further improved by feeding the amplified signal into a microcontroller unit (MCU).

2. It can eliminate noise in the input signal.

By acting as a filter for the input signal, the operational amplifier circuit is able to extract a signal with the frequency of interest. For example, when an op amp circuit is used in speech recognition or a voice recorder, it can extract frequencies close to the target frequency. Target sound while rejecting all other frequencies as noise.

Ⅲ. Types of operational amplifier

According to the parameters of integrated operational amplifiers, integrated operational amplifiers can be divided into the following categories:

1. Low-power operational amplifier

Since the biggest advantage of the integration of electronic circuits is that it can make complex circuits small and light, so with the expansion of the application range of portable instruments, we must use operational amplifiers with low power supply voltage and low power consumption.

Commonly used operational amplifiers include TL-022C, TL-060C, etc., whose working voltage is ±2V~±18V, and the consumption current is 50~250μA. At present, the power consumption of some products has reached the μW level. For example, the power supply of ICL7600 is 1.5V, and the power consumption is 10mW, which can be powered by a single battery.

2. Programmable control operational amplifier

In the process of using instruments and meters, we will all involve the problem of measuring range. In order to get a fixed voltage output, we have to change the magnification of the operational amplifier. For example: there is an operational amplifier with a magnification of 10 times, and when the input signal is 1mv, the output voltage is 10mv. When the input voltage is 0.1mv, the output is only 1mv. In order to get 10mv, we have to change the magnification to 100. Programmable operational amplifiers are created to solve this problem.

3. General purpose operational amplifier

General-purpose operational amplifiers are designed for general use. The main characteristics of this type of device are low price, large product volume and wide range, and its performance indicators can be suitable for general use.

For example: μA741 (single op amp), LM358 (dual op amp), LM324 (four op amps) and LF356 with FET as the input stage all belong to this type. They are the most widely used integrated operational amplifiers today.

4. High voltage and high power operational amplifier

The output voltage of the 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 milliamperes. To increase the output voltage or increase the output current, an auxiliary circuit must be added outside the integrated operational amplifier. The high-voltage and high-current integrated operational amplifier can output high voltage and high current without any additional external circuits.

The high-voltage and high-current integrated operational amplifier can output high voltage and high current without any additional external circuits. 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. In general industrial control, especially large-scale industrial control, will use this kind of high-voltage and high-power operational amplifier. In the absence of special requirements, we try to use general-purpose integrated operational amplifiers. This can not only reduce costs, but also easily ensure the supply of goods. When multiple operational amplifiers are used in a system, we try to use multi-operational amplifier integrated circuits, such as LM324, LF347, etc., which package four operational amplifiers together.

5. Low temperature drift operational amplifier

In automatic control instruments such as precision instruments and weak signal detection, we always hope that the offset voltage of the operational amplifier should be small and not change with temperature. Low temperature drift op amps are designed for this purpose. At present, the commonly used high-precision, low-temperature drift operational amplifiers include OP07, OP27, AD508, and the chopping zero-stable low-drift device ICL7650 composed of MOSFETs.

6. High-speed operational amplifier

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 unity gain bandwidth BWG must be large enough. General-purpose integrated operational amplifiers are not suitable for high-speed applications. The main features of high-speed operational amplifiers are high slew rate and wide frequency response.

Common operational amplifiers include LM318, μA715, etc., whose SR=50~70V/us, BWG>20MHz.

7. High resistance operational amplifier

These integrated op amps feature very high differential-mode input impedance and very low input bias current. Generally, rid>1GΩ~1TΩ, and IB is several picoamps to tens of picoamps. The main measure to achieve these indicators is to use the characteristics of the high input impedance of the FET to form the differential input stage of the operational amplifier with the FET. Using a field effect transistor as the input terminal, it has high input impedance and low input bias current, and also has the characteristics of high speed, broadband and low noise.

Common high-impedance operational amplifiers currently on the market include LF355, LF347 (four operational amplifiers), CA3130 and CA3140 (the input impedance is higher) and so on.

8. Precision operational amplifiers

A precision operational amplifier generally refers to an operational amplifier with an offset voltage lower than 1mV, and at the same time emphasizes that the drift value of the offset voltage with temperature should be less than 100V. Precision op amps perform much better than general op amps. The offset of a general operational amplifier is often a few mV, while a precision operational amplifier can be as small as 1uV.

The most commonly used precision operational amplifier is OP07, and its family such as OP27, OP37, OP177, OPA2333. There are many others. For example, many products of the American AD company are led by OPA.

9. Low temperature drift operational amplifier

In automatic control instruments such as precision instruments and weak signal detection, we always hope that the offset voltage of the operational amplifier should be small and not change with temperature. Low temperature drift op amps are designed for this purpose.

Commonly used high-precision, low-temperature drift operational amplifiers include OP-07, OP-27, AD508, and a chopping zero-stabilized low-drift device ICL7650 composed of MOSFETs.

Ⅳ. Selection of operational amplifiers

1. Before use, we need to understand the type and electrical parameters of the integrated operational amplifier, and figure out the package form, outer lead arrangement, pin connection, power supply voltage range, etc.

2. We try to use general-purpose integrated operational amplifiers. When multiple op amps are used in a system, we use multi-op amp ICs whenever possible.

3. When actually selecting an integrated operational amplifier, we also need to consider the nature of the signal source (whether it is a voltage source or a current source), the nature of the load, and whether the output voltage and current of the integrated operational amplifier meet the requirements.

Ⅴ. Operational amplifier application circuit

1. Amplifier Inverter

An inverter is also known as an inverting buffer and is the opposite of the previous voltage follower. If the two resistors are equal, the inverter does not amplify, but inverts the input signal. The input impedance is equal to R, and the gain is -1, giving Vout = -Vin.

2. Inverting amplifier

The inverting amplifier simultaneously inverts and amplifies the input signal at the ratio -RA/RB. The gain of the amplifier is controlled by negative feedback using the feedback resistor RA, and the input signal is fed to the inverting (-) input.

3. Voltage adder

A summer, also known as a summing amplifier, produces an inverting output voltage proportional to the sum of the input voltages V1 and V2. It can aggregate more inputs. If the values of the input resistors are equal (R1=R2=R), the total output voltage is given value and the gain is +1. If the input resistances are not equal, the output voltage is a weighted sum and becomes: Vout = -(V1(RA/R1)+V2(RA/R2)+etc).

4. Voltage Comparator

Comparators have many uses, but the most common is to compare an input voltage to a reference voltage. If the input voltage is higher than the reference voltage, the output is switched. If the input voltage is higher than the positive reference voltage Vin > Vref set by the voltage regulator, the output will change state. When the input voltage drops below the preset reference voltage and Vin<Vref, the output switches back. A basic comparator circuit can be easily converted to a Schmitt trigger by using positive feedback to reduce oscillations near the switching point.

5. Voltage subtractor

A subtractor, also known as a differential amplifier, uses inverting and noninverting inputs to produce an output signal. This signal is the difference between the two input voltages V1 and V2, allowing one signal to be subtracted from the other. We can add more inputs to subtract it out if needed.

When the resistors are the same (R=R3, RA=R4), the output voltage is a given value, and a voltage gain is a +1. When a circuit with unequal input resistance becomes an amplifier, the negative value of V1 is higher than V2, and the positive value of V1 is lower than V2.

6. Bridge amplifier

The inverting and non-inverting amplifier circuits above can be connected together to form a bridge amplifier configuration. The input signal is shared between the two op amps, and the output voltage signal is connected across the load resistor RL, which floats between the two outputs.

If the two op amp gains A1 and A2 are equal in magnitude to each other, the output signal will be doubled because it is actually a combination of the gains of the two separate amplifiers.

7. Non-inverting amplifier

A non-inverting amplifier does not invert or produce an inverted signal, but instead amplifies by a ratio of (RA+RB)/RB or usually 1+(RA/RB). The input signal is connected to the non-inverting (+) input.

8. Voltage follower

A voltage follower (also known as a buffer) does not amplify or invert an input signal, but instead provides isolation between two circuits. The input impedance is high and the output impedance is low, thus avoiding any loading effects within the circuit. When the output is connected directly back to one of the inputs, the overall gain of the buffer is +1 and Vout = Vin.

Ⅵ. Precautions for using operational amplifiers

1. Pay attention to the layout of the feedback loop

The components of the feedback loop must be close to the operational amplifier, and the PCB traces should be as short as possible, and at the same time, try to avoid interference sources such as digital signals and crystal oscillators. Unreasonable layout and wiring of the feedback loop will easily introduce noise, and in severe cases, it will cause self-excited oscillation.

2. Pay attention to power filtering

Power supply filtering for op amps cannot be ignored. The quality of the power supply directly affects the output. Especially for high-speed operational amplifiers, the power supply ripple has great interference with the output of the operational amplifier, and if it is not done well, it will become self-excited oscillation. So the best operational amplifier filter is to add a 0.1uF decoupling capacitor and a tens of uF tantalum capacitor next to the power supply pin of the operational amplifier, or connect a small inductor or magnetic bead in series, the effect will be better.

3. Do not connect the capacitor directly to the output of the operational amplifier

In the DC signal amplification circuit, sometimes in order to reduce noise, we directly connect the decoupling capacitor to the output of the operational amplifier. Although the DC signal is amplified, it is very unsafe to do so. When there is a step signal input or power-on moment, the output current of the operational amplifier will be relatively large. Moreover, the capacitance will change the phase characteristics of the loop, causing the circuit to self-oscillate.

The correct decoupling capacitor should form an RC circuit, that is, a resistor is connected in series at the output of the operational amplifier, and then the decoupling capacitor is connected in parallel. Doing so can greatly reduce the instantaneous output current of the op amp, and will not affect the phase characteristics of the loop, which can avoid oscillation.

4. Pay attention to the output swing of the op amp

It is impossible for any op amp to be an ideal op amp. It is impossible for their output voltage to reach the supply voltage. Generally, MOS-based operational amplifiers are rail-to-rail operational amplifiers. Its output can reach the power supply voltage under no-load conditions, but the output will carry a certain load. The greater the load, the more the output drops.

5. Pay attention to whether the input voltage exceeds the limit

If the input voltage is out of range, then the op amp will not work properly and some unexpected behavior will occur. However, some operational amplifiers are marked not with the input voltage range, but with the common-mode input voltage range. Since the input voltage of the non-inverting terminal and the inverting terminal are basically the same when the operational amplifier is working normally, the "input voltage range" and "common-mode input voltage range" are the same.

Ⅶ. Why must operational amplifiers introduce negative feedback?

The main purpose of introducing negative feedback is to make the amplifying circuit work in the linear region so that the output voltage does not exceed the maximum output voltage. If negative feedback is not introduced, the output is easily saturated due to the small range of the linear region of operation and the large voltage amplification factor, and there is no stability at all.

Although negative feedback is at the cost of reducing the magnification, its essential purpose is to improve the performance of the amplifier circuit, such as stabilizing the magnification, reducing nonlinear distortion, expanding the passband, etc., and the reduced magnification is sufficient for circuit design. Therefore, negative feedback is usually introduced in practical amplifiers.