Aug 15 2019

# Types of Operational Amplifier and Basis Comparison

## Introduction

This video explains the three golden rules when working with OpAmps and how to use them in order to understand and build all kind of OpAmp circuits. The main focus will be the noninverting and inverting amplifier and the comparator circuit.

Operational Amplifier Electrical Symbol

Vp: Non-inverting input

Vn: Inverting Input

Vn: Output

 Non-inverting phase Inverting phase Vp Vn Vo Vp Vn Vo ↑ grounding or stable level ↑ ↑ grounding or stable level ↓ ↓ grounding or stable level ↓ ↓ grounding or stable level ↑

1. Significance of in-phase input and inverting input.

1) When the inverse voltage is constant, the output waveform is the same as the non-inverting.

2) When the non-verting voltage is constant, the output waveform is opposite to the inverting end.

2. The magnification of Op Amp is infinite (∞).

3. The voltage at the inverting input of the operational amplifier is always equal to the voltage at the non-inverting input.

∵ Vin = Vp - Vn , Vout = A × Vin（A: magnification factor

and ∵ A = ∞，Vin = Vout / A

∴ Vin => 0, Vp = Vn

## Examples Analyses

Example 1:

Because the inverting phase voltage is always equal to the in-phase voltage, Vout is also 0.1V, because the op amp is infinite in input impedance and the output impedance is almost zero.

Example 2:

According to the basic principle that the voltage of inverting-phase terminal is always equal to the inverting terminal. When the inverting-phase voltage is 0.1V, and the partial voltage on the 1K resistor is 0.1V, the voltage division on the 100K resistor will be 10V, so the output voltage of Vo is the sum of the partial voltages of 100K and 1K resistors, that is 10.1V.

Example 3:

DC power supply, its input voltage is 24V. After filtered by C1, and regulated by R1 and D2 (voltage regulators), its voltage is regulated at 2.5V. At the same time, a 10K adjustable varistor is connected in parallel at both ends of the voltage regulator, and the adjustable range is: 0V ~ 2.5V. As shown in the figure, the in-phase terminal of the op amp is connected to the tap of the sliding varistor. Therefore, the voltage variation range of the non-inverting terminal is also 0V ~ 2.5V, and the inverting-phase end of the op amp is connected to R2 and R3, and the voltage variation range of R3 is 0V ~ 2.5V, then the voltage variation range on R2 is 0V ~ 12.5V, the output voltage is the sum of the voltages on R2 and R3, that is 0V~15V.

4. The input impedance of the op amp is infinite, which means that its input current is zero.

When the op amp is grounded at in phase, the voltage at the inverting-phase end will be 0V, and the voltage at the left side of the 100K resistor will be 1V and 0V on the right side, having a potential difference, there will be a current flowing through the 100K resistor. However, since the input impedance of the op amp is infinite, almost no current flows. So the current will flow through the 1000K resistor, then the voltage on the 1000K resistor is 10V, because the potential output is lower than the GND, so the output is -10V.

When the in-phase of op amp is connected to the 3V voltage, the inverting voltage is also 3V. The 2K resistor is 3V on the left side and 5V on the right side. Because of the potential difference, there is current flowing through the 2K resistor. However, since the input impedance of the op amp is infinite, there is almost no current flow on it. Then the current will flow through the 10K resistor, and the voltage on the 10K resistor is 10V, and the voltage at Vout to ground is Vout ⇒ A ⇒ B ⇒ GND, getting Vout = (-10V) + 3V = -7V.

## Basic Types of Op Amp

1. Universal om amp

It is designed for general purpose use. The main features of this type of device are low price, wide product range and so on for general use. Examples of μA741 (single op amp), LM358 (dual op amp), LM324 (four op amps) and LF356 with FET as the input stage fall into this category. They are the most widely used integrated operational amplifiers.

2. High resistance om amp

The characteristics of this integrated type are that the differential mode input impedance is very high, and the input bias current is very small, generally several picoamperes to several tens of picoamperes. The main measure to achieve these indicators is to use the high input impedance of the FET, and use the FET to form the differential input of the op amp. Using the FET as the input, the input impedance is high, and the input bias current is low, also has the advantages of high speed, wide bandwidth and low noise, but the input offset voltage is large. Some common integrated devices are LF355, LF347 and CA3130, CA3140 with higher input impedance.

3. Low temperature drift op amp

In automatic control instruments such as precision instruments and weak signal detection, there is a need that the offset voltage of the operational amplifier is small and does not change with temperature, thus low temperature drift op amps are designed for this purpose. At present, the commonly used low-temperature drift operational amplifiers with high precision include OP07, OP27, AD508, and chopper-stabilized low-drift device ICL7650 composed of MOSFET.

4. High-speed om amp

In fast A/D and D/A converters and video amplifiers, the conversion rate (also called slew rate SR) of the integrated operational amplifier is required to be high, and the unity gain bandwidth BWG must be large enough. The main features of it are high SR and wide frequency response. Common applications include LM318, μA715, etc., with SR=50~70V/us, BWG>20MHz.

5. Low-power consumption om amp

Since the biggest advantage of electronic integration makes complex circuits small and light, and the expansion of the scope of portable instruments, it is necessary to use an operational amplifier with low power supply voltage and low power consumption. Commonly used of this type are TL-022C, TL-060C, etc., and their operating voltage is ±2V~±18V, and the current consumption is 50~250μA. At present, some products have reached the power consumption level of μW. For example, the power supply of the ICL7600 is 1.5V, and the power consumption is 10mW, in addition, it can be powered by a single battery.

6. High-voltage and high-current om amp

The output voltage of an op amp is primarily limited by the power supply. In a conventional operational amplifier, the maximum value of the output voltage is generally only a few tens of volts, and the output current is only a few tens of milliamps. To increase the output voltage or the output current, an auxiliary circuit must be added to the outside of the op amp. High-voltage and high-current integrated operational amplifiers can make it without any additional circuit. For example, the D41 has a supply voltage of ±150V, and the μA791 has an output current of 1A.

7. Programmable control om amp

In the process of using the instrumentation, the range problem is involved. To obtain the output fixed voltage, the amplification factor of the op amp must be changed. For example, if the operational amplifier has a magnification of 10 times and the input signal is 1 mv, the output voltage is 10mv, when the input voltage is 0.1mv, the output is only 1mv. In order to get 10mv, the magnification factor must be changed to 100. The programmable control op amp is generated to solve this problem. For example, PGA103A, by controlling pins level to change the magnification.

Note: Confusion between the op amps and comparators

1) The basic concept is same between comparator and op amp.

Internal difference: The operational amplifier is a complementary output, which can output an undistorted analog signal. Generally, it can be used in closed loop, open loop or a small amount of positive feedback. It can also be used as a comparator, usually an OC (open collector) output, which is convenient for multiple parallel connection. The output switch signal requires a pull-up resistor, and most of them are used for open loop. In some occasions, a hysteresis is require by introducing a certain positive feedback.

The amplifier output has a loop to the input, that is, there is feedback, it is a closed loop, may be a resistor or a capacitor. Depending on the input, it is judged whether it is positive feedback or negative feedback. Connecting the in-phase end is positive feedback and the reverse terminal is negative feedback. In addition, introducing positive feedback, the system may oscillate, and if properly added, hysteresis (return difference) will be generated. So amplifiers typically introduce negative feedback to obtain a fixed magnification.

The concept of the loop: Signal-detection-standard comparison-controls a parameter of the input signal to the standard. This is a closed loop system and is a negative feedback system ( the input parameters are stable).

2) The amplifier is used to amplify small signals, and the emphasis is on proportional amplification. In contrast, the comparator is used to compare the input voltage difference between the positive and negative inputs, as long as the difference meets certain requirements, the output state changes immediately. Its important parameters are also mostly about the turning characteristics or we can understand that the comparator is a transitional circuit form characterized by an analog circuit and featuring digital signal input and output.

3) Comparator is a kind of operational amplifier without feedback (positive feedback or negative feedback). When the positive input is greater than the negative input, the output is infinite; when the positive input is less than the negative input, the output is infinitesimal, that is, the output of operational amplifier is calculated according to the feedback. In summary, there is no fundamental difference between the two.

4) Comparators are generally made using an op amp. When the op amp incorporates a negative feedback loop, the entire circuit itself can be viewed as an amplifying circuit with a certain gain. The figure below shows a classic op amp: Gain=Rf/Rin

G=Rf/Rin

The op amp can also be used as a comparator, just replacing the negative feedback with positive feedback. When the circuit adds positive feedback, the output voltage will saturate, but it will not and cannot exceed the supply voltage. The following figure shows the classic comparison circuit:

The resistor in the figure provides a reference voltage for the positive pole, and the output voltage is inverted when the negative voltage exceeds the positive voltage, as shown as the following figure.

In short, whether the circuit connected to the op amp is negative feedback or positive feedback, it can be used as an amplifier or a comparator depending on different cases, respectively.

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• Henry on 2019-8-16 16:34:57

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