[Hardware Design] Basics of Analog Electronics III--Integrated Operational Amplifier Circuit

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Preface: This chapter is a simple review of knowledge, suitable for knowledge review before learning hardware design, not suitable for use in exams

1. Integrated operational amplifier

1.1 Definition, composition and performance

① Definition

The integrated operational amplifier is a multi-stage direct-coupled amplifier circuit with very high magnification. It is the earliest and most widely used analog integrated circuit.
Features: high gain, high reliability, low cost, small size

② Composition

In principle, the integrated operational amplifier is essentially a direct-coupled multi-stage amplifier circuit with high voltage gain , high input resistance and low output resistance . The internal circuit of the integrated op amp is generally composed of four parts, , , and . Current or overheat protection circuit, current limiting circuit, voltage regulator circuit, etc.输入级中间级输出级偏置电路

③ performance

• Open-loop differential mode voltage gain: β-magnification (value is infinite, tens of thousands to hundreds of thousands of times)
• Input offset voltage: when U _o output voltage is 0, the value of U ₊ - U _- is generally within 10mv
• Input offset current: When the input resistance is infinite, the current is approximately equal to 0, but there is still an actual input current
• Input bias current: The bias current provided by the bias circuit to adjust the quiescent operating point
• Differential mode input resistance and output resistance
• Temperature Drift
  Input Offset Voltage Drift
  Input Offset Current Drift
• Common mode rejection ratio: Differential mode magnification/common mode magnification, the smaller the value, the better
  Maximum common mode input voltage
  Maximum differential mode input voltage

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1.2 Current source circuit

定义:

Current source is a unit circuit widely used in analog integrated circuits. The current source in the integrated operational amplifier provides a stable bias current for the amplifying circuit, and at the same time acts as an active load of the amplifying circuit to increase the gain of the amplifying circuit. Common current source circuits include mirror current source circuits, proportional current source circuits, and micro current source circuits.

① mirror current source

Multisim仿真图：

公式推导：
I_R1 = I_c1 + 2I_b
∵ I_c1 = βI_b ，I_c2 = βI_b
∵ I_c1 = I_c2

②Proportional current source

Multisim仿真图：

公式推导：
U _be1 + I _e1 R2 = U _be2 + I _e2 R3
∵ U _be1 ≈ U _be2
∴ I _e1 R2 = I _e2 R3
∵ I _e1 ≈ I _R1， I _e2 ≈ I _c2
∴ I _R1 R2 = I _c2 R3 ，R2/ R3 = _Ic2 / _IR1

③Micro current source

Multisim仿真图：

公式推导：
I _c1 ≈ I _c2 = (U _be2 - U _be1 )/R2

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1.3 Differential amplifier circuit

定义：

The differential amplifier circuit is also called the differential amplifier circuit, which is another type of basic amplifier circuit, which can effectively reduce the zero point drift caused by power fluctuations and transistor changes with temperature, so it has been widely used, especially a large number of applications In the integrated operational amplifier circuit, as the pre-stage of the multi-stage amplifier.

Multisim仿真图：

① When the bias voltages at both ends are different, the output voltage difference will be caused.

② Decreasing R2

can find that the voltage difference is reduced, so we can adjust the output voltage difference by adjusting R2

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1.4 Ideal operational amplifier

说明：
A _uo : Open-loop magnification, 80dB~140dB, almost infinite
R _id : Input resistance: $10^5$ ~ $10^{11}$
R_o: tens of ohms ~ hundreds of ohms
K_CMRR: common mode rejection ratio - 70dB~130dB

①Virtual short
The potentials of the two input terminals of the ideal op amp are equal (virtual short)
∵ u _o =A _uo U _i =A _uo (u ₊ - u _- )
∴ u ₊ -u _- = u _o /A _uo
∵ A _ud ≈ ∞
∴ u ₊ =u _-

②Virtual break
The input current of the ideal op amp is equal to zero (virtual break)
∵ Ideal R _id = ∞
∴ I ₊ = I _- = 0

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Second, the application of integrated operational amplifier

2.1 Inverse proportional operation circuit

Multisim仿真图：

公式推导：
∵ 虚断：
$\frac{V_1-U_{-}}{R_3}=\frac{U_{-}-U_o}{R_4}$
∵ 虚短：
U ₊ = U _-，而U ₊ = 0
∴ U _- = 0
∴ $\frac{V_1}{U_o}=-\frac{R_3}{R_4}=-\frac{1}{2}$

示波器图形：

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2.2 The same direction proportional operation circuit

Multisim仿真图：

公式推导：
∵ 虚断：
$\frac{0-U_{-}}{R_1}=\frac{U_{-}-U_o}{R_3}$
∵ 虚短：
U ₊ = U _-，而U ₊ = V ₁
∴ U _- = V ₁
∴ $-\frac{V_1}{R_1}=\frac{V_1-U_o}{R_3}$
化简可得：
$\frac{V_1}{U_o}=\frac{R_1}{R_3+R_1}=\frac{1}{3}$

示波器图形：

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2.3 Inverse addition operation circuit

Multisim仿真图：

公式推导：
∵ 虚断：
$\frac{V_1-U_{-}}{R_5}+\frac{V_2-U_{-}}{R_1}=\frac{U_{-}-U_o}{R_3}$
∵ 虚短：
U₊ = U_-，而U₊ = 0
∴ U_- = 0
∴ $\frac{V_1}{R_1}+\frac{V_2}{R_1}=-\frac{U_o}{R_3}$
Simplify:
$V_1+V_2=-\frac{U_o}{2}$

示波器图形：

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2.4 Inverse subtraction operation circuit

Multisim仿真图：

公式推导：
∵ 虚断：
$\frac{V_1-U_{-}}{R_1}=\frac{U_{-}-U_o}{R_3}$
∵ 虚短：
U₊ = U_-，而U₊ = $V_2\ast \frac{R_6}{R_6+R_2}$
∴ U_- = $V_2\ast \frac{R_6}{R_6+R_2}$

Simplify:
$V_1-V_2=-U_o$

示波器图形：

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2.5 Integral operation circuit

电路：

公式推导：

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2.6 Differential operation circuit

电路：

公式推导：

波形为：

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2.7 Voltage comparator circuit

Multisim仿真图：

函数发生器配置：

Turn on the circuit simulation, and you can see the LED lights flashing.

示波器波形：

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3. Instrument amplifier

3.1 Definition, principle, characteristics and classification

① Definition

An instrumentation amplifier is a precision differential voltage amplifier that is derived from and superior to an operational amplifier. The instrumentation amplifier integrates the key components inside the amplifier, and its unique structure makes it have the characteristics of high common mode rejection ratio, high input impedance, low noise, low linearity error, low offset drift, flexible gain setting and convenient use, etc. It is popular in data acquisition, sensor signal amplification, high-speed signal conditioning, medical equipment and high-end audio equipment.

②Principle

The instrumentation amplifier mainly consists of a two-stage differential amplifier circuit. Among them, the operational amplifiers A1 and A2 are in the same direction differential input mode, the same direction input can greatly increase the input impedance of the circuit, and reduce the attenuation of the weak signal by the circuit; the differential input can make the circuit only amplify the differential mode signal, but not the common mode signal. The input signal only acts as a follower, so that the ratio of the amplitude of the differential mode signal to the common mode signal (and the common mode rejection ratio CMRR) sent to the subsequent stage is improved.
In the differential amplifier circuit composed of operational amplifier A3 as the core component, under the condition that the CMRR requirement remains unchanged, the accuracy matching requirements for resistors R3 and R4, Rf and R5 can be significantly reduced, so that the instrumentation amplifier circuit is better than a simple differential amplifier circuit. Amplifying circuits have better common-mode rejection capabilities.
Under the conditions of R1=R2, R3=R4 and Rf=R5, the gain of the circuit is:
G=(1+2R1/Rg)Rf/R3.
It can be seen from the formula that the adjustment of the circuit gain can be realized by changing the resistance value of Rg.

③Features

• High Common Mode Rejection Ratio
• high input impedance
• low noise
• Low offset voltage and offset voltage drift
• low linear error
• Has a "Sense" terminal and a "Reference" terminal

④ Classification

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3.2 AD620 Instrumentation Amplifier

介绍：

• AD620 is a low-power, high-precision instrumentation amplifier. It only needs an external resistor to set various gains (1~1000).

• AD620 has the advantages of small size, low power consumption and high precision compared with the instrumentation amplifier (three op amp structure) composed of discrete components.

• The AD620 has found wide application in precision data acquisition systems such as gauges and sensor interfaces. It has also become the device of choice for medical instruments such as electrocardiograms and non-invasive blood pressure monitors.

应用：

Stress detection circuit:

Heart rate detection circuit:

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4. Power Amplifier

4.1 Definition and characteristics

① Definition

The power amplifying circuit is an amplifying circuit for the purpose of outputting relatively large power.

②Features

• The output signal voltage is large;
• The output signal current is large;
• The output resistance of the amplifying circuit is matched to the load.

③The difference between a voltage amplifier and a power amplifier:

• Voltage amplification - increase the amplitude of the input signal without distortion to drive the subsequent power amplification stage, usually working in a small signal state.
• Power amplification—under the condition that the signal is not distorted or slightly distorted, the output power is increased, and it usually works in a large signal state.

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4.2 Working mode

• Class A: The Q point is moderate, and the signal can pass through the entire cycle. Efficiency<50%
• Class B: Q point is in the cut-off area, half cycle conduction, efficiency ≤78%
• Class A and B: The Q point is close to the cut-off area, the conduction is greater than half a cycle, and the efficiency is between Class A and Class B

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4.3 Internal circuit and application

内部的两种电路：

应用：

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