1 What is an operational amplifier
The operational amplifier (op amp) is used for 调节和放大analog signals. The op amp is an integrated device containing a multi-stage amplifier circuit, as shown in the figure:
The left picture shows the same phase, the Vn terminal is grounded or a stable level, the Vp terminal level rises, the output terminal Vo level rises, and the Vp terminal level falls, the output terminal Vo level decreases; the right picture shows the reverse phase, The Vp terminal is grounded or a stable level, the level of the Vn terminal rises, the output terminal Vo level decreases, and the Vn terminal level decreases, the output terminal Vo level rises
2 Properties of operational amplifiers
理想运算放大器have the following properties:
- Infinite Input Impedance: An ideal op-amp input does not allow any current to flow in, i.e.
输入信号V+与V-两端点的电流信号恒为零, the input impedance is infinite - Output impedance close to zero: The output terminal of an ideal operational amplifier is a perfect voltage source, no matter how the current flowing to the amplifier load changes, the output voltage of the amplifier is constant, that is
输出阻抗为零 - Infinite open-loop gain: In the open-loop state of an ideal operational amplifier,
无限大的电压增益the differential signal at the input has - Infinite common-mode rejection ratio: The ideal operational amplifier can only respond to the difference in voltage between V+ and V- terminals (differential signal), ie
只放大V + − V − 的部份. The same part (common mode signal) of the two input signals will be completely ignored
共模信号：When double-ended input, the two signals are the same. 差模信号：When double-ended input, the phase difference of the two signals is 180 degrees
The integrated operational amplifier has two working states:
线性状态和非线性状态, when a negative feedback circuit is added to the integrated operational amplifier, it works in a linear state, and if a positive feedback circuit is added to the integrated operational amplifier or when it works in an open loop, it works in a nonlinear state
An integrated operational amplifier operating in a linear state has the following characteristics:
- It has virtual break characteristics and the current flowing into and out of the input terminal is 0, I-=I+=0A
- It has the characteristics of false short and the voltage of the two input terminals is equal, U+=U-
short and short
虚短：The open-loop magnification of the integrated operational amplifier is very large, and the open-loop voltage magnification of general-purpose operational amplifiers is above 80dB, but the output voltage of the operational amplifier is limited, generally between 10V and 14V. The modulus input voltage is less than 1 mV, so the two ends of the input can be approximately equipotential, 就相当于短路. The greater the magnification of the open-loop voltage, the closer the potentials of the two input terminals are equal. This characteristic is called virtual short
虚断：The integrated operational amplifier has the characteristics of high input impedance. Generally, the input resistance of the same input terminal and the reverse input terminal are above 1MΩ, so the current flowing into the operational amplifier at the input terminal is often less than 1uA, which is much smaller than the current of the external circuit at the input terminal. Therefore, the two input terminals of the op amp can usually be used here 视为开路, and the larger the input resistance of the op amp, the closer the open circuit is to both ends of the non-inverting and inverting inputs. When the op amp is in a linear state, according to this characteristic, the two input terminals can be regarded as an equivalent open circuit, referred to as a virtual break
An integrated operational amplifier operating in a nonlinear regime has the following characteristics:
- When the voltage of the non-inverting input terminal is greater than the voltage of the inverting input terminal, the output voltage is high
- When the voltage of the non-inverting input terminal is less than the voltage of the inverting input terminal, the output voltage is low
3 Classification of operational amplifiers
Operational amplifiers can be classified according to parameters as shown in the figure:
- General-purpose operational amplifier: the price is low, and its performance index is suitable for general application scenarios.
常用型号LM358、LM324 - Low-temperature drift operational amplifier: In automatic control instruments such as precision instruments and weak signal detection, the offset voltage should be small and not change with temperature.
常用型号OP07、AD508 - High-precision operational amplifier: less affected by temperature, low noise, high sensitivity, suitable for tiny signal amplification,
常用型号CF725M - High-impedance operational amplifier: the differential mode input impedance is very high, the input bias current is very small, generally Rid>1GΩ~1TΩ, Ib is several picoamperes to tens of picoamperes,
常用型号LF355、CA3130 - High-speed operational amplifier: with high conversion rate and wide frequency response, used for fast A/D and D/A converters,
常用型号LM318、AD8052 - Low-power operational amplifier: low supply voltage operation, low power consumption,
常用型号LM321、AD849 - High-voltage and high-power operational amplifier: The output voltage of the operational amplifier is limited by the power supply. If the ordinary operational amplifier wants to increase the output voltage or increase the output current, it needs to add an auxiliary circuit. The high-voltage and high-power op amp can output high voltage and high current without any external current.
常用型号PA44、A791 - Programmable control operational amplifier: variable magnification,
常用型号PGA103A、LTC6910
4 Parameters of the operational amplifier
- Common-mode input resistance: When the operational amplifier is operating in the linear region, the ratio of the input common-mode voltage range to the variation of the bias current within this range
- DC Common-Mode Rejection: A measure of the operational amplifier's ability to reject the same DC signal applied to both inputs
- AC Common Mode Rejection: Used to measure the op amp's ability to reject the same AC signal applied to both inputs
- Gain-bandwidth product: is a constant, defined in the characteristic curve of the open-loop gain versus frequency (-20dB/10 octave) roll-off area
- Input bias current: refers to the average current flowing into the input terminal when the operational amplifier works in the linear region
- Bias current temperature drift: the amount of change in the input bias current when the temperature changes
- Input Offset Current: The difference between the currents flowing into the two inputs
- Input offset current temperature drift: the amount of change in input offset current when the temperature changes
- Differential mode input resistance: the ratio of the change in input voltage to the corresponding change in input current, a change in voltage leads to a change in current
- Output impedance: When the operational amplifier works in the linear region, the internal equivalent small signal impedance at the output
- Output voltage swing: the peak value of the maximum voltage swing that can be achieved without clamping the output signal
- Power Consumption: Static power consumed at a given supply voltage
- Power Supply Rejection Ratio: A measure of the op amp's ability to maintain its output when the supply voltage varies
- Slew rate: the maximum value of the ratio of the change in output voltage to the time required for the change to occur
- Supply Current: The quiescent current drawn by the device at a specified supply voltage
- Unity-gain bandwidth: The maximum operating frequency of an op amp with an open-loop gain greater than 1
- Input offset voltage: Indicates the voltage difference that needs to be applied to the input terminal to make the output voltage zero
- Input offset voltage temperature drift: refers to the change of input offset voltage caused by temperature changes
- Input capacitance: Indicates the equivalent capacitance of any input terminal when the operational amplifier works in the linear region
- Input voltage range: the range of allowable input voltage when the operational amplifier works normally
- Input Voltage Noise Density: Can be viewed as a series noise voltage source connected to either input
- Input current noise density: can be seen as two noise current sources connected to each input and common
5 Applications of Operational Amplifiers
The inverting amplifier circuit is shown in the figure:
the relationship between the input and output voltages of the inverting amplifier circuit:
The non-inverting amplifier circuit is shown in the figure:
The relationship between the input and output voltages of the non-inverting amplifier circuit:
The addition circuit is shown in the figure:
The relationship between the input and output voltages of the adding circuit:
The subtraction circuit is shown in the figure:
The relationship between the input and output voltages of the subtraction circuit:
The integrating circuit is shown in the figure:
The relationship between the input and output voltages of the integrating circuit:
The differential circuit is shown in the figure:
Differential circuit input and output voltage relationship:
The differential amplifier circuit is shown in the figure:
The relationship between the input and output voltages of the differential amplifier circuit:
The voltage follower is shown in the figure:
The electromagnetic sampling amplifier circuit is shown in the figure:
I hope this article is helpful to everyone. If there is anything wrong with the above, please correct me.
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