Explanation of op amp parameters and selection of commonly used op amps

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Explanation of op amp parameters and selection of commonly used op amps [Copy link]

Integrated operational amplifiers have many parameters, among which the main parameters are divided into DC indicators and AC indicators, and all chips have limit parameters. This article takes NE5532 as an example to briefly explain each indicator. Except for the pictures below which are taken from the NE5532 data sheet, all other content is compiled from the Internet.

Limit parameters

are mainly used to determine the design of the operational amplifier power supply (how much V voltage is provided and how much the maximum current cannot exceed). The limit parameters of NE5532 are as follows:

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DC indicator

The main DC indicators of op amps include input offset voltage, temperature drift of input offset voltage (referred to as input offset voltage temperature drift), input bias current, input offset current, temperature drift of input bias current (referred to as input offset current temperature drift), differential open-loop DC voltage gain, common-mode rejection ratio, power supply voltage rejection ratio, output peak-to-peak voltage, maximum common-mode input voltage, and maximum differential-mode input voltage. The DC indicators of NE5532 are as follows:

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• Input offset voltage Vos:
  The input offset voltage is defined as the compensation voltage added between the two input terminals when the output voltage of the integrated operational amplifier is zero. The input offset voltage actually reflects the circuit symmetry inside the operational amplifier. The better the symmetry, the smaller the input offset voltage. The input offset voltage is a very important indicator of the operational amplifier, especially for precision operational amplifiers or when used for DC amplification. The input offset voltage has a certain relationship with the manufacturing process. The input offset voltage of the bipolar process (that is, the standard silicon process mentioned above) is between ±1~10mV; the input offset voltage will be larger when field effect transistors are used as the input stage. For precision operational amplifiers, the input offset voltage is generally below 1mV. The smaller the input offset voltage, the smaller the intermediate zero point offset during DC amplification, and the easier it is to handle. Therefore, it is an extremely important indicator for precision operational amplifiers.
• Input offset voltage temperature drift (abbreviated as input offset voltage temperature drift) ΔVos/ΔT: Input offset voltage temperature drift is defined as the ratio of input offset voltage change to temperature change within a given temperature range. This parameter is actually a supplement to input offset voltage, which is convenient for calculating the drift caused by temperature change in the amplifier circuit within a given working range. The input offset voltage temperature drift of general op amps is between ±10~20μV/℃, and the input offset voltage temperature drift of precision op amps is less than ±1μV/℃.
• Input bias current Ios: Input bias current is defined as the average bias current of the two input terminals when the output DC voltage of the op amp is zero. Input bias current has a greater impact on places where input impedance is required, such as high-impedance signal amplification and integration circuits. The input bias current has a certain relationship with the manufacturing process. The input bias current of the bipolar process (i.e. the standard silicon process mentioned above) is between ±10nA and 1μA; when field effect transistors are used as input stages, the input bias current is generally less than 1nA.
• Input offset current temperature drift (abbreviated as input offset current temperature drift) ΔIos/ΔT:
• Maximum common-mode input voltage Vcm:
  The maximum common-mode input voltage is defined as the common-mode input voltage when the common-mode rejection ratio of the op amp deteriorates significantly when the op amp works in the linear region. It is generally defined as the common-mode input voltage corresponding to the common-mode rejection ratio drop of 6dB as the maximum common-mode input voltage. The maximum common-mode input voltage limits the maximum common-mode input voltage range in the input signal. In the presence of interference, this issue needs to be paid attention to in circuit design. Common Mode Rejection Ratio CMRR: Common mode rejection ratio is defined as the ratio of the differential mode gain to the common mode gain of the op amp when the op amp works in the linear region. Common mode rejection ratio is an extremely important indicator, which can suppress the common mode interference signal in the differential mode input. Since the common mode rejection ratio is very large, the common mode rejection ratio of most op amps is generally tens of thousands of times or more. It is not convenient to compare directly with numerical values, so it is generally recorded and compared in decibels. The common mode rejection ratio of general op amps is between 80 and 120 dB. Power supply voltage rejection ratio PSRR: Power supply voltage rejection ratio is defined as the ratio of the change of the op amp input offset voltage to the power supply voltage when the op amp works in the linear region. The power supply voltage rejection ratio reflects the impact of power supply changes on the output of the op amp. For op amps with low power supply voltage rejection ratio, the power supply of the op amp needs to be carefully handled, otherwise the ripple of the power supply will be introduced to the output. Of course, an op amp with a high common-mode rejection ratio can compensate for part of the power supply voltage rejection ratio. In addition, when using dual power supplies, the power supply voltage rejection ratios of the positive and negative power supplies may be different. Output peak-to-peak voltage Vout: Output peak-to-peak voltage is defined as the maximum voltage amplitude that the op amp can output when the op amp is operating in the linear region and under a specified load when the op amp is powered by the current large power supply voltage. Except for low-voltage op amps, the output peak-to-peak voltage of general op amps is greater than ±10V. The output peak-to-peak voltage of general op amps cannot reach the power supply voltage, which is caused by the output stage design. The output stage of some modern low-voltage op amps has been specially processed so that the output peak-to-peak voltage is close to within 50mV of the power supply voltage when the load is 10k?, so it is called a full-scale output op amp, also known as a rail-to-rail op amp. It should be noted that the output peak-to-peak voltage of the op amp is related to the load. Different loads will result in different output peak-to-peak voltages. The positive and negative output voltage swings of the op amp are not necessarily the same. For practical applications, the closer the output peak-to-peak voltage is to the power supply voltage, the better. This can simplify the power supply design. However, current full-scale output op amps can only work at low voltages and are more expensive.
• Input impedance Rin:
  Input impedance reflects the impact of input on the performance of the op amp. When selecting an op amp, the larger the input impedance, the better.

AC indicators

The main AC indicators of op amps include open-loop bandwidth, unity gain bandwidth, conversion rate SR, full power bandwidth, settling time, equivalent input noise voltage, differential mode input impedance, common mode input impedance, and output impedance.

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[color=rgb(65, 131, There are many important parameters in the AC indicators, especially unity gain bandwidth and slew rate, which are particularly useful in the selection of small signal and large signal op amps respectively.

• Output impedance Rout:
  The input impedance reflects the load capacity of the output of the op amp, the smaller the better.
• Open-loop gain Av:
  The maximum gain that the op amp can achieve under open-loop conditions
• Open-loop bandwidth:
  The open-loop bandwidth is defined as the signal frequency corresponding to the open-loop voltage gain dropping 3db (or equivalent to 0.707 of the DC gain of the op amp) from the DC gain of the op amp when a constant amplitude sinusoidal small signal is input to the input of the op amp. This is used for very small signal processing. This parameter does not seem to be included in the NE5532 data sheet. Unit gain bandwidth GB (measured by gain bandwidth product GBW in NE5532) Unit gain bandwidth is defined as the signal frequency corresponding to a 3db drop in closed-loop voltage gain (or 0.707 of the input signal of the op amp) when a constant amplitude sinusoidal small signal is input to the input of the op amp under the condition that the closed-loop gain of the op amp is 1. Unit gain bandwidth is a very important indicator. When amplifying sinusoidal small signals, the unit gain bandwidth is equal to the product of the input signal frequency and the maximum gain at that frequency. In other words, when the signal frequency to be processed and the gain required by the signal are known, the unit gain bandwidth can be calculated to select the appropriate op amp. This parameter is used for op amp selection in small signal processing.
• Slew rate (conversion rate) SR: When the op amp is connected in a closed loop, a large signal (including step signal) is input to the input of the op amp, and the output rise rate of the op amp is measured from the output of the op amp. Since the input stage of the op amp is in a switching state during the conversion, the feedback loop of the op amp does not work, that is, the conversion rate has nothing to do with the closed-loop gain. Conversion rate is a very important indicator for large signal processing. For general op amps, the conversion rate SR <= 10V/μs, and the conversion rate SR of high-speed op amps> 10V/μs. The current high-speed op amp has a maximum conversion rate SR of 6000V/μs. This is used for op amp selection in large signal processing.
• Full power bandwidth:
  When the closed-loop gain of the op amp is 1x under rated load, a constant amplitude sinusoidal large signal is input to the input of the op amp, which makes the op amp output amplitude reach the maximum (allowing a certain distortion) signal frequency. This frequency is limited by the conversion rate of the op amp. Approximately, full power bandwidth = conversion rate/2πVop (Vop is the peak output amplitude of the op amp). Full power bandwidth is a very important indicator used in the selection of op amps for large signal processing.

51)] There are many important parameters in the AC indicators, especially the unity gain bandwidth and slew rate, which are particularly useful in the selection of small signal and large signal op amps respectively.

• Output impedance Rout:
  The input impedance reflects the load capacity of the output end of the op amp, the smaller the better.
• Open-loop gain Av:
  The maximum gain that the op amp can achieve under open-loop conditions
• Open-loop bandwidth:
  The open-loop bandwidth is defined as the signal frequency corresponding to the open-loop voltage gain dropping 3db (or equivalent to 0.707 of the DC gain of the op amp) from the DC gain of the op amp when a constant amplitude sinusoidal small signal is input to the input end of the op amp. This is used for very small signal processing. There seems to be no such parameter in the NE5532 data sheet. Unit gain bandwidth GB (measured by gain bandwidth product GBW in NE5532) Unit gain bandwidth is defined as the signal frequency corresponding to a 3db drop in closed-loop voltage gain (or 0.707 of the input signal of the op amp) when a constant amplitude sinusoidal small signal is input to the input of the op amp under the condition that the closed-loop gain of the op amp is 1. Unit gain bandwidth is a very important indicator. When amplifying sinusoidal small signals, the unit gain bandwidth is equal to the product of the input signal frequency and the maximum gain at that frequency. In other words, when the signal frequency to be processed and the gain required by the signal are known, the unit gain bandwidth can be calculated to select the appropriate op amp. This parameter is used for op amp selection in small signal processing.
• Slew rate (conversion rate) SR: When the op amp is connected in a closed loop, a large signal (including step signal) is input to the input of the op amp, and the output rise rate of the op amp is measured from the output of the op amp. Since the input stage of the op amp is in a switching state during the conversion, the feedback loop of the op amp does not work, that is, the conversion rate has nothing to do with the closed-loop gain. Conversion rate is a very important indicator for large signal processing. For general op amps, the conversion rate SR <= 10V/μs, and the conversion rate SR of high-speed op amps> 10V/μs. The current high-speed op amp has a maximum conversion rate SR of 6000V/μs. This is used for op amp selection in large signal processing.
• Full power bandwidth:
  When the closed-loop gain of the op amp is 1x under rated load, a constant amplitude sinusoidal large signal is input to the input of the op amp, which makes the op amp output amplitude reach the maximum (allowing a certain distortion) signal frequency. This frequency is limited by the conversion rate of the op amp. Approximately, full power bandwidth = conversion rate/2πVop (Vop is the peak output amplitude of the op amp). Full power bandwidth is a very important indicator used in the selection of op amps for large signal processing.

51)] There are many important parameters in the AC indicators, especially the unity gain bandwidth and slew rate, which are particularly useful in the selection of small signal and large signal op amps respectively.

• Output impedance Rout:
  The input impedance reflects the load capacity of the output end of the op amp, the smaller the better.
• Open-loop gain Av:
  The maximum gain that the op amp can achieve under open-loop conditions
• Open-loop bandwidth:
  The open-loop bandwidth is defined as the signal frequency corresponding to the open-loop voltage gain dropping 3db (or equivalent to 0.707 of the DC gain of the op amp) from the DC gain of the op amp when a constant amplitude sinusoidal small signal is input to the input end of the op amp. This is used for very small signal processing. There seems to be no such parameter in the NE5532 data sheet. Unit gain bandwidth GB (measured by gain bandwidth product GBW in NE5532) Unit gain bandwidth is defined as the signal frequency corresponding to a 3db drop in closed-loop voltage gain (or 0.707 of the input signal of the op amp) when a constant amplitude sinusoidal small signal is input to the input of the op amp under the condition that the closed-loop gain of the op amp is 1. Unit gain bandwidth is a very important indicator. When amplifying sinusoidal small signals, the unit gain bandwidth is equal to the product of the input signal frequency and the maximum gain at that frequency. In other words, when the signal frequency to be processed and the gain required by the signal are known, the unit gain bandwidth can be calculated to select the appropriate op amp. This parameter is used for op amp selection in small signal processing.
• Slew rate (conversion rate) SR: When the op amp is connected in a closed loop, a large signal (including step signal) is input to the input of the op amp, and the output rise rate of the op amp is measured from the output of the op amp. Since the input stage of the op amp is in a switching state during the conversion, the feedback loop of the op amp does not work, that is, the conversion rate has nothing to do with the closed-loop gain. Conversion rate is a very important indicator for large signal processing. For general op amps, the conversion rate SR <= 10V/μs, and the conversion rate SR of high-speed op amps> 10V/μs. The current high-speed op amp has a maximum conversion rate SR of 6000V/μs. This is used for op amp selection in large signal processing.
• Full power bandwidth:
  When the closed-loop gain of the op amp is 1x under rated load, a constant amplitude sinusoidal large signal is input to the input of the op amp, which makes the op amp output amplitude reach the maximum (allowing a certain distortion) signal frequency. This frequency is limited by the conversion rate of the op amp. Approximately, full power bandwidth = conversion rate/2πVop (Vop is the peak output amplitude of the op amp). Full power bandwidth is a very important indicator used in the selection of op amps for large signal processing.