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1. Unit gain bandwidth GB
Unit gain bandwidth is defined as: when the closed-loop gain of the op amp is 1, a constant amplitude sinusoidal small signal is input to the input of the op amp, and the closed-loop voltage gain is measured from the output of the op amp. The corresponding signal frequency is 3db (or equivalent to 0.707 of the op amp input signal). Unit gain bandwidth is a very important indicator. When amplifying a sinusoidal small signal, 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 is used for op amp selection in small signal processing.
2. The bandwidth of an op amp indicates its ability to process AC signals
. For small signals, it is generally expressed in terms of unit gain bandwidth. Unit gain bandwidth, also called gain bandwidth product, can roughly indicate the op amp's ability to process signal frequencies. For example, if the gain bandwidth of an op amp is 1MHz, and the actual closed-loop gain is 100, then the maximum frequency for processing small signals in theory is 1MHz/100=10KHz.
For large signal bandwidth, i.e., power bandwidth, it needs to be calculated based on the conversion speed. For DC signals
, bandwidth issues generally do not need to be considered, and accuracy and interference issues are the main considerations.
1. The bandwidth of an op amp is simply used to measure the frequency range of signals that an amplifier can process. The higher the bandwidth, the higher the signal frequency that can be processed and the better the high-frequency characteristics. Otherwise, the signal is easily distorted. However, this is for small signals. For large signals, the slew rate (or conversion rate) is generally used to measure.
2. For example, the amplification factor of an amplifier is n times, but it does not mean that the amplification capacity of all input signals is n times. When the signal frequency increases, the amplification capacity will decrease. When the output signal drops to 0.707 times the original output, that is, one square root of 2, or a decrease of 3dB, the frequency of the signal at this time is called the bandwidth of the op amp.
3. When the output signal amplitude is very small and below 0.1Vp-p, the main consideration is the influence of the gain bandwidth product.
That is, Gain Bandwidth = amplification factor * signal frequency.
When the output signal amplitude is large, the main consideration is the influence of the conversion rate Sr, the unit is V/uS.
In this case, the power bandwidth must be calculated, FPBW = Sr/2πVp-p.
That is, when designing the circuit, both the gain bandwidth and the power bandwidth must be met.
3. The main indicators and definitions of bandwidth and gain of op amps
1. 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 and measured from the output of the op amp. This is used for very small signal processing.
2. Unit gain bandwidth GB: The 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. The unit gain bandwidth is a very important indicator. When amplifying a sinusoidal small signal, 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 required gain of the signal are known, the unit gain bandwidth can be calculated to select the appropriate op amp. This is used for op amp selection in small signal processing.
3. Conversion rate (also called slew rate) SR: The conversion rate of an op amp is defined as the output rise rate of the op amp measured from the output of the op amp when a large signal (including step signal) is input to the input of the op amp under closed-loop conditions. 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 maximum conversion rate SR of high-speed op amps reaches 6000V/μs. This is used for op amp selection in large signal processing.
4. Full power bandwidth BW: Full power bandwidth is defined as the signal frequency at which the output amplitude of the op amp reaches the maximum (allowing certain distortion) when a constant amplitude sinusoidal large signal is input to the input of the op amp under the condition of the closed loop gain of the op amp being 1 times at rated load. 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 for selecting op amps in large signal processing.
5. Settling time: The definition of settling time is that when the closed-loop gain of the op amp is 1 times under the rated load, a large step signal is input to the input of the op amp, and the time required for the op amp output to increase from 0 to a given value. Because it is a large step signal input, there will be a certain jitter after the output signal reaches the given value. This jitter time is called the stabilization time. Stabilization time + rise time = settling time. For different output accuracies, the stabilization time varies greatly. The higher the accuracy, the longer the stabilization time. Settling time is a very important indicator for the selection of op amps in large signal processing.
6. Equivalent input noise voltage: Equivalent input noise voltage is defined as any irregular AC interference voltage generated at the output of a well-shielded op amp with no signal input. When this noise voltage is converted to the op amp input, it is called the op amp input noise voltage (sometimes also expressed as noise current). For broadband noise, the effective value of the input noise voltage of an ordinary op amp is about 10~20μV.
7. Differential input impedance (also called input impedance): Differential input impedance is defined as the ratio of the voltage change at the two input terminals to the corresponding input current change when the op amp works in the linear region. Differential input impedance includes input resistance and input capacitance. At low frequencies, it only refers to input resistance. General products also only give input resistance. The input resistance of an op amp using bipolar transistors as the input stage is no more than 10 megohms; the input resistance of an op amp using field effect transistors as the input stage is generally greater than 109 ohms.
8. Common-mode input impedance: Common-mode input impedance is defined as the ratio of the change in common-mode input voltage to the corresponding change in input current when the op amp is operating on an input signal (i.e. the same signal is input to both input terminals of the op amp). At low frequencies, it appears as a common-mode resistor. Usually, the common-mode input impedance of an op amp is much higher than the differential-mode input impedance, with a typical value of more than 108 ohms.
9. Output impedance: Output impedance is defined as the ratio of the voltage change to the corresponding current change when a signal voltage is applied to the output of the op amp when the op amp is operating in the linear region. At low frequencies, it only refers to the output resistance of the op amp. This parameter is tested in an open loop.
4. Performance indicators of operational amplifiers
1. Input offset voltage VIO (input offset voltage): When the input voltage is zero, divide the output voltage by the voltage gain and add a negative sign to get the offset voltage converted to the input. It is also the compensation voltage added to the input when the output voltage is zero. VIO characterizes the symmetry of the internal circuit of the op amp or reflects the mismatch of the input differential pair. Generally, Vos is about (1 to 10) mV, and Vos of high-quality op amps is below 1 mV.
2. Input offset voltage temperature drift: The ratio of the change in input offset voltage with temperature to the change in temperature within the specified operating temperature range. This parameter refers to the temperature coefficient of Vos within the specified operating range and is an important indicator for measuring the temperature effect of the op amp. Generally, it is about (10-30) uV/degree Celsius, and high-quality ones can be <0.5uV/C (degree Celsius).
3. Input offset current IIO (input offset current): At zero input, the difference in the base current of the differential pair of tubes in the differential input stage, II0 = |IB1-IB2|. It is used to characterize the degree of asymmetry of the differential input current. Usually, Ios is (0.5~5)nA, and high-quality ones can be less than 1nA.
4. Input offset current temperature drift: The ratio of the change in input offset current with temperature to the change in temperature within the specified operating temperature range. It refers to the temperature coefficient of II0 within the specified operating range, and is also an important indicator for measuring the impact of temperature on op amps. It is usually about (1 to 50) nA/C, and a high-quality one is about a few pA/C.
5. Input bias current IB (input bias current): The average value of the bias current at the two input terminals of the op amp. More precisely, it is the average current flowing into the input terminals when the operational amplifier operates in the linear region. It is used to measure the input current of the differential amplifier pair.
6. Maximum differential mode input voltage: The maximum differential mode input voltage that the two input terminals of the op amp can withstand. When this voltage is exceeded, the differential tube will experience reverse breakdown. For NPN tubes made by planar technology, the value is about 5V, and the Vidmax of lateral PNP tubes can reach more than +-30V.
7. Maximum common mode input voltage: The allowable range of common mode input voltage under the condition of ensuring the normal operation of the operational amplifier. When the common mode voltage exceeds this value, the input differential pair tube becomes saturated and the amplifier loses its common mode rejection capability.
5. Dynamic technical indicators of operational amplifiers
1. Open loop differential voltage gain: The ratio of the output voltage to the input voltage change without external feedback.
2. Differential input resistance: The input resistance of the op amp when a differential signal is input. It is the dynamic resistance seen from the two differential input terminals under the open loop condition of the op amp.
3. Common mode input resistance Ric (common mode input resistance): It is defined as the resistance to ground when the two input terminals of the operational amplifier are connected in parallel. For integrated operational amplifiers with transistors as input stages, Ric is usually about two orders of magnitude higher than Rid. When using field effect transistors, the values of Ric and Rid of the input stage operational amplifier are equivalent.
4. Common mode rejection ratio: Same as the definition in differential amplifier circuit, it is the ratio of differential mode voltage gain to common mode voltage gain, usually expressed in decibels. KCMR=20lg(Avd/Avc) (dB). It is a parameter to measure the symmetry of input differential amplifier and characterize the ability of integrated operational amplifier to suppress common mode interference signal. The larger the value, the better. Usually KCMR is about (70~100) decibels, and high quality can reach 160 decibels.
5.—3dB bandwidth: The bandwidth defined when the differential mode voltage gain of the operational amplifier drops by 3dB. The larger the value, the better.
6. Unit gain bandwidth (BW?G) (unit gain band width): The frequency corresponding to when it drops to 1 is defined as the unit gain bandwidth. It is similar to the characteristic frequency of the transistor.
7. Conversion rate (slew rate): Also known as the rise rate, it reflects the responsiveness of the op amp to a rapidly changing input signal. The expression of conversion rate is. The larger the SR, the better the responsiveness of the op amp to a rapidly changing input signal. The larger the signal amplitude and the higher the frequency, the larger the SR of the integrated op amp is required.
8. Equivalent input noise voltage Vn: When the input is short-circuited, the noise voltage at the output is converted to the value at the input. This value often corresponds to a certain frequency band.
6. Introduction to the main parameters of the op amp
This section uses the integrated operational amplifier in the "China Integrated Circuit Encyclopedia" as the main reference material, and also refers to other relevant materials. Integrated operational amplifiers have many parameters, among which the main parameters are divided into DC indicators and AC indicators.
The main DC indicators 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 main AC indicators 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.
1. DC indicator
input offset voltage VIO: 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.
Temperature drift of input offset voltage (abbreviated as input offset voltage temperature drift) αVIO: The temperature drift of input offset voltage is defined as the ratio of the change of input offset voltage to the temperature change within a given temperature range. This parameter is actually a supplement to the 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 IIB: Input bias current is defined as the average bias current at the two input terminals when the output DC voltage of the op amp is zero. Input bias current has a great influence on places that require input impedance, 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 IIO: Input offset current is defined as the difference between the bias currents at its two input terminals when the output DC voltage of the op amp is zero. The input offset current also reflects the circuit symmetry inside the op amp. The better the symmetry, the smaller the input offset current. Input offset current is a very important indicator for op amps, especially for precision op amps or when used for DC amplification. The input offset current is approximately one percent to one tenth of the input bias current. Input offset current has an important influence on small signal precision amplification or DC amplification, especially when a larger resistor (such as 10k? or greater) is used outside the op amp. The impact of input offset current on accuracy may exceed the impact of input offset voltage on accuracy. The smaller the input offset current, 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 op amps.
Temperature drift of input offset current (abbreviated as input offset current temperature drift): The temperature drift of input bias current is defined as the ratio of the change of input offset current to the temperature change within a given temperature range. This parameter is actually a supplement to the input offset current, which is convenient for calculating the drift size of the amplifier circuit caused by temperature change within a given operating range. The input offset current temperature drift is generally only given in the precision op amp parameters, and it only needs to be paid attention to when it is used for DC signal processing or small signal processing.
Differential open-loop DC voltage gain: The differential open-loop DC voltage gain is defined as the ratio of the output voltage of the op amp to the differential input voltage when the op amp works in the linear region. Since the differential open-loop DC voltage gain is very large, the differential open-loop DC voltage gain of most op amps is generally tens of thousands of times or more, and it is not convenient to compare directly with numerical values, so it is generally recorded and compared in decibels. The differential open-loop DC voltage gain of a general op amp is between 80 and 120 dB. The differential open-loop voltage gain of an actual op amp is a function of frequency. In order to facilitate comparison, the differential open-loop DC voltage gain is generally used.
Common-mode rejection ratio: The common-mode rejection ratio is defined as the ratio of the differential gain of the op amp to the common-mode gain when the op amp works in the linear region. The common-mode rejection ratio is an extremely important indicator that can suppress differential-mode input == mode interference signals. 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, and 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 a general op amp is between 80 and 120 dB.
Power supply voltage rejection ratio: The power supply voltage rejection ratio is defined as the ratio of the input offset voltage of the op amp to the change of 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. At present, the power supply voltage rejection ratio can only be about 80dB. Therefore, when used for DC signal processing or small signal processing analog amplification, the power supply of the op amp needs to be carefully handled. 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: 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 the specified load and 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 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, which can simplify the power supply design. However, the current full-scale output op amps can only work at low voltages and are relatively expensive.
Maximum common-mode input voltage: 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 operates in the linear region. It is generally defined as the common-mode input voltage corresponding to a 6dB drop in the common-mode rejection ratio 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 taken into account in circuit design.
Maximum differential input voltage: The maximum differential input voltage is defined as the maximum input voltage difference allowed between the two input terminals of the op amp. When the input voltage difference allowed between the two input terminals of the op amp exceeds the maximum differential input voltage, the op amp input stage may be damaged.
2. Main AC indicators
Open-loop bandwidth: The open-loop bandwidth is defined as the signal frequency corresponding to the open-loop voltage gain dropping 3db (or 0.707 of the DC gain of the op amp) from the DC gain of the op amp when a constant-amplitude sinusoidal signal is input to the input of the op amp and measured from the output of the op amp. This is used for very small signal processing.
Unit gain bandwidth GB: 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 is used for op amp selection in small signal processing.
Conversion rate (also called slew rate) SR: The conversion rate of an op amp is defined as the output rise rate of the op amp measured from the output of the op amp when a large signal (including step signal) is input to the input of the op amp under closed-loop conditions. 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 maximum conversion rate SR of high-speed op amps reaches 6000V/μs. This is used for op amp selection in large signal processing.
Full power bandwidth BW: Full power bandwidth is defined as the signal frequency at which the output amplitude of the op amp reaches the maximum (allowing a certain amount of distortion) when a constant amplitude sinusoidal large signal is input to the input of the op amp under the condition of a rated load and a closed loop gain of 1. 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 for selecting op amps in large signal processing.
Settling time: Settling time is defined as the time required for the output of the op amp to increase from 0 to a given value when a large step signal is input to the input of the op amp under the condition that the closed-loop gain of the op amp is 1 times at rated load. Since it is a large step signal input, there will be a certain jitter after the output signal reaches the given value. This jitter time is called the stabilization time. Stabilization time + rise time = settling time. For different output accuracies, the stabilization time varies greatly. The higher the accuracy, the longer the stabilization time. Settling time is a very important indicator for selecting op amps in large signal processing.
Equivalent input noise voltage: Equivalent input noise voltage is defined as any irregular AC interference voltage generated at the output of a well-shielded op amp with no signal input. When this noise voltage is converted to the op amp input, it is called the op amp input noise voltage (sometimes also expressed as noise current). For broadband noise, the effective value of the input noise voltage of an ordinary op amp is about 10~20μV.
Differential input impedance (also called input impedance): Differential input impedance is defined as the ratio of the voltage change at the two input terminals to the corresponding input current change when the op amp is operating in the linear region. Differential input impedance includes input resistance and input capacitance. At low frequencies, it only refers to input resistance. General products also only give input resistance. The input resistance of an op amp using bipolar transistors as the input stage is no more than 10 megohms; the input resistance of an op amp using field effect transistors as the input stage is generally greater than 109 ohms.
Common-mode input impedance: Common-mode input impedance is defined as the ratio of the change in common-mode input voltage to the corresponding change in input current when the op amp is operating on an input signal (i.e. the same signal is input to both input terminals of the op amp). At low frequencies, it appears as a common-mode resistor. Usually, the common-mode input impedance of an op amp is much higher than the differential-mode input impedance, with a typical value of more than 108 ohms.
Output impedance: Output impedance is defined as the ratio of the voltage change to the corresponding current change when a signal voltage is applied to the output of the op amp when the op amp is operating in the linear region. At low frequencies, it only refers to the output resistance of the op amp. This parameter is tested in an open loop.
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