Basic introduction Chinese name: operationalamplifier mbth: operational amplifier? Abbreviation: main parameters of operational amplifier: common mode rejection, gain bandwidth product and other attributes: high magnification circuit unit material: vacuum tube birth time: 1930, development history, principle, classification, universal type, high resistance type, low temperature drift type, high speed type, low power consumption type, high voltage and high power type, programmable type, parameters, common mode input resistance, DC common mode rejection type, AC type. Bias current temperature drift, input bias current, input offset current temperature drift (TCIOS), differential mode input resistance, output impedance, output voltage swing, power consumption, power supply rejection ratio, slew rate, power supply current, unity gain bandwidth, input offset voltage, input capacitance, input voltage range, input voltage noise density (en), and input current. The first amplifier designed with vacuum tube in the history of development was completed around 1930. This amplifier can do addition and subtraction. The first purpose of operational amplifier design is to convert analog voltage into digital for addition, subtraction, multiplication and division, and it also becomes the basic component of analog computer. The application of ideal operational amplifier in circuit system design far exceeds the calculation of addition, subtraction, multiplication and division. Nowadays, operational amplifiers, whether using transistors or vacuum tubes, discrete components or integrated circuits, have gradually approached the requirements of ideal operational amplifiers. Early operational amplifiers were designed with vacuum tubes, but at present most of them are integrated circuit components. However, if the demand for amplifiers in the system exceeds the demand for integrated circuit amplifiers, discrete components are usually used to realize these special operational amplifiers. At the end of 1960s, Fairchild Semiconductor introduced the first widely used integrated circuit operational amplifier, model μA709, designed by Bob Widlar. However, 709 was quickly replaced by the new product μA74 1, with better performance, more stability and easier use. 74 1 operational amplifier has become a unique symbol in the development history of microelectronics industry. After decades of evolution, it has not been replaced. Many IC manufacturers are still producing 74 1. Up to now, μA74 1 is still a typical textbook for explaining the principle of operational amplifier in the department of electronic engineering in colleges and universities. As shown in the figure, the operational amplifier has two input terminals A (inverting input terminal), B (noninverting input terminal) and one output terminal O. Also known as reverse input, non-reverse input and output respectively. When the voltage U- is applied to the A terminal and the common terminal (the common terminal is the point where the voltage is zero), it is equivalent to the reference node in the circuit. ), and its actual direction is higher from the A terminal than the common terminal, the actual direction of the output voltage U is from the common terminal to the O terminal, that is, the direction is just the opposite. When the input voltage U+ is applied between the B terminal and the common terminal, the actual directions of U and U+ with respect to the common terminal are exactly the same. For the convenience of distinguishing, terminal A and terminal B are marked with "-"and "+"respectively, but they should not be mistaken for the positive and negative polarities of the voltage reference direction. The positive and negative polarities of voltage should be marked separately or indicated by arrows. Inverting amplifier and noninverting amplifier are as follows: Operational amplifier Operational amplifier can generally be regarded as a high-gain direct-coupled voltage amplifier with a signal output connection port (Out) and two high-resistance inputs, so it can be used to make noninverting, inverting and differential amplifiers. The power supply mode of operational amplifier is divided into dual power supply and single power supply. For an operational amplifier with dual power supply, its output can change on both sides of zero voltage, and when the differential input voltage is zero, the output can also be set to zero. The output of a single power supply operational amplifier varies within a certain range between power supply and ground. The input potential of an operational amplifier is usually higher than the negative power supply and lower than the positive power supply. Specially designed operational amplifier can allow the input potential to change in the whole range from negative power supply to positive power supply, even slightly higher than positive power supply or slightly lower than negative power supply. This operational amplifier is called a rail-to-rail input operational amplifier. The output signal of the operational amplifier is directly proportional to the signal voltage difference between the two input terminals. In the audio part, the output voltage =A0(E 1-E2), where A0 is the low-frequency open-loop gain of the operational amplifier (e.g. 100dB, i.e. 10000 times), and E 1 is the input signal voltage of the noninverting terminal. Classification According to the parameters of integrated operational amplifiers, integrated operational amplifiers can be divided into the following categories. General purpose operational amplifier is designed for general purpose. The main characteristics of this kind of device are low price, large quantity and wide product range, and its performance index can be suitable for general use. Examples include μA74 1 (single operational amplifier), LM358 (double operational amplifier), LM324 (quad operational amplifier) and LF356 with FET as input stage. They are the most widely used integrated operational amplifiers at present. Operational amplifier High-resistance integrated operational amplifier is characterized by very high differential mode input impedance and very small input bias current, generally RID >；; 1Ω ~1Ω, IB is several picoamps to several tens of picoamps. The main measure to achieve these goals is to use the characteristics of high input impedance of field effect transistor to form the differential input stage of operational amplifier. Using FET as input stage not only has high input impedance and low input bias current, but also has high speed, wide frequency band and low noise, but also has large input offset voltage. Common integrated devices include LF355 and LF347 (four operational amplifiers), and CA3 130 and CA3 140 with high input impedance. Low temperature drift type in precision instruments, weak signal detection and other automatic control instruments, it is always hoped that the offset voltage of operational amplifier will be small and will not change with temperature. The low temperature drift operational amplifier is designed for this purpose. At present, the commonly used high-precision low-temperature drift operational amplifiers include OP07, OP27, AD508 and ICL7650, a chopper-stabilized zero drift device composed of MOSFET. High-speed type In fast A/D and D/A converters and video amplifiers, it is required that the conversion rate SR of integrated operational amplifiers must be high and the unit gain bandwidth BWG must be large enough. For example, a general-purpose integrated operational amplifier is not suitable for high-speed applications. The main characteristics of high-speed operational amplifier are high slew rate and wide frequency response. Common operational amplifiers are LM3 18, μA7 15 and so on. ，Sr = 50 ~ 70V/US，BWG >； 20 MHz. Low power consumption type Because the biggest advantage of electronic circuit integration is that it can make complex circuits small and light, with the expansion of the application range of portable instruments, it is necessary to use operational amplifiers with low power supply voltage and low power consumption. Commonly used operational amplifiers are TL-022C and TL-060C. Their working voltage is 2V ~18V, and their current consumption is 50 ~ 250μ A. At present, the power consumption of some products has reached μW level. For example, the power supply of ICL7600 is 1.5V, and the power consumption is 10mW, which can be supplied by a single battery. Operational amplifier The output voltage of high-voltage and high-power operational amplifier is mainly limited by power supply. In an ordinary operational amplifier, the maximum output voltage is generally only tens of volts, and the output current is only tens of milliamps. In order to increase the output voltage or current, an auxiliary circuit must be added outside the integrated operational amplifier. The high-voltage and high-current integrated operational amplifier can output high-voltage and high-current without any additional circuit. For example, the power supply voltage of D4 1 integrated operational amplifier can reach 150V, and the output current of μA79 1 integrated operational amplifier can reach1a. Programmable control always involves the range problem in the use of instruments. In order to obtain an output with a fixed voltage, the amplification factor of the operational amplifier must be changed. For example, if the magnification of an operational amplifier is 10 times and the input signal is 1mv, the output voltage is 10mv, and when the input voltage is 0. 1mv, the output is only 65438. Program-controlled operational amplifier is produced to solve this problem. For example, PGA 103A can change the amplification by controlling the level of pin 1 2. Parameter common-mode input resistance This parameter represents the ratio of the input common-mode voltage range to the variation of bias current in the linear region of the operational amplifier. DC common-mode rejection This parameter is used to measure the rejection ability of an operational amplifier to the same DC signal acting on two inputs. Ac common-mode rejection CMRAC is used to measure the rejection ability of an operational amplifier to the same AC signal acting on two inputs, and it is a function of differential-mode open-loop gain divided by common-mode open-loop gain. Gain Bandwidth Product Gain Bandwidth product is a constant, which is defined as the area where the characteristic curve of open-loop gain versus frequency drops by -20dB/ 10 times. Input bias current This parameter refers to the average current flowing into the input terminal in the linear region of the operational amplifier. Bias current temperature drift This parameter indicates the change of input bias current when the temperature changes. TCIB is usually expressed as pa/c, and input offset currentThis parameter refers to the difference between the currents flowing into two input terminals. Input offset current temperature drift (TCIOS) This parameter indicates the change of input offset current when the temperature changes. TCIOS usually uses pa/c to represent the differential mode input resistance. This parameter represents the ratio of the change of input voltage to the corresponding change of input current, and the change of voltage leads to the change of current. When one input is measured, the other input is connected to a fixed common-mode voltage. Output impedance This parameter refers to the internal equivalent small signal impedance of the output terminal of the operational amplifier in the linear region. Output voltage swing This parameter refers to the peak-to-peak value of the maximum voltage swing that can be achieved without clamping the output signal. VO is generally defined at a specific load resistance and power supply voltage. Power consumption refers to the static power consumed by the device at a given power supply voltage, and Pd is usually defined under no-load condition. Power supply rejection ratio of operational amplifier This parameter is used to measure the ability of operational amplifier to keep its output unchanged when the power supply voltage changes. PSRR is usually expressed by the change of input offset voltage caused by the change of power supply voltage. Conversion Rate This parameter refers to the maximum value of the ratio between the change of output voltage and the time required for the change to occur. SR generally takes v/&; Microscopic; S is expressed as a unit, sometimes as a positive change and a negative change respectively. Power supply current This parameter is the static current consumed by the device at the rated power supply voltage, and these parameters are usually defined under no-load conditions. Unit gain bandwidth This parameter refers to the maximum operating frequency of the operational amplifier when the open-loop gain is greater than 1. Input Offset Voltage This parameter represents the voltage difference that needs to be applied at the input terminal to make the output voltage zero. Temperature drift of input offset voltage (TCVOS) This parameter refers to the change of input offset voltage caused by temperature change, usually expressed as&; Microscopic; V/c is the unit. The input capacitance CIN represents the equivalent capacitance of any input terminal (the other input terminal is grounded) in the linear region of the operational amplifier. Input voltage range This parameter refers to the allowable input voltage range when the operational amplifier works normally (the expected result can be obtained), and VIN is usually defined under the specified power supply voltage. Input voltage noise density (en) For an operational amplifier, the input voltage noise can be regarded as a series noise voltage source connected to any input terminal. EN is usually expressed by nV/root Hz and is defined at a specific frequency. Input current noise density (iN) For an operational amplifier, the input current noise can be regarded as two noise current sources, which are connected to each input terminal and the common terminal respectively, usually expressed in units of pA/ root Hz and defined at a specified frequency. Ideal operational amplifier parameters: differential mode magnification, differential mode input resistance, common mode rejection ratio and infinite upper limit frequency; Input offset voltage and its temperature drift, input offset current and its temperature drift and noise are all zero. Operational amplifiers are widely used devices, which can be used as precision AC and DC amplifiers, active filters, oscillators and voltage comparators when connected to an appropriate feedback network. Measuring operational amplifier is a very high gain amplifier with differential input and single-ended output, which is often used in high-precision analog circuits, so its performance must be measured accurately. However, in open-loop measurement, its open-loop gain may be as high as 107 or higher, and pickup, stray current or Zeebek (thermocouple) effect may produce very small voltage at the input of the amplifier, so the error will be hard to avoid. Using servo loop can greatly simplify the measurement process and force the input of the amplifier to zero, so that the amplifier under test can measure its own error. Figure 1 shows a multifunctional circuit using this principle, which uses an auxiliary operational amplifier as an integrator to establish a stable loop with extremely high DC open-loop gain. This switch helps to perform the following tests. The circuit shown in figure 1 can minimize most measurement errors and support accurate measurement of a large number of DC and a small number of AC parameters. The additional "auxiliary" operational amplifier does not need to have better performance than the operational amplifier under test, and its DC open-loop gain should reach 106 or higher. If the offset voltage of the device under test (DUT) may exceed several millivolts, the auxiliary operational amplifier shall be powered by 15V (if the input offset voltage of DUT may exceed10mvolts, the resistance of resistor R3 of 99.9kω needs to be reduced. ) Figure 1 DUT The power supply voltages +V and–V are equal in amplitude and opposite in polarity. Of course, the total power supply voltage is 2× V, and the circuit uses symmetrical power supply, even a "single power supply" operational amplifier, because the ground of the system is based on the intermediate voltage of the power supply. As an integrator, the auxiliary amplifier is configured as an open loop (maximum gain) at DC, but its input resistance and feedback capacitance limit its bandwidth to several Hz. This means that the DC voltage at the output of the DUT is amplified by the auxiliary amplifier with the highest gain and applied to the noninverting input of the DUT through the 1000: 1 attenuator. Negative feedback drives the DUT output to ground potential. (Actually, the actual voltage is the offset voltage of the auxiliary amplifier, or more precisely, the voltage drop caused by the offset voltage plus the bias current of the auxiliary amplifier on the resistor of100kΩ, but it is very close to the ground potential, so it doesn't matter, especially considering that the voltage change at this point is unlikely to exceed a few mV during measurement). The voltage at the test point TP 1 is 1000 times of the corrected voltage (the amplitude is equal to the error) applied to the input terminal of the DUT, which is about tens of mV or higher, so it can be measured very easily. The offset voltage (Vos) of an ideal operational amplifier is 0, that is, when the two inputs are connected together and the intermediate supply voltage remains unchanged, the output voltage is also the intermediate supply voltage. In fact, the offset voltage of an operational amplifier varies from a few microvolts to a few millivolts, so a voltage within this range must be applied to the input terminal to keep the output at an intermediate potential. Figure 2 shows the most basic test configuration-offset voltage measurement. When the voltage on TP 1 is 1000 times of the offset voltage of the DUT, the output voltage of the DUT is at ground potential. An ideal operational amplifier has infinite input impedance and no current flows into its input. However, in practice, a small amount of "bias" current will flow into the inverting and noninverting inputs (IB–and Ib+ respectively), which will generate significant offset voltage in high impedance circuits. Depending on the type of operational amplifier, this bias current may be several Fa (1Fa =10–15A, which flows through an electron every few microseconds) to several Na; In some ultra-fast operational amplifiers, it even reaches 1-2 μA a. Figure 3 shows how these currents are measured. This circuit is basically the same as the offset voltage circuit in Figure 2, except that two series resistors R6 and R7 are added at the input of the DUT. These resistors can be short-circuited by switches S 1 and S2. When both switches are closed, the circuit is exactly the same as that in Figure 2. When S 1 is turned off, the bias current at the inverting input terminal flows into Rs, and the voltage difference increases to the offset voltage. By measuring the voltage change of TP1(=1000ib –× rs), IB–can be calculated. Similarly, when S 1 is closed and S2 is open, Ib+ can be measured. If the voltage of TP 1 is measured first when S 1 and S2 are closed, and then the voltage of TP 1 is measured again when S 1 and S2 are open, the "input" can be calculated by the change of this voltage. The resistance values of R6 and R7 depend on the current to be measured. Figure 2 Figure 3 If the value of Ib is about 5 pA, a large resistor will be used. It will be very difficult to use this circuit, and other technologies may be needed, including the rate at which Ib charges the low leakage capacitor (used to replace Rs). When S 1 and S2 are closed, Ios will still flow into the resistance of100Ω, resulting in Vos error, but it can usually be ignored in calculation, unless Ios is large enough to produce an error of 1% larger than the measured Vos. The open-loop DC gain of an operational amplifier may be very high. Gains above 107 are not uncommon, but gains of 250,000 to 2,000,000 are more common. The DC gain is measured by switching R5 between the output terminal of the DUT and the reference voltage of 1 V through S6, forcing the output of the DUT to change by a certain amount (1 V in Figure 4, but it can be designated as 10 V if the device is powered by a sufficiently large power supply). If R5 is+1 V, the DUT output must be–1v to keep the input of the auxiliary amplifier constant around 0. The voltage variation of TP 1 is attenuated by 1000: 1 and input to the DUT, resulting in the output variation of 1 V, so it is easy to calculate the gain (=1000×1v/TP/kloc-). Fig. 4 In order to measure the open-loop AC gain, it is necessary to inject a small AC signal into the input of the DUT and measure the corresponding output signal (TP2 in fig. 5). After completion, the auxiliary amplifier continues to stabilize the average DC level at the output of the DUT. In fig. 5, the AC signal is applied to the input of the DUT through the attenuator of10,000:1. Such a large attenuation value must be used for low-frequency measurement, and the open-loop gain may be close to DC value. (For example, when the gain is 1 1,000,000, the 1 V rms signal will apply 100 μV to the input of the amplifier, and the amplifier will try to provide 100 V rms output, resulting in saturation of the amplifier. Therefore, the frequency of AC measurement is generally several hundred Hz when the open-loop gain drops to 1; When low frequency gain data is needed, great care should be taken when measuring at low input amplitude. The simple attenuator shown in the figure can only work at the frequency below 100 kHz, and even if the stray capacitance is handled carefully, it cannot exceed this frequency. If higher frequencies are involved, more complex circuits need to be used. The common mode rejection ratio (CMRR) of operational amplifier refers to the ratio of the apparent change of offset voltage caused by the change of common mode voltage to the change of applied common mode voltage. In DC, it is generally between 80 dB and 120 dB, but it will decrease at high frequency. The t circuit of fig. 5 is very suitable for measuring CMRR (fig. 6). Instead of applying a common-mode voltage to the input of the DUT to avoid the low-level effect from damaging the measurement, it changes the power supply voltage (in the same direction as the input, that is, the common-mode direction), while the rest of the circuit remains unchanged. In the circuit shown in fig. 6, the offset voltage is measured at TP 1, the supply voltage is v (+2.5 V and–2.5v in this example), and the two supply voltages are shifted up by+1 V again (to +3.5 V and–1.5v). The change of offset voltage corresponds to the change of common-mode voltage of 1 V, so DC CMRR is the ratio of offset voltage to1v. Figure 6 CMRR measures the change of offset voltage relative to common-mode voltage, while the total supply voltage remains the same. On the other hand, power supply rejection ratio (PSRR) refers to the ratio of the change of offset voltage to the change of total power supply voltage, and the common-mode voltage keeps the middle power supply voltage unchanged (Figure 7). The circuits used are exactly the same, except that the total power supply voltage changes, while the common mode level remains unchanged. In this example, the power supply voltage is switched from +2.5V and–2.5 V to +3 V and–3 V, and the total power supply voltage is changed from 5 V to 6 V, and the common-mode voltage remains at the middle power supply voltage. The calculation method is the same (1000× TP11v). Fig. 7 In order to measure AC CMRR and PSRR, it is necessary to modulate the power supply voltage with voltage, as shown in fig. 8. DUT continues to work in DC open loop, but the exact gain is determined by AC negative feedback (100 times in the figure). In order to measure AC CMRR, the positive and negative power supplies of DUT are modulated by AC voltage with peak amplitude of 1 V, and the modulation of the two power supplies is in phase, so the actual power supply voltage is a stable DC voltage, but the common-mode voltage is a 2V peak-to-peak sine wave, which leads to the output of DUT including the AC voltage measured at TP2. Fig. 8 If the AC voltage amplitude of TP2 is x V peak (2 V peak-to-peak), the CMRR converted to the input of DUT (that is, before amplifying 100 times AC gain) is x/ 100 V, and CMRR is the ratio of this value to 1 V peak. The measurement method of AC PSRR is to apply AC voltage to positive and negative power supplies with a phase difference of 180, so as to modulate the amplitude of power supply voltage (in this case, it is also 1 V peak, 2 V peak), while the common-mode voltage keeps a stable DC voltage. The calculation method is very similar to the previous parameters. To sum up, of course, there are many other parameters of the operational amplifier that may need to be measured, and there are many methods to measure the above parameters, but as shown in this paper, the most basic DC and AC parameters can be reliably measured by simple basic circuits, which are easy to build and understand without any problems.