How to Convert a Dual-Supply Circuit to a Single-Supply Circuit

We often see a lot of very classic operational amplifier application atlas, but these applications are based on dual power supply. Many times, circuit designers must use a single power supply, but they do not know how to double the power circuit Convert to single power circuit.

When designing single-supply circuits, you need to be more careful than dual-supply circuits. Designers must fully understand what is described in this article.

1.1 Power Supply and Single Power Supply

All op amps have two power supply pins. Generally, in the literature, their designations are VCC+ and VCC-, but sometimes their designations are VCC+ and GND. This is because some data book authors attempt to distinguish this type of marking as a difference between single-supply op amps and dual-supply op amps. However, this does not mean that they must use it as such - they may work at other voltages. When the op amp is not powered by the default voltage, refer to the op amp's data sheet, especially the absolute maximum supply voltage and voltage swing specifications.

Most analog circuit designers know how to use an op amp with dual supply voltages. For example, the circuit on the left side of Figure 1, a dual supply consists of a positive supply and a negative supply of equal voltage. It is usually plus or minus 15V, positive and negative 12V and positive and negative 5V are also often used. Both the input voltage and the output voltage are given by reference. The positive and negative voltage swing amplitude limit Vom and the maximum output swing are also included.

A single-supply circuit (right in Figure 1) connects the power pins of the op amp to the positive supply and ground. The positive supply pin is connected to VCC+, the ground or VCC- pin is connected to GND. The voltage after the positive voltage is divided into half is connected to the input pin of the op amp as virtual ground. At this time, the output voltage of the op amp is also the virtual ground voltage. The output voltage of the op amp is centered on the virtual ground and swings at the Vom. Inside.

Some new op amps have two different maximum output voltages and lowest output voltages. This kind of op amp data sheet will specifically specify Voh and Vol respectively. It is important to note that many designers will use the virtual ground to refer to the input voltage and output voltage. However, in most applications, the input and output are referenced to the ground, so the designer must input and output. Add DC blocking capacitors to isolate the DC voltage between virtual ground and ground. (See section 1.3)

Usually the single-supply voltage is typically 5V, and the output voltage swing of the op amp will be lower. In addition, the operational voltage of the op amp can also be 3V or even lower. For this reason, the op amps used in single-supply circuits are basically Rail-To-Rail op amps, which eliminates the missing dynamic range.

It is important to point out that input and output may not all be able to withstand the voltage of Rail-To-Rail. Although the device is specified as Rail-To-Rail, if the output or input of the op amp does not support rail-to-rail, voltages near the input or near-output voltage limits may degrade the op amp's functionality. Carefully reference the data sheet whether the inputs and outputs are rail-to-rail. In order to ensure that the function of the system will not be degraded, this is the obligation of the designer.

1. 2 virtual ground

A single-supply op amp requires an external virtual ground. Typically, this voltage is VCC/2. The circuit in Figure 2 can be used to generate a VCC/2 voltage, but it will reduce the system's low-frequency characteristics.

R1 and R2 are equivalent and are selected by the allowable power dissipation and allowable noise. Capacitor C1 is a low-pass filter to reduce the noise from the power supply. Buffered op amps can be ignored in some applications.

In the following, there are some circuits where the virtual ground must be generated by two resistors, but in fact this is not the perfect method. In these examples, the resistance values ​​are all greater than 100K. When this happens, the circuit diagrams are noted.

1. 3 AC coupling

Virtual ground is greater than the DC level of the power supply ground. This is a small, local ground level. This creates a potential problem: the input and output voltages are generally referenced to the ground, if the output of the source is connected directly To the input of the op amp, this will produce an unacceptable DC offset. If this happens, the op amp will not respond properly to the input voltage because it will cause the signal to exceed the op amp's allowed input or output range.

The solution to this problem is to use AC coupling between the signal source and the op amp. Using this method, both the input and output devices can reference the system ground, and the op amp circuit can reference virtual ground. When more than one op amp is used, it is not necessary to use the coupling capacitance between the following conditional levels: The reference ground of the first-level op amp is the reference ground of the virtual ground second-level op amp and also the virtual ground. There is no gain at every level of the op amp. Any DC bias will be multiplied by the gain in any one stage, and may make the circuit beyond its normal operating voltage range.

If there is any doubt, assemble a prototype with a coupling capacitor and then take one of them one at a time to see if the electrical work is working. Unless the input and output are reference virtual ground, there must be a coupling capacitor to isolate the signal source and op amp inputs as well as the op amp output and load. A good solution is to disconnect the input and output, and then check the DC voltage across the two input pins of all op amps and the output pin of the op amp. All voltages must be very close to the virtual ground voltage. If not, the output of the front stage must be isolated with a capacitor. (or circuit problem)

1.4 Combined Op Amp Circuit

In some applications, a combination op amp can be used to save cost and board space, but it inevitably causes coupling between each other, which can affect filtering, DC bias, noise, and other circuit characteristics. Designers usually start with independent functional prototypes such as amplification, DC biasing, filtering, and so on. Combine them after checking each unit module. Unless otherwise specified, all filter units in this paper have a gain of 1.

1. 5 Select Resistor and Capacitor Values

Everyone who has just started to do analog design wants to know how to select the parameters of the components. Should the resistor use 1 Euro or 1 Megaohm? In general, the common application of resistance in the K-European to 100K-European level is more appropriate. High-speed applications have resistance values ​​ranging from 100 Euros to 1K Euros, but they increase power consumption. In portable design, the resistance is in the 1M to 10MΩ range, but they will increase the system noise. The basic equation used to select the value of the resistor and capacitor for adjusting the circuit parameters is given in each figure. If you do a filter, the accuracy of the resistor should be 1% E-96 series (see Appendix A). Once the magnitude of the resistance value is determined, select the standard E-12 series capacitor.

E-24 series capacitors are used to make parameter adjustments, but they should not be used as much as possible. The capacitance used to adjust the circuit parameters should not be 5%, and it should be 1%.

2.1 Zoom in

There are two basic types of amplifier circuits: non-inverting amplifiers and inverting amplifiers. Their ac-coupled version is shown in Figure 3. For AC circuits, reverse means that the phase angle is shifted by 180 degrees. This circuit uses a coupling capacitor - Cin. Cin is used to prevent the circuit from generating DC amplification, so that the circuit will only amplify the AC. If the Cin is omitted in the DC circuit, the DC amplification must be calculated.

In high frequency circuits, it is very important not to violate the bandwidth limitations of the op amp. In practical applications, the gain of the primary amplifier circuit is usually 100 times (40dB), and a high amplification will cause the circuit to oscillate, except when the board is installed. If you want to get a large amplification amplifier, using two equal gain op amps or multiple equal gain op amps is much better than using one op amp.

2.2 Attenuation

The traditional inverting attenuator composed of operational amplifiers is shown in Figure 4.

R2 is less than R1 in the circuit. This method is not recommended because many op amps are not suitable for working under a magnification of less than 1x. The correct method is to use the circuit in Figure 5.

A set of normalized R3 values ​​in Table 1 can be used to generate different levels of attenuation. For the resistance that is not in the table, it can be calculated by the following formula

R3=(Vo/Vin)/(2-2(Vo/Vin))

If there is a value in the table, it is processed as follows:

Select a value between 1K and 100K for Rf and Rin as the base value.

Divide Rin by two to get RinA and RinB.

Multiplying the base values ​​by 1 or 2 yields Rf, Rin1, and Rin2, as shown in Figure 5.

Select a suitable scale factor for R3 in the table and multiply it by the base value.

For example, if Rf is 20K, both RinA and RinB are 10K, then a -2dB attenuation can be obtained with a 12.1K resistor.

The in-phase attenuators in Figure 6 can be used as voltage attenuation and non-inverting buffers.

2.3 Adder

Figure 7 is an inverting adder, which is a basic audio mixer. However, this circuit is rarely used in a real audio mixer. Because this will approach the operational limits of the op amp, we actually recommend increasing the power supply voltage to improve the dynamic range.

The in-phase adder is achievable, but it is not recommended. Because the impedance of the signal source will affect the gain of the circuit.

2.4 Subtractor

Just like an adder, Figure 8 is a subtractor. A common application is to remove the original vocal in a stereo tape and leave the sound (the original vocal level is the same in both channels at the time of recording, but the sound is slightly different).

2.5 Analog Inductors

The circuit in Figure 9 is a circuit that inverts the capacitor and is used to model the inductance. The inductor will resist the change of current, so when a DC level is added to the inductor, the current rise is a slow process, and the voltage drop across the resistor in the inductor is particularly important.

Inductance makes it easier to pass low frequencies through it. Its characteristics are just opposite to the capacitance. An ideal inductor is without resistance. It allows direct current to pass without any restrictions, and has infinite impedance for signals whose frequency is infinite.

If the DC voltage is suddenly applied to the inverting input of the op amp through resistor R1, the output of the op amp will not change because this voltage is also applied to the non-inverting output with the overcapacitance C1. The output of the output shows a very high impedance, just like a true inductor.

As the capacitor C1 continues to charge through the resistor R2, the voltage on R2 continues to drop, and the op amp draws current through the resistor R1. As the capacitor continues to charge, the voltage on the two input and output pins of the final op amp eventually goes to virtual ground (Vcc/2).

When the capacitor C1 is completely filled, the resistor R1 limits the current flowing through it, which shows a resistor connected in series with the inductor. This series resistor limits the Q of the inductor. The DC resistance of a true inductor will generally be much smaller than the simulated inductor. Here are some limitations of analog inductors:

A section of the inductor is connected to the virtual ground;

The Q value of the analog inductor cannot be made very high, depending on the series resistance R1;

Analog inductors do not store energy like real inductors. Real inductors can cause high reverse-phase spikes due to the effect of a magnetic field, but the voltage of the analog inductor is limited by the output voltage swing of the op amp, so the pulse responds. Limited by voltage swing.

2.6 Instrumentation Amplifier

Instrumentation amplifiers are used in applications where a small signal level signal is needed to amplify the DC signal. It is derived from the subtractor topology. Instrumentation amplifiers take advantage of the high impedance of the non-inverting input. The basic instrument amplifier is shown in Figure 10.

This circuit is the basic instrumentation amplifier. Other instrumentation amplifiers are also shown in the figure. Here, the input is also powered by a single power supply. This circuit is actually a single power strain gauge. The disadvantage of this circuit is that it requires exactly the same resistance, otherwise the common-mode rejection ratio of this circuit will be very low.

The circuit in Figure 10 can simply remove three resistors, just like the circuit in Figure 11.

The gain of this circuit is very good. But this circuit also has one drawback: that is, the two resistors in the circuit must be replaced together, and they must be equal. Another disadvantage is that the first-level op amp does not produce any useful gain.

In addition, two amplifiers can also be used to form an instrument amplifier, as shown in Figure 12.

But this instrumentation amplifier is not recommended because the first op amp has less than one magnification, so he may be unstable, and the signal on Vin - takes more time than the signal on Vin + Reach the output.

This section provides a very in-depth description of active filters made up of op amps. In many cases, in order to block the DC level due to virtual ground, a capacitor is cascaded at the input of the op amp. This capacitor is actually a high-pass filter. In a sense, a single-supply op amp circuit like this has such a capacitor. The designer must determine that the capacitance of this capacitor must be more than 100 times larger than the capacity of other capacitors in the circuit. This ensures that the amplitude-frequency characteristics of the circuit are not affected by this input capacitance. If this filter also has amplifying effect, the capacity of this capacitor is preferably more than 1000 times of the capacity of other capacitors in the circuit. If the input signal already contains the DC bias of VCC/2, this capacitor can be omitted.

The output of these circuits includes the DC bias of VCC/2. If the circuit is the last stage, then the output capacitor must be connected in series.

There is an agreement about filter design, where the filters are all composed of single-supply op amps. The implementation of the filter is simple, but the designer must pay attention to the following points:

1. Inflection point (center) frequency of the filter

2. The gain of the filter circuit

3. Q-values ​​of band-pass filter and band-stop filter

4. Types of low-pass and high-pass filters (Butterworth, Chebyshev, Bessell)

Unfortunately, it is impossible to get a perfectly ideal filter that cannot be made up of an op amp. Even if possible, designers have to use very complex calculations to complete the design of the filter due to the negative mutual inductance between the components. The more complex the control requirements for waveforms typically means that more op amps are needed, which will be determined based on the maximum distortion the designer can accept. Or it can be finalized through several experiments. If the designer wants to use a minimum number of components to implement the filter, then there is no choice but to use a conventional filter, which can be calculated.

3.1 First-order filter

First-order filters are the simplest circuits, they have 20dB frequency-doubled amplitude-frequency characteristics

3.1.1 Low-pass filter

A typical low-pass filter is shown in Figure 13.

3.1.2 High-pass filter

A typical high-pass filter is shown in Figure 14.

3.1.3 Venturi filter

The venturi filter has the same gain for all frequencies, but it can change the phase angle of the signal and it can also be used as a phase angle correction circuit. The circuit in Fig. 15 has a phase shift of 90 degrees for a signal having frequency F, a phase shift of 0 degree for direct current, and a phase shift of 180 degrees for high frequency.

3.2 Second-order filter

Second-order filter circuits are generally named after their inventors. A few of them are still in use today. Some second-order filter topologies can be composed of low-pass, high-pass, band-pass, band-stop filters, and some are not. Not all filter topologies are listed here, just list them that are easy to implement and easy to adjust.

The second-order filter has an amplitude-frequency characteristic of 40 dB per double frequency.

The common bandpass and bandstop filters of the same topology use the same components to adjust their Q values, and they change the filter between Butterworth and Chebyshev filters. It must be understood that only the Butterworth filter can accurately calculate the inflection frequency. Chebyshev and Bessell filters can only be fine-tuned on the basis of the Butterworth filter.

The bandpass and bandstop filters we usually use have very high Q values. If you need to implement a very wide band-pass or band-stop filter, you need to use high-pass filters and low-pass filters in series. The pass characteristic for the band pass filter will be the overlapping part of the two filters, and the pass characteristic for the band stop filter will be the non-overlapping part of the two filters. The inverse Chebyshev and Elliptic filters are not described here because they are not already in the range of circuit sets that need to be introduced.

Not all filters can produce the result that we envisaged - for example, the final attenuation of the filter in the stopband will be larger in the multifeedback filter than in the Sallen-Key filter. Since these features are beyond the scope of the circuit map, please go to the textbooks to find the respective advantages and disadvantages of each circuit. However, the circuit described here is not used in a very special case, and the results are acceptable.

3.2.1 Sallen-Key Filter

The Sallen-Key filter is a popular, widely used second-order filter. His cost is low, requiring only one op amp and four passive components. But changing to a Butterworth or Chebyshev filter is not so easy. This circuit is a unity-gain circuit. Changing the gain of the Sallen-Key filter also changes the amplitude-frequency characteristics and type of the filter. In fact, the Sallen-Key filter is a Butterworth filter with a gain of 1.

3.2.2 Multiple Feedback Filters

The multiple feedback filter is a versatile, low cost and easy to implement filter. Unfortunately, the design-time calculations are somewhat complicated and are not covered here. See the detailed description of the multiple feedback filter in reference item [1]. If a unit-gain Butterworth filter is needed, the circuit here can give an approximate result.

3.2.3 Double T Filter

The double T filter can be implemented with either one op amp or two op amps. He is based on a passive network of three resistors and three capacitors. The matching of these six components is critical, but fortunately this is still a very easy process. This network can consist of resistors of the same value and capacitors of the same value. With the formula in the figure, R3 and C3 can be calculated at the same time. Should try to use the same batch of components, they have very similar characteristics.

3.2.3.1 Single Op Amps Implementation

If the band-pass filter is composed of elements whose parameters are very close, it is easy to oscillate. The resistance to virtual ground is best chosen in the E-96 1% series, which can destroy the oscillation conditions.

3.2.3.2 Implementation of dual op amps

A typical dual op amp is shown in Figure 20 to Figure 22.

Operational amplifiers (OP, OPA, OPAMP) are DC-coupled, differential mode (differential mode) inputs, and usually high-gain (gain) voltages with single-ended outputs. Amplifiers, because they were first used mainly for addition, multiplication, and other arithmetic circuits, were named. An ideal operational amplifier must have the following characteristics: an infinite input impedance, an output impedance equal to zero, an infinite open loop gain, an infinite common mode rejection ratio, and an infinite bandwidth. The most basic operational amplifier is shown in Figure 1-1. An op amp module typically includes a positive input (OP_P), a negative input (OP_N), and an output (OP_O).

When an operational amplifier is commonly used, its output is connected to its inverting input node to form a negative feedback configuration. The reason is that the operational amplifier's voltage gain is very large, ranging from a few hundred to tens of thousands of times. The use of negative feedback can ensure the stable operation of the circuit. However, this does not mean that op amps cannot be connected to positive feedback. Conversely, in many systems that need to generate oscillatory signals, operational amplifiers that are configured to return to feedback are very common components.

Open loop

Open-loop operational amplifiers are shown in Figure 1-2. When an ideal operational amplifier operates in an open-loop manner, the relationship between the output and the input voltage is as follows:

Vout = ( V+ -V-) * Aog

Among them, Aog represents the open-loop differential gain of the operational amplifier (open-loop differential gai). Since the open loop gain of the operational amplifier is very high, even if the differential signal at the input is small, the output signal will still be "saturation" (saturation). Causes nonlinear distortion to appear.

Closed-loop negative feedback

Connect the op amp's inverting input to the output, and the amplifier circuit is in a negative-feedback configuration. In this case, the circuit can often be simply called a closed-loop amplifier. The closed-loop amplifier can be divided into inverting amplifiers and non-inverting amplifiers according to the input signal entering the end of the amplifier.

The inverting closed loop amplifier is shown in Figure 1-3. Assuming that this closed-loop amplifier uses an ideal op amp, because its open-loop gain is infinite, the two inputs of the op amp are virtual ground. The relationship between the output and the input voltage is as follows:

Vout = -(Rf / Rin) * Vin

Non-inverting closed loop amplifiers are shown in Figure 1-4. Assuming that loop amplifier using an ideal operational amplifier, due to its open-loop gain is infinite, so the voltage difference between the two input terminals of the operational amplifier is almost zero, the relationship of output and input voltage as follows: Vout = ((R2 / R1) + 1) * Vin

Closed loop positive feedback

Connecting the forward input and output of the operational amplifier, the amplifier circuit is in a positive feedback condition. Because the positive feedback configuration works in an extremely unstable state, it is often used in applications that require the generation of oscillating signals.

Optimum op amps and ideal op amp conditions

When analyzing and integrating op amp application circuits, the integrated op amp can be considered as an ideal operational amplifier in most cases. The ideal operational amplifier, as its name implies, is to idealize the technical indicators of the integrated operational amplifier. Since the actual operational amplifier's technical specifications are closer to the ideal operational amplifier, the error caused by idealization is very small and can be ignored in general engineering calculations.

The ideal operational amplifier specifications are as follows:

1. Open-loop differential mode voltage amplification Aod = ∞;

2. Input resistance Rid = ∞; output resistance Rod =0

3. Input bias current IB1 = IB2 = 0;

4. Offset Voltage UIO, Offset Current IIO, Offset Voltage Temperature Drift , offset current temperature drift Both are zero;

5. Common mode rejection ratio CMRR = ∞;;

6. -3dB bandwidth fH = ∞ ;

7. No internal interference and noise.

The actual operational amplifier parameters can be treated as ideal operational amplifiers by reaching the following levels:

Voltage amplification reaches 104 to 105 times; input resistance reaches 105Ω; output resistance is less than several hundred ohms;

The current in the external circuit is much larger than the bias current; the offset voltage, offset current and temperature drift are small, the drift of the circuit is within the allowable range, and the stability of the circuit can meet the requirements; when the minimum signal is input, there is a certain letter Noise ratio, common mode rejection ratio is greater than or equal to 60dB; the bandwidth is in line with circuit bandwidth requirements.

Virtual and short-circuited meanings in operational amplifiers

Two important conclusions can be drawn when the ideal op amp is operating in the linear region:

Short

Because the ideal op amp has a large voltage amplification, and the op amp operates in the linear region, it is a linear amplifier circuit, the output voltage does not exceed the linear range (ie, the finite value), so the op amp non-inverting input and inverting input potential Very close to equal. When the op amp supply voltage is ±15V, the maximum value of the output is generally 10~13V. Therefore, the voltage difference between the two input terminals of the op amp is less than 1mV, and the two input terminals are short-circuited. This feature is called imaginary and short. Obviously this is not a true short circuit. It is just a reasonable approximation of the allowable error range when analyzing the circuit.

Virtually broken

Because the input resistance of the op amp is generally more than a few hundred kilohms, the current flowing into the operational amplifier's non-inverting input and inverting input is very small, which is several orders of magnitude smaller than the current in the external circuit. The current flowing into the op amp can often be Ignoring, this is quite an op amp's input open circuit, this feature is called virtual broken. Obviously, the op amp's input cannot be truly open.

Using the concepts of "virtuality and short-circuit" and "virtual power-off" can simplify the analysis process of the application circuit when analyzing the linear application circuit of the op amp. Operational circuits composed of operational amplifiers require a certain functional relationship between input and output, so both of these conclusions can be applied. If the op amp is not working in the linear area, there is no "virtual short" or "virtual" feature. If the potentials of the two input terminals of the operational amplifier are measured to reach several millivolts or more, the op amp is often not operating in the linear region or has been damaged.

Important indicators

Input offset voltage UIO

An ideal integrated op amp, when the input voltage is zero, the output voltage should also be zero (no zero adjustment device). However, in fact, it is difficult for the differential input stage of the integrated operational amplifier to be completely symmetrical. Usually, when the input voltage is zero, there is a certain output voltage. The input offset voltage refers to the compensation voltage applied to the input terminal to make the output voltage zero. Actually, when the input voltage is zero, the output voltage is divided by the voltage amplification factor. The value converted to the input is called the input offset voltage. That is, the size of UIO reflects the symmetry of the op amp and the potential matching condition. The smaller the UIO, the better. The magnitude is between 2mV and 20mV. UIO of ultra-low offset and low-drift op amps generally input offset current IIO from 1μV to 20μV.

When the output voltage is zero, the difference between the quiescent currents at the differential base of the differential input stage is called the input offset current IIO, ie,

Due to the presence of the internal resistance of the signal source, changes in the IIO can cause changes in the input voltage, making the op amp output voltage non-zero. The smaller the IIO, the better the symmetry of the input-level differential pair is, typically about 1nA~0.1μA. Input bias current IIB

When the integrated op amp output voltage is zero, the average value of the static bias currents at the two inputs of the op amp is defined as the input bias current.

From the use point of view, the bias current is small, and the output voltage change caused by the change of the internal resistance of the signal source is also smaller, so the input bias current is an important technical indicator. The general IIB is about 1nA~0.1μA.

Input offset voltage temperature drift â–³UIO/â–³T

Input offset voltage drift is the ratio of the input offset voltage to the temperature change over the specified operating temperature range. It is an important indicator to measure the temperature drift of the circuit and cannot be compensated by an external zero adjustment device. The smaller the input offset voltage drift, the better. The input op amp voltage drift of general op amps is between ±1mV/°C~±20mV/°C.

Input offset current temperature drift â–³IIO/â–³T

In the specified operating temperature range, the ratio of the input offset current change with temperature and the temperature change is called input offset current temperature drift. Input offset current drift is a measure of the current drift of the amplifier circuit and cannot be compensated by an external zero-adjustment device. High-quality op amps have a few pA per degree.

Maximum differential mode input voltage Uidmax

The maximum differential mode input voltage Uidmax refers to the maximum differential mode input voltage that the two inputs of the op amp can withstand. Above this voltage, the input stage of the op amp will enter the non-linear area, causing the performance of the op amp to deteriorate significantly and even cause damage. According to different processes, Uidmax is about ±5V~±30V.

Maximum common mode input voltage Uicmax

The maximum common-mode input voltage, Uicmax, refers to the maximum common-mode input voltage the op amp can withstand under the normal operational conditions of the op amp. When the common-mode voltage exceeds this value, the operating point of the input differential pair enters the nonlinear region, the amplifier loses common-mode rejection, and the common-mode rejection ratio drops significantly.

The maximum common-mode input voltage Uicmax is defined as the common-mode input voltage value that causes the output voltage to produce a 1% following error when the op amp is connected to a voltage follower at the nominal supply voltage; or it is defined as the common-mode added at a drop of 6dB. Input voltage value.

The open-loop differential mode voltage amplification Aud refers to the ratio of the change of the output voltage to the change of the input voltage at the input port of the op amp when the integrated op amp operates in the linear region and accesses the specified load. The Aud of the op amp is between 60 and 120 dB. Different functions of the op amp, Aud difference.

The differential-mode input resistance Rid is the input resistance of the op amp when a differential-mode signal is input. The larger Rid, the smaller the impact on the signal source, the input resistance Rid of the op amp is generally more than a few hundred kilohms.

The common mode rejection ratio (KCMR) of an op amp is the same as that defined in a differential amplifier circuit. It is the ratio of the differential mode voltage amplification factor to the common mode voltage amplification factor, and is usually expressed in decibels. Different functions of the op amp, KCMR is not the same, some in the 60 ~ 70dB, and some up to 180dB. The larger the KCMR, the stronger the ability to suppress common-mode interference.

Open loop bandwidth BW

The open-loop bandwidth, also called -3dB bandwidth, refers to the frequency fH that the operational amplifier's differential mode voltage amplification Aud drops by 3dB in the high frequency band.

The unity gain bandwidth BWG refers to the frequency fT corresponding to the increase of the signal frequency when Aud drops to 1, that is, the signal frequency fT when Aud is 0 dB. It is an important parameter for integrating the op amp. The fT=7Hz of the 741 op amp is relatively low.

Slew Rate SR (Slew Rate)

Conversion rate SR

Refers to the amplifier circuit in the voltage amplification factor is equal to 1 conditions, the input of large signals (such as step signals), the amplifier circuit output voltage of the maximum rate of change of time, see Figure 7-1-1. It reflects the op amp's ability to respond to rapidly changing input signals. The expression of the conversion rate SR is

The slew rate SR is an important indicator when working with large signals and high frequency signals. At present, the general general-purpose op amp slew rate is about 1 to 10 V/μs.

Unity gain bandwidth BWG (fT)

Common mode rejection ratio KCMR

Differential mode input resistance

Open-loop differential mode voltage amplification Aud

Open loop bandwidth:

The open-loop bandwidth is defined as the input of a constant-amplitude sinusoidal small signal to the input of the op amp. The open-loop voltage gain measured from the output of the op amp drops 3 dB from the DC gain of the op amp (or the DC equivalent of the op amp). The gain of 0.707) corresponds to the signal frequency. This is for very small signal processing.

Unit gain bandwidth GB:

The unity-gain bandwidth is defined as the op amp's closed-loop gain is 1 times. A constant-amplitude sinusoidal small signal is input to the input of the op amp, and the closed-loop voltage gain measured from the output of the op amp is decreased by 3db (or equivalent The signal frequency corresponding to the 0.707 input signal of the op amp. The unity-gain bandwidth is an important indicator. For sinusoidal small-signal amplification, the unity-gain bandwidth is equal to the product of the input signal frequency and the maximum gain at that frequency. In other words, it is necessary to know the signal frequency and signal to be processed. After the increase, the unity gain bandwidth can be calculated to select the appropriate op amp. This is used for op amp selection in small signal processing.

Slew Rate (also called Slew Rate) SR:

Op Amp slew rate is defined as that, when the op amp is connected to a closed-loop condition, a large signal (including a 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 during the conversion, the input stage of the op amp is in the on-off state, the feedback loop of the op amp is inactive, ie the slew rate is independent of the closed-loop gain. The slew rate is a very important indicator for large signal processing. For a general op amp slew rate SR<=10V/μs, the high speed op amp slew rate SR>10V/μs. The highest conversion rate of current high-speed op amps reaches 6,000V/μs. This is used for op amp selection in large signal processing.

Full power bandwidth BW:

The full power bandwidth is defined as that, at a rated load, when the closed-loop gain of the op amp is 1×, a constant amplitude sinusoidal large signal is input to the input of the op amp, which maximizes the output amplitude of the op amp (allowing certain distortion) The frequency of the signal. This frequency is limited by the conversion rate of the op amp. Approximately, full power bandwidth = slew rate/2πVop (Vop is the op amp's peak output amplitude). Full power bandwidth is a very important indicator for large-signal processing op amp selection.

Establishment time:

The settling time is defined as: when the rated load is equal to the closed loop gain of the op amp, the signal of one step is input to the input of the op amp, and the output of the op amp is increased from 0 to a given value. The time needed. Because it is a large step signal input, there will be some jitter after the output signal reaches a given value. This jitter time is called the settling time. Settling time + rise time = settling time. For different output accuracy, there is a big difference in the settling time. The higher the precision, the longer the settling time. Settling time is a very important indicator for large signal processing op amp selection.

Equivalent input noise voltage:

The equivalent input noise voltage is defined as the op amp with good shielding, no signal input, and any irregular ac disturbance voltage generated at its output. When this noise voltage is converted to the input of the op amp, it is called the op amp input noise voltage (sometimes also expressed as noise current). For broadband noise, the input noise voltage of an ordinary op amp is about 10~20μV.

Differential input impedance (also called input impedance):

The differential-mode input impedance is defined as the ratio of the voltage change at the two inputs to the current change at the corresponding input when the op amp is operating in the linear region. The differential-mode input impedance includes the input resistance and the input capacitance, and at low frequencies only the input resistance. The general product also only gives the input resistance. The input resistance of an op amp with a bipolar transistor as the input stage is not greater than 10 megohms; the input resistance of an op amp with a FET as the input stage is generally greater than 109 ohms.

Common-mode input impedance:

The common-mode input impedance is defined as the ratio of the common-mode input voltage variation to the corresponding input current variation when the op amp is operating on the input signal (ie, the op amp's two inputs are inputting the same signal). At low frequencies it manifests as common-mode resistance. Typically, the op amp's common-mode input impedance is much higher than the differential-mode input impedance, typically more than 108 ohms.

Output impedance:

输出阻抗定义为，运放工作在线性区时，在运放的输出端加信号电压，这个电压变化量与对应的电流变化量的比值。在低频时仅指运放的输出电阻。这个参数在开环测试。