Analysis of Basic Circuits of Common Op Amps

/ 2019-06-06 / Tech

We often see many very classic op amp application atlases, but these applications are based on dual power supplies. Many times, circuit designers must use a single power supply, but they don't know how to double The circuit of the power supply is converted into a single power supply circuit.


Being more careful than designing a single-supply circuit, the designer must fully understand what is described in this article.


1.1 Power supply and single power supply


All op amps have two power supplies Pins, generally in the data, are labeled VCC+ and VCC-, but sometimes their flags are VCC+ and GND. This is because some data source authors attempt to distinguish this difference from a single-supply op amp to a dual-supply op amp. However, this is not to say that they must use it that way - they may be able to work under other voltages. When the op amp is not powered by the default voltage, you need to refer to the op amp's data sheet, especially the absolute maximum supply voltage and voltage swing instructions.  


The vast majority of analog circuit designers know how to use an op amp under dual supply voltage conditions, such as the one on the left of Figure 1, a dual supply. It consists of a positive supply and a negative supply of equal voltage. Generally, positive and negative 15V, plus or minus 12V and plus or minus 5V are also frequently used. Both the input voltage and the output voltage are given in reference, and include the swing amplitude limit Vom of the positive and negative voltages and the maximum output swing.  


The power supply for the single-supply circuit (right in Figure 1) is connected to the positive supply and ground. The positive supply pin is connected to VCC+, and the ground or VCC- pin is connected to GND. The voltage divided into half by the positive voltage is connected to the input pin of the op amp as the 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 the swing is at Vom. Inside.


There are some new op amps with two different maximum output voltages and lowest output voltages. Voh and Vol are specified separately in the data sheet for this op amp.It is important to note that there are quite a few designers who will use the virtual ground to refer to the input voltage and output voltage, but in most applications, the input and output are reference ground, so the designer must be at the input and output. Place a DC blocking capacitor to isolate the DC voltage between the virtual ground and ground. (See Section 1.3)


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


It is important to note that the inputs and outputs are not necessarily capable of withstanding the voltage of the 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, a voltage near the input or near the output voltage limit may degrade the function of the op amp, so Carefully refer to the data sheet for whether the inputs and outputs are rail-to-rail. This is to ensure that the function of the system will not degrade, which is the designer's obligation.


1. 2 virtual ground


A single-supply op amp needs to provide a virtual ground externally. Normally, this voltage is VCC/2.The circuit of Figure 2 can be used to generate a voltage of VCC/2, but it will reduce the low frequency characteristics of the system.


R1 and R2 are equivalent, through the power supply The allowable consumption and the allowed noise are chosen. Capacitor C1 is a low pass filter used to reduce the noise generated from the power supply. Buffered op amps can be ignored in some applications.


In the following, some circuits have virtual grounds that must be generated by two resistors, but this is not the perfect method. In these examples, the resistance values are greater than 100K, and when this happens, it is noted in the circuit diagram.


1. 3 AC coupling


The virtual ground is greater than the DC level of the power ground, this is a Small, local ground levels, which creates a potential problem: the input and output voltages are typically reference ground. If the output of the signal source is directly connected to the input of the op amp, this will result in an unacceptable DC offset. If this happens, the op amp will not respond correctly to the input voltage as this will cause the signal to exceed the input or output range allowed by the op amp.


The solution to this problem is to couple the signal source to the op amp. Using this method, both the input and output devices can be referenced to the system ground, and the op amp circuit can refer to the virtual ground. When more than one op amp is used, it is not necessary to use a coupling capacitor between the following conditions:The reference ground of the first stage op amp is the reference of the virtual ground second stage op amp. The virtual stage also has no gain for each stage of the two stage op amps. Any DC offset will be multiplied by the gain in any stage and may cause the circuit to exceed its normal operating voltage range.


If you have any questions, assemble a prototype with a coupling capacitor and then remove one of them at a time to see if the electrical work is working. Unless the input and output are referenced to virtual ground, there must be a coupling capacitor to isolate the source and op amp inputs as well as the op amp output and load. A good solution is to disconnect the inputs and outputs and then check the DC voltage on the two input pins of all op amps and the output pins 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 by capacitors. (or there is a problem with the circuit)


1. 4 Combination op amp circuit


In some applications, Combined op amps can be used to save cost and board space, but inevitably cause mutual coupling.Can affect filtering, DC offset, noise, and other circuit characteristics. Designers typically start with independent functional prototypes such as amplification, DC offset, filtering, and more. Unit each unit module after it has been verified. All filter units in this article have a gain of one unless otherwise stated.


1. 5 Select the value of the resistor and capacitor


Everyone who is just starting a simulation design I want to know how to choose the parameters of the component. Should the resistor be 1 ohm or 1 megaohm? In general, the resistance value in the ordinary application is suitable from K to 100K. High-speed applications have resistance values from 100 ohms to 1K ohms, but they increase power consumption. The resistance in portable designs ranges from 1 megahertz to 10 megaohms, but they will increase the noise of the system. The basic equations used to select the values of the resistors and capacitors used to adjust the circuit parameters are given in each figure. If you are making 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.


Use E-24 series capacitors for parameter adjustment, but you should try not to use them. The capacitor used for circuit parameter adjustment should not be used with 5%, and should be used at 1%.


2.1 Zoom


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, the 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 only amplifies the AC. If Cin is omitted in the DC circuit, then DC amplification must be calculated.


In high-frequency circuits, it is 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 the higher amplification factor will cause the circuit to oscillate unless it is very careful when laying out the board. If you want to get a large amplifier with a magnification comparison,Using two equal gain op amps or multiple equal gain op amps is much better than using an op amp.


2.2 Attenuation

The traditional inverting attenuator consisting of an operational amplifier 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 operation when the magnification is less than 1x. Correct The method is to use the circuit of Figure 5.


The set of normalized R3 values in Table 1 can be used to generate different levels of attenuation. For resistance values not listed in the table, the following formula can be used to calculate  




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


Select a value between 1K and 100K for Rf and Rin, which is used as the base value. 


Divide Rin by two to get RinA and RinB.  


Multiply the base values by 1 or 2 to get Rf, Rin1, and Rin2, as shown in Figure 5.


Select an appropriate scale factor for R3 in the table and multiply it by the base value.


For example, if Rf is 20K, RinA and RinB are both 10K, then a -1dB attenuation can be obtained with a 12.1K resistor.




The in-phase attenuator in Figure 6 can be used as a voltage attenuation and in-phase buffer.




2.3 Adder


Figure 7 is a counter Adder, he is a basic audio mixer, but this circuit is rarely used in real audio mixers. Because this will approach the operational limits of the op amp, we actually recommend increasing the power supply voltage to improve the dynamics. Scope.


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


2.4 Subtractor


As with the adder, Figure 8 is a subtractor. A common application is to remove the original vocal from the stereo tape and leave the accompanying sound ( The original vocal level in the two channels is the same when recording, but the sound is slightly different.)


2.5 Analog Inductance


The circuit in Figure 9 is a circuit that reverses the operation of the capacitor. It is used to simulate the inductor. The inductor resists the change in current, so when a DC The rise in current when applied to the inductor is a slow process, and the voltage drop across the resistor is particularly important.


Inductors It will be easier to pass the low frequency through it. Its characteristic is exactly opposite to the capacitance. An ideal inductor has no resistance. It can make the DC power pass without any limit, and has infinite impedance to the signal with infinite frequency.


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 the same Capacitor C1 is also applied to the non-inverting output, and the output of the op amp exhibits a high impedance, just like a true inductor.


As capacitor C1 is continuously charged through resistor R2, the voltage on R2 drops and the op amp draws current through resistor R1. As the capacitor is continuously charged, the voltage on the two input and output pins of the final op amp eventually tends to virtual ground (Vcc/2).


When capacitor C1 is fully charged, resistor R1 limits the current flowing through it, which exhibits a series-connected resistance in the inductor. This series of resistors limits the Q of the inductor. The DC resistance of a true inductor is typically much smaller than the analog inductor. There are some limitations on analog inductors:


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


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


The analog inductor does not store energy like a real inductor, the true inductance due to The action of the magnetic field can cause very high inversion spike voltages, but the voltage of the analog inductor is limited by the swing of the op amp output voltage, so the response pulse is limited by the voltage swing.


2.6 Instrumentation Amplifier


The instrumentation amplifier is used for small-level signal DC signals. In the case of amplification, he is derived from the subtractor topology. The instrumentation amplifier takes advantage of the high impedance of the non-inverting input. The basic instrumentation amplifier is shown in Figure 10.


This circuit is the basic instrumentation amplifier circuit. The other instrumentation amplifiers are also shown in the figure. The input terminals here also use a single power supply. This circuit is actually a single-supply strain gauge. The disadvantage of this circuit is that it requires exactly equal resistance, otherwise the common-mode rejection ratio of this circuit will be 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 a disadvantage: that is, the two resistors in the circuit must Replace them together, and they must be equivalent. There is also a disadvantage that the first stage op amp does not produce any useful gain.


Use two more The op amp can also form an instrumentation amplifier, as shown in Figure 12.


so he may be unstable, and the signal on Vin - takes more time than the signal on Vin + to reach the output.


This section An active filter consisting of an op amp is introduced in great depth. In many cases, in order to block the DC level caused by the virtual ground, a capacitor is inserted in 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 capacity 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 amplification, the capacity of this capacitor is preferably more than 1000 times the capacity of other capacitors in the circuit. If the input signal already contains the DC offset of VCC/2, this capacitor can be omitted.


The output of these circuits contains the DC bias of VCC/2. If the circuit is the last stage, then the output capacitor must be serialized.


There is a protocol for filter design, where the filters are each composed of a single-supply op amp. The implementation of the filter is very simple, but the following designers must pay attention:


1. The inflection point (center) frequency of the filter

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2. Gain of filter circuit


3. Q value of bandpass filter and bandstop filter


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


Unfortunately, to get a perfectly ideal filter is not possible with an op amp . Even if possible, the designer must use very complex calculations to complete the filter design due to the negative mutual inductance between the various components. Often the more complex control requirements for waveforms mean that more op amps are needed, which is determined by the maximum distortion that the designer can accept. Or it can be finally determined through several experiments. If the designer wants to implement the filter with the fewest components, then there is no choice but to use a conventional filter, which can be obtained by calculation.


3.1 First-order filter


The first-order filter is the simplest circuit. They have a 20dB frequency-frequency characteristic per octave


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.

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3.1.3 Venturi filter


The Wenn filter has the same gain for all frequencies, But it can change the phase angle of the signal and also be used to make the phase angle correction circuit. The circuit in Figure 15 has a phase shift of 90 degrees for the signal with frequency F and 0 degree for the phase shift of DC. The phase shift is 180 degrees.


3.2 Second-order filters


Second-order filter circuits are generally named after their inventors. A few of them are still in use today. There are some second-order filter topologies that can form low-pass, high-pass, band-pass, and band-stop filters, while others don't. Not all filter topologies are listed here, just Those that are easy to implement and easy to adjust are listed.


The second-order filter has a 40dB frequency-frequency characteristic per octave.


The usual bandpass and bandstop filters of the same topology use the same components to adjust their Q values, and they make the filters in Butterworth and Changes between Chebyshev filters. It must be known that only the Butterworth filter can accurately calculate the knee frequency. The Chebyshev and Bessell filters can only be fine-tuned based on the Butterworth filter.


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


Not all filters can produce the results we envision - for example, the filter will have a final attenuation amplitude in the multi-feedback filter Larger than in the Sallen-Key filter. Since these features are beyond the scope of the circuit atlas, please go to the textbook to find the advantages and disadvantages of each circuit. However, the circuit described here is used in a very special case and the results are acceptable.

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