Basic analysis method for op amps: virtual open circuit, virtual short circuit. For unfamiliar op amp application circuits, use this basic analysis method.

Op amps are widely used devices. When connected to appropriate feedback networks, they can be used as precision AC and DC amplifiers, active filters, oscillators, and voltage comparators.

- Application of op amps in active filtering

The figure above is a typical active filter circuit (Saron-Kayl circuit, a type of Butterworth circuit). The advantage of active filtering is that it can make signals greater than the cutoff frequency decay more quickly, and the filtering characteristics do not require high capacitance and resistance.

The design points of this circuit are: under the condition of meeting the appropriate cutoff frequency, the resistance values of R233 and R230 should be selected as consistent as possible, and the capacitance of C50 and C201 should be selected as consistent (when the resistance and capacitance values of the two-stage RC circuit are equal, it is called a Saron-Kayl circuit), so that the types of devices can be normalized while meeting the filtering performance. Among them, the resistor R280 prevents the input from being suspended, which will cause abnormal output of the op amp.

The three most commonly used second-order active low-pass filter circuits for filtering are: Butterworth, monotonically decreasing, flat and smoothest curve;

The most used in Butterworth low-pass filtering is the Saron-Kayl circuit, which is the simulated circuit.

For a filter, you need to know its cutoff frequency, or you can write the transfer function and frequency response.

If the filter also has an amplification function, you need to know the gain of the filter.

When the resistance and capacitance values of the two-stage RC circuit are equal, it is called a Serenka circuit. A negative feedback is introduced into the second-order active circuit to make the output voltage drop rapidly in the high frequency range.

The passband gain of the second-order active low-pass filter circuit is 1+Rf/R1, which is the same as the first-order low-pass filter circuit;

Note that the unit of m is ohm and the unit of N is u

So the cutoff frequency is calculated to be

Chebyshev, rapidly decaying, but with ripples in the passband;

Bessel (elliptical), phase shift is proportional to frequency, and group delay is essentially constant.

2. Application of op amp in voltage comparator

This circuit is actually a combination of a zero-crossing comparator and a deep amplifier circuit.

The output is amplified by (1+R292/R273). The higher the amplification factor, the steeper the rising edge of the square wave.

There is also a key component resistance value in this circuit that needs to be paid attention to, that is R275, which determines the rising speed of the square wave.

3. Design of constant current source circuit

As shown in the figure, the constant current principle analysis process is as follows:

U5B (the lower op amp in the above figure) is a voltage follower, so V1=V4;

According to the virtual short principle of the operational amplifier, for the op amp U4A (the upper op amp in the above figure): V3=V5;

Combining the above equations, we get:

When the reference voltage Vref is fixed at 1.8V, the resistor R30 is 3.6, and the current output is constant at 0.5mA.

This constant current source circuit can be used to design constant current sources of other currents. The basic idea is: all resistors need to use high-precision resistors with consistent resistance values. The input reference voltage (using a special reference voltage chip) is divided by the resistance value to obtain the output current.

However, in actual use, in order to protect the constant current source circuit, a diode and a resistor are generally connected in series at the output end. The first benefit of this is to prevent external interference from entering the constant current source circuit, causing damage to the constant current source circuit, and secondly, to prevent the external load from being short-circuited, so as not to damage the constant current source circuit.

5. Thermal resistance measurement circuit

The circuit in the figure above is a typical thermal resistor/couple measurement circuit. The measurement idea is: a 1-10mA constant current source is added to the load, which will generate a certain voltage on the load, and the voltage is actively filtered. After processing, the signal is adjusted (signal amplification or attenuation), and finally the signal is sent to the ADC interface.

When using this circuit, pay attention to applying protection at the input end. TVS can be connected in parallel, but pay attention to the impact of capacitors on measurement accuracy. Of course, if in some low-cost occasions, the above circuit diagram can be simplified to the following circuit

In the use of operational amplifiers, voltage follower is a common application. The benefits of this circuit are: first, it reduces the impact of the load on the signal source; second, it improves the signal’s ability to carry load.

**7.Application of single power supply**In the actual use of op amps, we generally use dual power supplies to maintain the frequency characteristics of the op amps. However, sometimes in actual use, we only have a single power supply and can also achieve normal operation of the op amp.

First, we use the op amp follower circuit to achieve a VCC/2 voltage divider:

Of course, if the requirements are not very high, we can directly divide the voltage with resistors to obtain +VCC/2, but due to the characteristics of resistor voltage division, its dynamic response speed will be very slow, so please use it with caution.

After obtaining +VCC/2, we can use a single power supply to achieve signal amplification function, as shown below:

In this circuit, R66=R67//R68, and the output gain of the signal is G=-R67/R68.

The specific application is shown in the figure below: the op amp is powered by a single +5V_AD, and the voltage of the AD chip is 3.3V (obtained by the reference voltage chip REF3033). The 3.3V is divided by resistors and followed by the op amp to obtain 1.65V, which is given to the op amp’s in-phase input terminal.