Application of LM324 quad operational amplifier

　　This circuit can divide the input AC signal into three outputs. The three signals can be used for indication, control, analysis and other purposes respectively. The impact on the signal source is minimal. Because the input resistance of the operational amplifier Ai is high, the output terminal of the operational amplifier A1-A4 is directly connected to the negative input terminal, and the signal is input to the positive input terminal, which is equivalent to the situation of Rf=0 in the non-phase amplification state, so the
voltage amplification factor of each amplifier is equal to is 1, which has the same effect as an emitter follower composed of discrete components.

R1 and R2 form a 1/2V+ bias. When static, the voltage at the output terminal of A1 is 1/2V+, so the output terminals of the op amp A2-A4 are also 1/2V+. Through the DC blocking effect of the input and output capacitors, the AC signal is taken out.

      Active bandpass filter

Many spectrum analyzers of audio equipment use this circuit as a band-pass filter to select signals in different frequency bands, and use the number indicate the amplitude of the signal. The center frequency of this active bandpass filter, the voltage gain Ao=B3/2B1 at the center frequency fo, the quality factor, 3dB bandwidth B=1/(п*R3*C) can also be determined according to the design Q , fo and Ao values ​​to find the parameter values ​​of each component of the bandpass filter.
R1=Q/(2пfoAoC), R2=Q/((2Q2-Ao)*2пfoC), R3=2Q/(2пfoC).

In the above formula, when fo=1KHz, C is 0.01Uf. This circuit can also be used for general frequency selective amplification.

This circuit can also use a single power supply, just bias the positive input terminal of the op amp at 1/2V+ and connect the lower end of resistor R2 to the positive input terminal of the op amp.

     Comparators

When the feedback resistance of the op amp is removed, or when the feedback resistance tends to infinity (i.e. open-loop state), theoretically, the open-loop amplification factor of the op amp is also infinite (actually very large, such as the LM324 op amp open loop The amplification is 100dB, which is 100,000 times). At this time, the op amp forms a voltage comparator, and its output is either high level (V+) or low level (V- or ground). When the positive input terminal voltage is higher than the negative input terminal voltage, the op amp outputs a low level.

In the figure, two operational amplifiers are used to form an upper and lower voltage limit comparator. The resistors R1 and R1ˊ form a voltage dividing circuit to set the comparison level U1 for the operational amplifier A1. The resistors R2 and R2ˊ form voltage dividing circuit for the operational amplifier A2. Set comparison level U2. Input voltage U1 is added between the positive input terminal of A1 and the negative input terminal of A2 at the same time. When Ui > U1, the operational amplifier A1 outputs high level; when Uiwill light up.

If U1>U2 is selected, when the input voltage Ui exceeds the [U2, U1] interval, the LED lights up, which is a voltage double-limit indicator.

If U2 > U1 is selected, when the input voltage is within the range [U2, U1], the LED lights up. This is a "window" voltage indicator.

This circuit is used in conjunction with various sensors. With slight modifications, it can be used for double-limit detection, short circuit, open circuit alarm, etc. of various physical quantities.

      monostable trigger

See attached figure 5. This circuit can be used in some automatic control systems. Resistors R1 and R2 form a voltage divider circuit to provide bias voltage U1 for the negative input terminal of op amp A1 as a comparison voltage reference. In static state, capacitor C1 is fully charged, and the voltage U2 at the positive input terminal of op amp A1 is equal to the power supply voltage V+, so A1 outputs a high level. When the input voltage Ui becomes low level, the diode D1 is turned on, and the capacitor C1 is rapidly discharged through D1, causing U2 to suddenly drop to the ground level. At this time, because U1>U2, the op amp A1 outputs a low level. When the input voltage becomes high, the diode D1 is cut off, and the power supply voltage R3 charges the capacitor C1. When the charging voltage on C1 is greater than U1, U2>U1, the A1 output becomes high level again, thus ending a monostable trigger. Obviously, increasing U1 or increasing the values ​​of R2 and C1 will increase the monostable delay time, and vice versa will shorten it.

                                  Figure 5 Figure 6

If diode D1 is removed, this circuit has a power-on delay function. When it is first powered on, U1>U2, the output of op amp A1 is low level. As capacitor C1 continues to charge, U2 continues to increase . When U2>U1, the output of A1 becomes high level. Refer to Figure 6.