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Jul 27 2020

# LM324 Op Amp: Detailed Analysis of 10 Simple Circuits

## I Introduction

The LM324 is widely used in various circuits, that is because, the LM324 quad op-amp has a wide range of power supply voltage, small static power consumption, can be used with a single power supply, etc.

In this blog, We will analyze 10 very practical and easy to understand LM324 circuits for you！ Figure 1. LM324

Catalog

 I Introduction II LM324 Inverting AC Amplifier Circuit III LM324 Non-inverting AC Amplifier Circuit IV LM324 AC Signal Three Distribution Amplifier Circuit V LM324 Active Bandpass Filter Circuit VI LM324 Temperature Measurement Circuit VII LM324 Comparator Circuit VIII LM324 Monostable Trigger Circuit IX LM324 Step Wave Generator Circuit X LM324 High Sensitivity Sniffer Circuit XI LM324 Responder Circuit

## II LM324 Inverting AC Amplifier Circuit

The circuit is shown in Figure 2 below. This amplifier can replace the transistor for AC amplification and can be used for preamplification of the amplifier. The circuit does not need to be debugged. The amplifier is powered by a single power supply, which is composed of R1 and R2 to form a 1/2V+ bias, and C1 is a vibration suppression capacitor. Figure 2. Inverting AC Amplifier Circuit

The amplifier voltage amplification factor Av is only determined by the external resistors Ri and Rf:

Av=-Rf/Ri

The negative sign indicates that the output signal and the input signal have opposite phases. According to the value Av=-10 given in the figure, the input resistance of this circuit is Ri. Generally, Ri is first equal to the internal resistance of the signal source, and then Rf is selected according to the required magnification. Co and Ci are coupling capacitances.

## III LM324 Non-inverting AC Amplifier Circuit

See Figure 3 below. The Non-inverting AC amplifier is characterized by high input impedance. R1 and R2 form a 1/2V+ voltage divider circuit, which biases the op amp through R3.

The voltage amplification factor Av of the circuit is also only determined by the external resistance:

Av=1+Rf/R4

The circuit input resistance is R3, and the resistance of R4 ranges from several thousand ohms to tens of thousands of ohms. Figure 3. Non-inverting AC Amplifier Circuit

## IV LM324 AC Signal Three Distribution Amplifier Circuit

This op ap LM324 circuit can divide the input AC signal into three outputs, and the three signals can be used for indication, control, analysis and other purposes, and has little effect on the signal source.

Due to the high input resistance of the op amp Ai, the op amps A1-A4 all directly connect the output terminal to the negative input terminal, and the signal is input to the positive input terminal, which is equivalent to the case of Rf=0 in the same-phase amplification state.

Therefore, the voltage amplification factor of each amplifier is 1, which is the same as the emitter follower composed of discrete components. Figure 4. AC Signal Three Distribution Amplifier Circuit

R1 and R2 form a 1/2V+ bias. When static, the voltage at the output of A1 is 1/2V+, so the output of the op amp A2-A4 is also 1/2V+. The AC signal is taken out by the DC blocking function of the input and output capacitors to form three  distribution outputs.

## V LM324 Active Bandpass Filter Circuit

The spectrum analyzers of many audio devices use this circuit as a band-pass filter to select signals of different frequency bands, and use the number of light-emitting diodes on the display to indicate the magnitude of the signal amplitude. The center frequency of this active band-pass filter is , and the voltage gain Ao=B3/2B1 at the center frequency fo, .3dB bandwidth B=1/(п*R3*C) can also be based on the Q, fo, Ao values determined by the design, to find the component parameter values of the band-pass filter

• R1=Q/(2пfoAoC)
• R2=Q/((2Q2-Ao)*2пfoC)
• R3=2Q/(2пfoC)

In the above formula, when fo=1KHz, C takes 0.01Uf, this circuit can also be used for general frequency selection amplification. Figure 5. Active Bandpass Filter Circuit

This op ap LM324 circuit can also use a single power supply, just bias the positive input of the op amp to 1/2V+ and connect the lower end of the resistor R2 to the positive input of the op amp.

## VI LM324 Temperature Measurement Circuit

See Figure 6, the temperature probe uses a silicon triode 3DG6, connect it into a diode form. The temperature coefficient of the emitter junction voltage of a silicon transistor is about -2.5mV/°C, that is, every time the temperature rises by 1 degree, the emitter junction voltage will drop by 2.5mV.

The op amp A1(op ap LM324) is connected in the form of in-phase DC amplification. The higher the temperature, the smaller the voltage drop of the transistor BG1, the lower the voltage at the non-inverting input of the op amp A1, and the lower the voltage at the output. Figure 6. Temperature Measurement Circuit

This is a linear amplification process. We only need to connect a measuring or processing circuit to the output of A1 to indicate the temperature or perform other automatic control.

## VII LM324 Comparator Circuit

When the feedback resistance of the op amp is removed, or when the feedback resistance tends to infinity (that is, the open-loop state), in theory, the open-loop magnification of the op amp is also infinite (in fact, it is very large. For example, the open-loop amplification of the LM324 op amp is 100dB, or 100,000 times). At this time, the op ap LM324 will form a voltage comparator whose output is either high level (V+) or low level (V- or ground). When the voltage at the positive input is higher than the voltage at the negative input, the op ap LM324 outputs a low level. Figure 7. LM324 Comparator Circuit

In Figure 7, two op amps are used to form a voltage comparator. Among them, resistors R1, R1ˊ constitute a voltage divider circuit, setting the comparison level U1 for the op amp A1; resistors R2, R2ˊ constitute a voltage divider circuit, setting the compare level U2 for the op amp A2. The input voltage U1 is simultaneously applied between the positive input terminal of A1 and the negative input terminal of A2. When Ui> U1, the op amp A1 outputs a high level; when Ui <SPAN>, the op amp A2 outputs a high level.

As long as there is an output high level of the operational amplifiers A1 and A2, the transistor BG1 will be turned on and the light-emitting diode LED will be lit.

If you select U1>U2, the LED lights up when the input voltage Ui exceeds the range of [U2, U1], which is a voltage double limit indicator.

If you select U2> U1, the LED lights up when the input voltage is within the range of [U2, U1], which is a "window" voltage indicator.

This op ap LM324 circuit is used in conjunction with various sensors. With a little modification, it can be used for double limit detection of various physical quantities, short circuit, open circuit alarm, etc.

## VIII LM324 Monostable Trigger Circuit

As shown in Figure 8, this circuit can be used in some automatic control systems. Resistors R1 and R2 form a voltage divider circuit to provide a bias voltage U1 for the negative input of the op amp A1(op ap LM324) as a comparison voltage reference. When static, the capacitor C1 is fully charged, and the op amp A1 positive input voltage U2 is equal to the power supply voltage V+, so A1 outputs a high level.

When the input voltage Ui becomes low, the diode D1 conducts, and the capacitor C1 quickly discharges through D1, causing U2 to suddenly drop to 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 turned off, and the power supply voltage R3 charges the capacitor C1. When the charging voltage on C1 is greater than U1, both U2>U1 and A1 output become high level, 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.  Figure 8. Monostable Trigger Circuit

If the diode D1 is removed, this circuit has a power-on delay function. When power is turned on, U1>U2, the operational amplifier A1 outputs a low level. As the capacitor C1 continues to charge, U2 continues to rise. When U2>U1, the A1 output changes to a high level.

## IX LM324 Step Wave Generator Circuit Figure 9. Step Wave Generator Circuit

Figure 9 is a op ap LM324 practical circuit of a ladder wave generator composed of a current-type op amp. The op amp A1(op ap LM324) and peripheral components form a rectangular wave generating circuit and output a pulse train.

Op amp A2 and its peripheral components are integrating-holding circuits. The integrating capacitor integrates the input pulses and maintains the steps of the input pulses. What is obtained at the output is the accumulation of each step, that is, the step wave. Op amp A3 is a voltage comparator. When the step wave voltage rises to about 80% of the power supply voltage, A3 reverses.

The op amp A4 and its peripheral components are monostable circuits. The inversion of A3 causes it to output a pulse (about 100 UFS), which is used as a reset pulse to reset A2, thereby completing a ladder cycle.

## X LM324 High Sensitivity Sniffer Circuit

With this device, you can hear very weak sounds in the distance, its strong directivity and high sensitivity. For example, you can use it to hear the whispers of athletes and coaches on the sports ground. Figure 10. High Sensitivity Sniffer Circuit

The working principle of the LM324 high sensitivity sniffer circuit:

The circuit is shown in Figure 10. The microphone installed in the special tube receives the sound in a certain direction (the sound in other directions is suppressed) and sends it to the amplifier for amplification. The amplifier consists of two stages. The first stage consists of one of the LM324's four op amps and has a gain of 110 times. The second stage consists of another op amp and has a gain of 500 times.

Such a high amplification capacity is enough to amplify a very weak sound signal, which is output by the headphones. It can be used to hear the faint sound that the human ear cannot hear directly from far away.

Notes:

• Four operational amplifiers are integrated in LM324, only A and D are used here, the wiring method can refer to the above figure;
• R1=R2, the value range is between 10K---100K;
• Power supply +6V---9V, two (or three) battery clips can be used in series;
• The sensitivity of this unit is extremely high. Do not speak near the MIC during the test.

## XI LM324 Responder Circuit

We can use the op amp LM324 to design and manufacture a successful answering device circuit according to the principle of "simple circuit, low cost, and easy access to components", as shown in the following figure. Figure 11. Responder Circuit

The circuit principle in the figure of LM324 responder circuit above:

By turning on the power and adjust RP , the inverting input of each op amp will have a certain voltage. Because the in-phase terminals of each op amp are grounded through the R junction of R1～R4 and R5 and BG, each op amp outputs a low level; When AN1 is pressed, R6 and R1 divide voltage (because the voltage of C cannot be abruptly changed, BG has not been turned on), so that the non-inverting input terminal of the operational amplifier IC-1 generates a certain voltage. This voltage is higher than the voltage at the inverting input terminal, and the op amp IC-1 outputs a high level, which is fed back to the non-inverting input terminal via LED1 and locks itself. At the same time, the current is connected to the ground via the R1, R5, BC be. On the one hand keep LED1 on; on the other hand provide base current for BG.

After the delay function of C is over, BG is saturated and turned on. Even if the other buttons are pressed again, the non-inverting input terminal of the corresponding op amp will not output a high level because there is no higher voltage, thus ensuring that the person who presses the button first answers successfully. After resetting by AN, the second round of rush answering can be carried out.

Before debugging this circuit, use a larger-capacity capacitor C to adjust RP so that the voltage at the inverting input of each op ap LM324 is about 4V, and then reduce the capacity of C as much as possible when each channel can be reliably triggered.

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