1.LM74 1 What chip is it?

LM74 1 operational amplifier is a DC-coupled high-gain electronic voltage amplifier, which is one of the most commonly used operational amplifier integrated circuits, and can perform digital operation and amplification functions at the same time.

The main function of LM74 1 operational amplifier is to perform mathematical operations in various circuits. Operational amplifiers have large gain and are usually used as voltage amplifiers. LM74 1 can work under single power supply or dual power supply voltage.

Output voltage = gain * input voltage.

The LM74 1 circuit adopts internal compensation, which is relatively simple and difficult to self-excite. Its operation is stable and simple, and a perfect protection circuit is designed, which is not easy to be damaged.

Because of its high quality and reliable performance, the operational amplifier LM74 1 is very suitable for comparators, multivibrators, DC amplifiers, summing amplifiers, integrators or differentiators and active filters.

II. LM74 1 Pin and Function

1, LM74 1 chip pin.

2.LM74 1 Pin and Function.

The LM74 1 consists of 8 pins. Two pins are used for power supply, such as Vcc- and VCC+. The number 74 1 in the name indicates that there are 7 valid pins, 4 pins (pins 2, 3, 4 and 7) can accept input, and 1 pin (pin 6) is the output pin. The triangle in IC represents the integrated circuit of operational amplifier. The function of each pin is as follows:

1, power supply pins (pin 4 and pin 7):

Pin 4 and pin 7 are negative voltage and positive voltage power supply terminals respectively. The power supply for IC operation comes from these two pins, and the voltage level between these two pins can be in the range of 5V to 18V.

2. Input pins (pin 2 and pin 3):

Pin 2 and pin 3 are input pins of the operational amplifier IC. Pin 2 is an inverting input and pin 3 is a noninverting input. When the voltage of two pins is greater than the voltage of three pins, that is, the terminal voltage of the inverting input is high, the output signal is low. Similarly, when the voltage at pin 3 is greater than the voltage at pin 2, that is, the voltage at the noninverting input is high, the output signal is at a high level.

3. Output pin (pin 6):

Pin 6 is the output pin of the operational amplifier IC74 1. The output voltage of this pin depends on the level of the input pin and the feedback method used. When the voltage at this pin is high, it means that the output voltage is close to the positive supply voltage. Similarly, when the pin voltage is low, it means that the output voltage is close to the negative output voltage.

4. Offset empty pins (pin 1 and pin 5):

Pin 1 and pin 5 are used for the offset voltage in the operational amplifier IC74 1. Because the voltage gain of operational amplifier IC74 1 is high, even the smallest change of inverting and non-inverting input voltages, or other external interference caused by construction process or abnormality, will affect the output voltage. To overcome this effect, a voltage offset value can be applied on pin 1 and pin 5, which is usually achieved by using a potentiometer.

5. Unconnected pin (pin 8):

Pin 8 is not connected to any circuit inside the operational amplifier IC74 1. It is just a pin to fill the gap in the 8-pin standard package.

Thirdly, the internal circuit of LM74 1 is explained.

The standard operational amplifier IC74 1 consists of 20 transistors and 1 1 resistors. All these transistors and resistors are integrated on a single chip.

Here, the inverting and non-inverting terminals are connected to transistors Q 1 and Q2, respectively. Transistors Q 1 and Q2 both serve as NPN emitters. Outputs of transistors Q 1 and Q2 are connected to a pair of transistors Q3 and Q4. This type of configuration isolates the two inputs of transistors Q3 and Q4 and prevents possible feedback.

The voltage fluctuation at the input of operational amplifier will affect the current in the internal circuit and the effective range of transistors in the circuit. To prevent this, two current mirrors are used. Transistor pairs (Q8, Q9) and (Q 12, Q 13) are connected in a certain way to form two mirror circuits.

Transistors Q8 and Q 12 are used as regulating transistors, and the voltage level of emitter-base (EB) junction is set for the corresponding transistor pair. The voltage level can be accurately adjusted to several millivolts to allow the required amount of current.

The first mirror circuit developed by Q8 and Q9 is coupled to the input circuit, and the second mirror circuit developed by Q 12 and Q 13 is coupled to the output circuit. In addition, the third mirror circuit developed by Q 10 and Q 1 1 is used as a high impedance connection between the input and the negative power supply. The reference voltage it provides has no load influence on the input circuit.

Transistor Q 16 and resistors 4.5KΩ and 7.5KΩ form a voltage level shifter circuit, which reduces the voltage level of the input amplifier circuit by Vin and then transmits it to the next circuit. This is done to prevent signal distortion in the output amplifier section.

Transistors Q 15, Q 19 and Q22 are designed as class A amplifiers, and transistors Q 14, Q 17 and Q20 constitute the output stage of operational amplifier IC74 1.

In order to balance any irregularities in the input phase of the poor channel, transistors Q5, Q6 and Q7 will form a configuration with offsets of null+ve and -ve, and balance the inverting and noninverting inputs accordingly.

Fourth, the working principle of LM74 1.

The working principle of 1 and LM74 1.

When using LM74 1, it is necessary to provide a pair of positive and negative power supply voltages +VDC and -Vdc with the same size on pin 7 and pin 4. Once there is a voltage difference between pin 2 and pin 3, that is, between two input terminals, the voltage difference will be amplified at the output terminal.

The operational amplifier has the characteristic that the output voltage value will never be greater than the positive power supply voltage +VDC or less than the negative power supply voltage -Vdc. If the input voltage difference is greater than the range of external power supply voltage +VDC to -Vdc, its value will be equal to +VDC or -Vdc.

Therefore, the output voltage of a general operational amplifier has a characteristic curve as shown in the following figure, and it will be saturated when the output voltage reaches +VDC and -Vdc.

The basic operation of LM74 1, if the voltage is input at the noninverting input terminal, the amplified output with the same polarity will be obtained at the output terminal; If the same voltage signal is input at the inverting input, the signal output with the same amplification but opposite polarity will be obtained at the output.

When the voltage is input to two input terminals of the amplifier at the same time, the voltage value (V2) of the inverting input terminal is subtracted from the voltage value (V 1) of the noninverting input terminal, and the ratio of the output multiplied by the output terminal (V 1-V2) can be obtained.

2. Power supply.

The power supply itself has two sets of external jacks to provide two sets of power output. When positive and negative output voltages are required, you can use the voltage adjustment button on the power supply. For example, in order to generate a voltage of 15 VDC, it is necessary to first connect the anode of one of the two power outputs to the cathode of the other power output.

The other two unconnected output terminals are power output terminals, then turn on the power and press the tracking button on the dashboard, and then use the adjustment knob on the panel to adjust the required voltage 15 VDC.

In the process of adjustment, it can be found that although only one set of power output adjustment knobs is turned, the two sets of voltage output values change at the same time, and the displayed numbers are the same, but one end is positive and the other end is negative. One end is the negative electrode, which is also the output of 15Vdc. The principle is similar to two batteries connected in series.

However, if the amplifier circuit and sensitive components are to be integrated into the test equipment, the power supply cannot be used to supply power to the operational amplifier, and a self-made 15 VDC power supply circuit is needed.

The manufacturing method is to use a bridge rectifier and a voltage stabilizing IC with appropriate specifications to form a rectifier circuit, and convert the commonly used 1 10V power supply into 15Vdc power supply. The circuit diagram is as follows. 1 10V power supply is rectified by bridge, and the voltage value is adjusted to 15VDC with three-terminal voltage regulators IC78 15 and 79 15, in which 78 15 is regulated to+15VDC and 79/5v DC.

V. LM74 1 parameters.

Electrical characteristics of 1 and LM74 1 operational amplifiers.

One of the main features of LM74 1 operational amplifier is overload protection. Most importantly, it supports the overload protection of inverting and noninverting pins. Different from other operational amplifiers, it has the following characteristics:

No latch circuit requirements.

No oscillation.

PDIP, CDIP and TO99 packages are available.

2.LM74 1 operational amplifier parameters.

The input impedance is greater than 100 kω.

The output impedance is less than 100ω.

The frequency range is between 0HZ and 1MZ.

Low offset voltage and current.

The voltage gain is about 2 million.

Power supply: In order to work normally, it needs at least 5V power supply, and can handle the voltage of 18V at the highest.

Input impedance: about 2mΩ.

Output impedance: about 75 Ω.

Voltage gain: the minimum frequency range is 2000000.

Slew rate (the rate at which an operational amplifier can detect voltage changes): 0.5V/? s .

Input offset: 2mV-6mV.

Output load: it is recommended to be greater than 2kΩ.

Maximum output current: 20mA.

Note: In order to use the operational amplifier as a voltage amplifier, it is recommended to use high input impedance and low output impedance. This impedance makes the operational amplifier IC74 1 almost an ideal voltage amplifier. The above specifications are general and may vary from manufacturer to manufacturer.

6. What can replace LM74 1?

Substitution/Equivalent/Other Part Number:

MC 1439、LM748、LM709C、LM20 1 .

TLC27 1、CA3 140E、TL08 1CN、TLO6 1CP、TL07 1CP、LF35 1N .

LM74 1A, LM74 1C, LM709C, LM20 1, MC 1439 and LM748.

Seven. LM74 1 Circuit Description

1, noninverting operational amplifier

In the noninverting operational amplifier IC74 1, pin 3 and pin 6 are used as input and output pins. The input voltage is provided through pin 3, and the output comes from pin 6, maintaining the same polarity as the input voltage. When the input voltage is positive, the output is positive; when the input voltage is negative, the output is negative. Therefore, the amplifier is named non-inverting amplifier.

The gain of the noninverting amplifier is given by:

Gain (av) = 1+(R2/r 1).

Where R2 is the feedback resistance.

By adjusting the values of R 1 and R2, the required magnification can be obtained. When the feedback resistor R2 is zero, the gain is 1, and the operational amplifier acts as a voltage follower or a unity gain buffer.

2. Inverting operational amplifier.

In the inverting operational amplifier IC74 1, pin 2 and pin 6 are used as input and output pins. The input voltage is provided through pin 2, and the output is from pin 6, resulting in polarity inversion. When the input voltage is positive, the output is negative, and when the input voltage is negative, the output is positive, so the amplifier is called an inverting amplifier.

The gain of the inverting amplifier is given by:

Gain (av) =-(R2/r 1).

Where R2 is the feedback resistance.

Here, the negative sign indicates the polarity inversion of the output voltage. By adjusting the values of R 1 and R2, the required magnification can be obtained.

3. Example of unity gain amplifier

One of the uses of operational amplifier is unity gain amplifier or buffer amplifier. Unity gain amplifier can be used as follower and inverter.

The follower provides a gain of 1, and the output is the same as the input. On the other hand, in addition to providing unity gain, the inverter will also reverse the polarity of the input. The output resistance of the operational amplifier can be ignored. Therefore, the circuit provides as much current as possible according to the requirements of the load.

In this circuit, we give an input voltage of 6V. After that, we connect a feedback resistor. The output voltage we get is exactly 6V. Because the gain of the amplifier is uniform. Therefore, the output on the oscilloscope is 6V. According to this equation:

Vout=Vinx gain Vout = 6x 1 = 6V//, because the gain = 1 and vin = 6V.

4. An example of LM741square wave generator.

The square wave generator converts the AC sine wave into a square wave. But we can also call it a zero-crossing detection circuit. In short, its main function is to generate a square wave from a sine wave.

In this example, LM74 1 is used as a comparator to compare the voltage amplitude of the zero reference and the sine wave. Whenever a sine wave passes through zero level, we will get a square wave at the output. The comparator produces outputs of+15 and-15 volts. But we use an edge detection circuit to convert the output of the operational amplifier into a square wave.

5. Optical sensor circuit.

In this optical sensor circuit, LM74 1IC is used as a multivibrator. 120K variable resistor is used to adjust the sensitivity or activation point of LED.

The circuit uses LDR sensor to sense light. When the light level on the LDR reaches the preset level set by the variable resistor 120K, the LED will be activated. The working voltage is 9VDC.

6. Dark sensor circuit.

Like the above circuit, LM74 1 is also used in the dark inductance circuit, which is in unstable multivibrator mode, but this time, in order to detect darkness, the middle pin of the variable resistor is connected to the pin three/noninverting input of the IC.

Now, when the level or darkness of the LDR surface reaches the preset level 120K set by the variable resistor in the circuit, the LED will be activated. The power supply of this circuit is also 9V.

7. Other applications.

The practical applications of LM74 1 include: advertising conversion, audio amplifier, programmable logic controller, video signal conditioning unit, processor, sensor data charge, digital-to-analog converter in telephone, temperature sensor and controller, error amplifier, communication circuit, mobile phone charger, receiver, modulator and synthesizer.