# Multivibrator circuit structure and working principle

In pulse technology, a pulse source is often needed to meet the needs of digital operations, information transmission, and system testing. Multivibrator is one of the more common pulse sources. Its output waveform is similar to a square wave, so it is also called a square wave generator. Since the square wave is composed of many sine waves of different frequencies, it has been called “multi-harmonic”.

Multivibrator is a kind of self-excited oscillator that can generate rectangular wave, also called rectangular wave generator. After the power is turned ON, rectangular pulses can be automatically generated without external pulses!

Multivibrator: An oscillator that uses deep positive feedback to alternately turn on and off two Electronic devices through resistance-capacitance coupling, thereby self-excited to generate a square wave output. Often used as a square wave generator.

The composition of the multivibrator

1) Operational Amplifier composition

In pulse technology, a pulse source is often needed to meet the needs of digital operations, information transmission, and system testing. Multivibrator is one of the more common pulse sources. Its output waveform is similar to a square wave, so it is also called a square wave generator. Since the square wave is composed of many sine waves of different frequencies, it has been called “multi-harmonic”.

Generally speaking, nonlinear wave generators such as triangle wave, ramp wave, sawtooth wave and square wave are composed of the following three parts: integrator (also called timing circuit), comparator and logic circuit. As shown in the block diagram. The functions of these three parts can be completed by only one or two integrated operational amplifiers. The characteristics of this circuit are:

1. Suitable for in the audio range, for a fixed

Frequency application,

2. Change R: the frequency can be adjusted,

3. The stability of the frequency mainly depends on the stability of the Capacitor C and the Zener diode, so even if cheap components are used, a multivibrator with a relatively small frequency drift can be obtained, ②

2) Integrated gate circuit composition

The simple way to design a multivibrator with a gate circuit is to connect an odd number of gates end to end. But this kind of oscillator has a low jing degree and an oscillation rate that cannot be designed arbitrarily. It is only related to the delay time of an odd number of gates. The RC timing multivibrator has a simple structure, high timing jing, and the oscillation frequency can be freely designed. Figure 1 (a) is a circuit diagram of a multi-skilled oscillator with RC timing. GA and GB are CMOS inverters, R1 and C1 are timing components, and Rs are series resistors. Figure 1 (b) is the waveform diagram of each point, the working process can be illustrated by the circuit shown in Figure 2. When the power is turned on, the potential at point ⑧ rises, and the potential at point ④ also rises. When the potential of point ④ rises to the Vtv level of the GA gate, GA is opened, point ② jumps to low level, and point ③ rises to VDD level. Then C1 discharges through GA’s “P” tube, R1, C1, and GA’s “n” tube. In the process of discharging, the potential of point ④ drops according to the time constant of R1 and C1. When the potential of point ④ drops to the VRA level, G The gate is closed, point ② jumps to a level close to VDD, point ⑧ jumps to a level close to 0V, and point ④ jumps to a level of (VRA-VDD). Then C1 is charged through the “p” tube of G, and the “n” tube of C1, R1, and GA. Just oscillate like this. Complementary output waveforms are obtained at points ③ and ②.

Three) The composition of body tube frequency stabilization

Transistor multivibrators (hereinafter referred to as bistables) are mostly used in digital devices such as electronic computers and electronic switches, or in measuring instruments such as counters and digital voltmeters.

As a bistable circuit, the Eocles-oJrdan type circuit is very different from the current switch type circuit. Due to its simple circuit structure and low power consumption, almost all circuits except for high-speed operation It adopts Eccles-Jordan type circuit.

Four) 555 integrated circuit composition

The 555 timer is a medium-scale integrated device that combines analog and digital functions. The device has low cost and reliable performance. It only needs a few external resistors and Capacitors to achieve multiple functions. Because the circuit is simple and reliable, it is widely used in many fields such as signal generators, audio alarm circuits, electronic toys, home appliance control, and so on. In the experimental teaching of electronic technology in colleges and universities, the 555 multivibrator experiment is a classic must-do content.

5) Other composition methods

The multivibrator can also be composed of discrete components or an integrated Schmitt trigger. The circuit structure is as follows:

Multivibrator function and working principle

1. Change the two resistance coupling branches of the bistable trigger circuit to capacitive coupling branches. Then, the circuit does not have a stable state, and becomes an unstable circuit.

2. Startup: Due to the slight difference in circuit parameters, the positive feedback causes one tube to saturate and the other to cut off, resulting in a transient steady state. Suppose that BG1 is saturated and BG2 is cut off.

working principle:

1. Positive feedback: At the moment when BG1 is saturated, VC1 changes from EC to close to zero, forcing the base potential VB2 of BG2 to drop to close to -EC, so BG2 is reliably cut off.

2. A transient steady state:

C1 discharge
C2 charging

3. Flip: When VB2 rises to 0.5V as C1 discharges, BG2 starts to conduct, and BG1 is turned off through positive feedback, and BG2 is saturated.

4. The second transient steady state:

C2 discharge
C1 charging

5. The self-excited oscillation is formed by cyclically reciprocating.

6. Oscillation period: T=T1 T2=0.7 (RB2*C1 RB1*C2)=1.4RB*C

7. Oscillation frequency: F=1/T=0.7/RB*C

8. Waveform improvement: it can be the same as the monostable circuit, using a correction diode circuit.

The role of multivibrator

Multivibrators are used to output square waves, especially square waves with a certain duty cycle. According to the relevant theory of signal and system. The square wave is superimposed with many harmonics. ?? The role of a multivibrator is a device that generates a multivibrator signal, which is composed of related components.

Application of multivibrator

Multivibrator function and working principle

The above picture is a simple temperature control alarm circuit composed of a multivibrator. In the picture, IEO is the reverse saturation current flowing from the collector area through the base area to the emission area when the base of the Transistor T is open, called the penetration current. ICEO is one of the thermal stability parameters of the Transistor. At room temperature, the ICEO of the silicon tube is smaller than that of the germanium tube; the IEO of the germanium tube increases as the temperature rises, and the IEO of the germanium tube increases faster with the increase in temperature. When choosing a transistor, it is generally hoped that the IEO is as small as possible, but this circuit uses a germanium tube that has a large penetrating current and is sensitive to temperature changes, and uses its IEO to control the voltage of the 555 timer reset terminal 4 pins. In the picture, the 555 timer and R1, R2 and C form a multivibrator, and its reset terminal 4 feet RD is grounded through R3. Under normal temperature, the penetrating current IEO of the germanium tube is small, generally 10～50μA, and the Voltage generated on 3 is low, then the voltage of the 555 reset terminal 4 pin RD is low, then the 555 is in the reset state, and the multivibrator stops. Vibrate. When the temperature rises or there is a fire alarm, the ICEO increases, and the voltage generated on R3 rises, making the 555 reset terminal 4 pin RD high, the multivibrator starts to oscillate, and the speaker sounds an alarm.

Different transistors in the temperature control alarm circuit have a large difference in IEO value, so the resistance value of R3 needs to be changed to adjust the temperature control point. The method is to first place the temperature measuring element T at the temperature that requires an alarm, and adjust R3 to make the circuit just sound the alarm. The tone of the alarm depends on the oscillation frequency of the multivibrator, which is determined by the components R1, R2, and C. Changing the value of these components can change the tone, but R1 is required to be greater than 1kΩ.

Multivibrator is a kind of self-excited oscillator that can generate rectangular wave, also called rectangular wave generator. After the power is turned on, rectangular pulses can be automatically generated without external pulses!

Multivibrator: An oscillator that uses deep positive feedback to alternately turn on and off two electronic devices through resistance-capacitance coupling, thereby self-excited to generate a square wave output. Often used as a square wave generator.

The composition of the multivibrator

1) Operational amplifier composition

In pulse technology, a pulse source is often needed to meet the needs of digital operations, information transmission, and system testing. Multivibrator is one of the more common pulse sources. Its output waveform is similar to a square wave, so it is also called a square wave generator. Since the square wave is composed of many sine waves of different frequencies, it has been called “multi-harmonic”.

Generally speaking, nonlinear wave generators such as triangle wave, ramp wave, sawtooth wave and square wave are composed of the following three parts: integrator (also called timing circuit), comparator and logic circuit. As shown in the block diagram. The functions of these three parts can be completed by only one or two integrated operational amplifiers. The characteristics of this circuit are:

1. Suitable for in the audio range, for a fixed

Frequency application,

2. Change R: the frequency can be adjusted,

3. The stability of the frequency mainly depends on the stability of the capacitor C and the Zener diode, so even if cheap components are used, a multivibrator with a relatively small frequency drift can be obtained, ②

2) Integrated gate circuit composition

The simple way to design a multivibrator with a gate circuit is to connect an odd number of gates end to end. But this kind of oscillator has a low jing degree and an oscillation rate that cannot be designed arbitrarily. It is only related to the delay time of an odd number of gates. The RC timing multivibrator has a simple structure, high timing jing, and the oscillation frequency can be freely designed. Figure 1 (a) is a circuit diagram of a multi-skilled oscillator with RC timing. GA and GB are CMOS inverters, R1 and C1 are timing components, and Rs are series resistors. Figure 1 (b) is the waveform diagram of each point, the working process can be illustrated by the circuit shown in Figure 2. When the power is turned on, the potential at point ⑧ rises, and the potential at point ④ also rises. When the potential of point ④ rises to the Vtv level of the GA gate, GA is opened, point ② jumps to low level, and point ③ rises to VDD level. Then C1 discharges through GA’s “P” tube, R1, C1, and GA’s “n” tube. In the process of discharging, the potential of point ④ drops according to the time constant of R1 and C1. When the potential of point ④ drops to the VRA level, G The gate is closed, point ② jumps to a level close to VDD, point ⑧ jumps to a level close to 0V, and point ④ jumps to a level of (VRA-VDD). Then C1 is charged through the “p” tube of G, and the “n” tube of C1, R1, and GA. Just oscillate like this. Complementary output waveforms are obtained at points ③ and ②.

Three) The composition of body tube frequency stabilization

Transistor multivibrators (hereinafter referred to as bistables) are mostly used in digital devices such as electronic computers and electronic switches, or in measuring instruments such as counters and digital voltmeters.

As a bistable circuit, the Eocles-oJrdan type circuit is very different from the current switch type circuit. Due to its simple circuit structure and low power consumption, almost all circuits except for high-speed operation It adopts Eccles-Jordan type circuit.

Four) 555 integrated circuit composition

The 555 timer is a medium-scale integrated device that combines analog and digital functions. The device has low cost and reliable performance. It only needs a few external resistors and capacitors to achieve multiple functions. Because the circuit is simple and reliable, it is widely used in many fields such as signal generators, audio alarm circuits, electronic toys, home appliance control, and so on. In the experimental teaching of electronic technology in colleges and universities, the 555 multivibrator experiment is a classic must-do content.

5) Other composition methods

The multivibrator can also be composed of discrete components or an integrated Schmitt trigger. The circuit structure is as follows:

Multivibrator function and working principle

1. Change the two resistance coupling branches of the bistable trigger circuit to capacitive coupling branches. Then, the circuit does not have a stable state, and becomes an unstable circuit.

2. Startup: Due to the slight difference in circuit parameters, the positive feedback causes one tube to saturate and the other to cut off, resulting in a transient steady state. Suppose that BG1 is saturated and BG2 is cut off.

working principle:

1. Positive feedback: At the moment when BG1 is saturated, VC1 changes from EC to close to zero, forcing the base potential VB2 of BG2 to drop to close to -EC, so BG2 is reliably cut off.

2. A transient steady state:

C1 discharge
C2 charging

3. Flip: When VB2 rises to 0.5V as C1 discharges, BG2 starts to conduct, and BG1 is turned off through positive feedback, and BG2 is saturated.

4. The second transient steady state:

C2 discharge
C1 charging

5. The self-excited oscillation is formed by cyclically reciprocating.

6. Oscillation period: T=T1 T2=0.7 (RB2*C1 RB1*C2)=1.4RB*C

7. Oscillation frequency: F=1/T=0.7/RB*C

8. Waveform improvement: it can be the same as the monostable circuit, using a correction diode circuit.

The role of multivibrator

Multivibrators are used to output square waves, especially square waves with a certain duty cycle. According to the relevant theory of signal and system. The square wave is superimposed with many harmonics. ?? The role of a multivibrator is a device that generates a multivibrator signal, which is composed of related components.

Application of multivibrator

Multivibrator function and working principle

The above picture is a simple temperature control alarm circuit composed of a multivibrator. In the picture, IEO is the reverse saturation current flowing from the collector area through the base area to the emission area when the base of the transistor T is open, called the penetration current. ICEO is one of the thermal stability parameters of the transistor. At room temperature, the ICEO of the silicon tube is smaller than that of the germanium tube; the IEO of the germanium tube increases as the temperature rises, and the IEO of the germanium tube increases faster with the increase in temperature. When choosing a transistor, it is generally hoped that the IEO is as small as possible, but this circuit uses a germanium tube that has a large penetrating current and is sensitive to temperature changes, and uses its IEO to control the voltage of the 555 timer reset terminal 4 pins. In the picture, the 555 timer and R1, R2 and C form a multivibrator, and its reset terminal 4 feet RD is grounded through R3. Under normal temperature, the penetrating current IEO of the germanium tube is small, generally 10～50μA, and the voltage generated on 3 is low, then the voltage of the 555 reset terminal 4 pin RD is low, then the 555 is in the reset state, and the multivibrator stops. Vibrate. When the temperature rises or there is a fire alarm, the ICEO increases, and the voltage generated on R3 rises, making the 555 reset terminal 4 pin RD high, the multivibrator starts to oscillate, and the speaker sounds an alarm.

Different transistors in the temperature control alarm circuit have a large difference in IEO value, so the resistance value of R3 needs to be changed to adjust the temperature control point. The method is to first place the temperature measuring element T at the temperature that requires an alarm, and adjust R3 to make the circuit just sound the alarm. The tone of the alarm depends on the oscillation frequency of the multivibrator, which is determined by the components R1, R2, and C. Changing the value of these components can change the tone, but R1 is required to be greater than 1kΩ.

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