A multivibrator uses deep positive feedback, to turn on and off two electronic devices alternately through the resistance-capacitance coupling, thereby self-excites and generates a square wave output. It's often used as a square wave generator...

I Introduction

A multivibrator uses deep positive feedback, to turn on and off two electronic devices alternately through the resistance-capacitance coupling, thereby self-excites and generates a square wave output. It's often used as a square wave generator.

A multivibrator is a kind of self-excited oscillator that can generate a rectangular wave, also called a rectangular wave generator. "Multi" means that in addition to the fundamental waves, the rectangular wave also contains rich higher harmonics. The multivibrator has no steady-state, only two transient states. During operation, the state of the circuit automatically alternates between these two transient states, thereby generating rectangular wave pulse signals, which are often used as pulse signal sources and clock signals in sequential circuits.

II Multivibrator Circuit Structure

1. Op-amp Multivibrator Circuit

In pulse technology, a pulse source is often needed to meet the needs of digital computing, information transmission, and system testing. Multivibrator is one of the 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's "multi-harmonic".

Generally speaking, nonlinear wave generators such as triangle wave, ramp wave, sawtooth wave, and square wave generators are composed of the following three parts: integrator (also called timing circuit), comparator, and logic circuit. As shown in the block diagram of Figure 1, the function of these three parts can be completed by only one or two integrated operational amplifiers.

Figure 1. Op-amp Multivibrator Circuit

The characteristics of this circuit are:

(1) It is suitable for the audio in a fixed frequency range.

(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 we use cheap components, a multivibrator with a relatively small frequency drift can be obtained.

2. Integrated Gate Multivibrator Circuit

The simplest 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 low precision, and the oscillation rate cannot be designed at will, which is only related to the delay time of the odd number of gates. The RC timing multivibrator has a simple structure, high timing accuracy, and the oscillation frequency can be freely designed.

Figure 2(a) is a multivibrator circuit with RC timing. GA and GB are CMOS inverters, R1 and C1 are timing components, and Rs is a series resistor. Figure 2(b) is the waveform diagram of each point, and the working process is illustrated by the circuit shown in Figure 2.

Figure 2. Multivibrator Circuit with RC Timing

When the power is turned on, the electric 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, the potential at point ② jumps to a low level, and point ③ rises to the VDD level.

Then C1 discharges through GA's "P" tube, R1, C1, and GA's "n" tube. In the process of discharging, the potential at point ④ decreases according to the time constant of R1 and C1. When the potential at point ④ drops to the VRA level, the gate G is closed, the potential at point ② jumps to a level close to VDD, point ⑧ jumps to close to 0V, and point ④ jumps to a level of (VRA-VDD).

Then C1 is charged through the "p" tube of G, C1, R1, and the "n" tube of GA. Complementary output waveforms are obtained at points ③ and ②.

3. 555 Timer Mutivibrator Circuit

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 warning circuits, electronic toys, home appliance control, and so on. In the college electronic technology experiment teaching, the 555 multivibrator experiment is a classic must-do content.

Other Circuit Structures

The multivibrator can also be composed of discrete components or an integrated Schmitt trigger.

III Multivibrator Types

1. Astable Multivibrator

Figure 3. Astable Multivibrator Circuit

The figure illustrates the configuration of a typical astable multivibrator circuit.

In the basic operation mode, this circuit operates in the following two states:

(1) State One

Q1 is turned on, and the collector voltage of Q1 is close to 0V. C1 is discharged by the current flowing through R2 and Q1_CE. Due to the reverse voltage provided by capacitor C1, Q2 is turned off. C2 is charged through R4 and Q1_BE. The output voltage is high (but because C2 is charged by R4, it is slightly lower than the power supply voltage).

This state continues until the discharge of C1 is completed. Since R2 provides base bias, Q2 turns on: this circuit enters state two

(2) State Two

Q2 is turned on, and the collector voltage of Q2 (that is, the output voltage) changes from a high level to close to 0V. Due to the reverse voltage provided by the capacitor C2, Q1 is instantly cut off, and Q1 is cut off, making the collector voltage of Q1 rise to a high level. And C1 is charged by R1 and Q2_BE, and C2 is discharged through R3 and Q2_CE. Because the capacitor C2 provides a reverse voltage, Q1 is turned off.

This state continues until the discharge of C2 is finished. Because R3 provides a bias voltage to the base of Q1, Q1 turns on: the circuit enters state one.

In the circuit startup process, when the circuit is just connected to the power supply, both transistors are off. However, when the base voltages of the two transistors rise together since it is impossible to control the turn-on delay of each transistor during the manufacturing process, one of the transistors must be turned on first. So this circuit enters one of the states, guaranteeing a continuous oscillation.

For the oscillating period, roughly speaking, the duration of state one (output high potential) is related to R1 and C1, and the duration of state two is related to R2 and C2. Because R1, R2, C1, and C2 can all be freely configured, the oscillating period and duty cycle can be freely determined.

However, the duration of each state is determined by the initial state of the capacitor at the beginning of charging (the voltage across the capacitor), which is related to the discharge amount in the previous state. The amount of discharge in the previous stage is determined by the current through R1 and R4 in the process and the duration of the discharge process... In a word, when the circuit is just started, it takes a long time to charge the capacitor (generally, the capacitor terminals are completely discharged when it is not started), but the duration of each subsequent stage will be shorter and stabilized.

Because the multivibrator uses the current charging process to control the period, the oscillation period is also related to the amount of current flowing out of the multivibrator at the output.

Due to the influence of various unstable factors on the oscillation period of the multivibrator, more accurate timing integrated circuits are usually used in practice to replace the simple multivibrator circuit.

2. Monostable Multivibrator

Figure 4. Monostable Multivibrator Circuit Diagram

The basic collector coupled transistor monostable multivibrator circuit and its related waveforms are shown above. When the power is on, the base of transistor TR2 is connected to Vcc through a bias resistor RT and makes the transistor fully on and into a saturated state. At the same time, TR1 is closed in the process, which represents the steady-state of the circuit with zero output. Therefore, the current flowing into the saturation base of TR2 will be: Ib = (Vcc-0.7)/RT.

If a negative trigger pulse is now applied to the input, the fast decay edge of the pulse will pass directly through the capacitor C1 to the base of the transistor, and TR1 will turn on it through the blocking diode. The voltage of the TR1 collector quickly drops below zero volts, effectively making the capacitor CT reverse charge become -0.7v. This action causes the transistor TR2 to have a negative base voltage at X, thereby completely turning off the transistor. This represents the second state of the circuit, the unstable state, and the output voltage is equal to Vcc.

The timing capacitor CT starts to discharge this -0.7v current through the timing resistor RT, trying to charge to the power supply voltage Vcc. The negative voltage at the bottom of the transistor TR2 starts to combine with RT and CT. When the base voltage of TR2 increases back to Vcc, the transistor turns on, thus turning off the transistor TR1 again. As a result, it automatically returns to its original stable state in the monostable multivibrator, waiting for the second negative triggering pulse to restart the process again.

The monostable multivibrator can generate short pulses or long rectangular waveforms. The rising edge of the waves rises with time and the externally applied trigger pulse, and its falling edge depends on the RC time constant of the feedback element. This RC time constant can be changed over time to generate a series of pulses. These pulses have a controlled fixed time delay compared to the original trigger pulse, as shown below.

Figure 5. Monostable Multivibrator Waveform

The time constant of the monostable multivibrators can be changed by changing the value of capacitor CT, resistor RT or both. The monostable multivibrator is usually used to increase the pulse width or generate a time delay in the circuit. The frequency of the output signal is always the same as the frequency of the trigger pulse input, the only difference is the pulse width.

3. Bistable Multivibrator

A bistable multivibrator, or flip-flop, is a storage element with two stable states, which can record binary digital signals "1" and "0". We can change the circuit state by applying a signal to one or more control input terminals, and there will be 1 or 2 outputs. Flip-flops are the basic logic units that constitute sequential logic circuits and various complex digital systems. Flip-flops and latches are the basic components of digital electronic systems used in computers, communications, and many other types of systems.

Figure 6. Bistable Circuit

Types of Flip-flops

Triggers can be divided into several common types: SR (set-reset), D (data or delay), T (toggle), and JK type. All of the above types of flip-flops can use characteristic equations to derive the next (ie next clock pulse) output (Qnext) with the existing input and output signals (Q).

(1) RS Flip-flop

The basic RS flip-flop, also known as the SR latch, is the simplest type of flip-flops and a basic component of various other types of flip-flops. The input and output ends of two NAND gates or NOR gates are cross-coupled or connected end to end to form a basic RS flip-flop.

The characteristic equation is Qnext=S+R'Q, and RS=0.

(2) D Flip-Flop

The D flip-flop has one input, one output, and one clock frequency input. When the clock frequency changes from 0 to 1, the output value will be equal to the input value. Such flip-flops can be used to prevent errors caused by noise and can increase the amount of data processed through the pipeline.

The truth table is as follows:

(3) JK Flip-flop

The JK flip-flop, is also called JK latch, has two inputs, and the output value is determined by the following formula.

Qnext=K'Q+JQ'

The structure of JK flip-flop is similar to RS flip-flop. The difference is that the RS flip-flop does not allow R and S to be 1 at the same time, while the JK flip-flop allows. When J and K become 1 at the same time, the output value state will be reversed. In other words, if was 0, it becomes 1; if it was 1, it becomes 0.

(4) T flip-flop

T flip-flop (Toggle Flip-Flop, or Trigger Flip-Flop) has an input and an output. When the clock frequency changes from 0 to 1, if T and Q are not the same, the output value will be 1. When the input terminal T is 1, the state Q of the output terminal is reversed; when the input terminal T is 0, the state Q of the output terminal remains unchanged. Connect the J and K input points of the JK flip-flop can form a T flip-flop.

IV Multivibrator Application

The picture below is a simple temperature-controlled alarm circuit composed of a multivibrator. In the picture, ICEO is the reverse saturation current that flows from the collector area through the base area to the emitter area when the base of the transistor T is open.

Figure 7. Multivibrator in Temperature Control Alarm Circuit

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 tub. The ICEO increases as the temperature rises, and the ICEO of the germanium tube increases faster with the increase in temperature. When we choose a transistor, it is generally hoped that ICEO is as small as possible.

However, this circuit uses a germanium tube that has a large ICEO sensitive to temperature changes and uses its ICEO to control the voltage of the 555 timer reset terminal at pin 4.

In the picture, the 555 timers and R1, R2, and C form a multivibrator, and the R'D' at its reset terminal 4 is grounded through R3.

Under normal temperature, the ICEO of the germanium tube is small, generally 10-50μA. The voltage generated on pin 3 is low, and the voltage of 555 reset terminal R'D' is low, then 555 is in the reset state, and the multivibrator stops oscillating.

When the temperature rises or there is a fire alarm, the ICEO increases, and the voltage generated on R3 rises, making the voltage of 555 reset terminal R'D become a high level. The multivibrator starts to oscillate, and the speaker sounds an alarm.

Different transistors in the temperature control alarm circuit have a large difference in ICEO value, so the resistance value of R3 needs to be changed to adjust the temperature control point. The method is:

First, place the temperature measuring element T at the temperature required to alarm.

Next, adjust R3 to make the circuit sound an 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.