Astable multivibrators are essential components in the field of electronics, serving as foundational elements in various applications, including timing circuits, waveform generation, and oscillators. When integrated with E-MOSFETs (Enhancement-Mode Metal-Oxide-Semiconductor Field-Effect Transistors), these circuits exhibit distinct characteristics and advantages. This blog post explores the workings, applications, and benefits of astable multivibrators powered by E-MOSFETs, specifically focusing on the IRF540N N-channel enhancement MOSFET.
E-MOSFET Basics
E-MOSFETs are a type of field-effect transistor known for their ability to amplify and switch electronic signals efficiently. They consist of three terminals: gate, drain, and source. In enhancement mode, these transistors typically remain off until a positive voltage is applied to the gate. The IRF540N N-channel MOSFET is commonly used not only for constructing astable oscillators but also in MOSFET-based amplifiers.Astable Multivibrator Circuit with E-MOSFET
Components Required
- E-MOSFETs: IRF540N (usually a pair)
- Resistors
- Capacitors
- Battery (3 to 9V)
- Lamps or LEDs
- Connecting wires
Working Principle
- Initialization: At the start, one E-MOSFET conducts while the other remains off.
- State Change: Capacitors charge and discharge through resistors, causing continuous switching between the conducting and non-conducting states of the E-MOSFETs.
- Output Waveform: The circuit generates a square wave with equal high and low durations.
Circuit Diagram of astable multivibrator using MOSFET is shown below.
Here I am just sharing the PCB design and Modelling of LM741 Astable Multivibrator that I wrote about in Astable Multivibrator with Op-Amp 741. I did this to get more experience in PCB design of various kinds of circuits. Although simple but because of faster repetition of small circuits, I think I will be later on be completing larger circuits in smaller time. Here first I changed some components with PCB layout and 3D parts to make a realistic prototype. That is the product will look exactly like this. I changed the battery part, added a 2-Terminal block, rerouted the PCB traces, changed the direction of the 2 terminal block and then changed the color of the PCB board from default green color to a red color. I have uploaded the video while I was doing these things on youtube.
Of course I have no plan to fabricate this. In my blog post fabricating PCB at home I wrote about how one can create PCB at home and what chemicals you will need. One if ferride oxide, the other is ethanol and the other is nail polish remover. Besides these 3 things you can create a professional looking PCB at home. I have even made a 15mils trace PCB circuits successfully. The other thing that you need is a laser printer and photo papers.
As an electronics engineer at least he/she should have knowledge how to DIY produce a circuit at home. For example, its like doctor should know how to perform basic operation or knowledge of using equipment.
Besides this, it advantageous to learn PCB design for sending to PCB fabrication house. Also one can share the PCB design and 3D model of the product to potential customer or just for fun. In the video, I have shown at the end how to export the 3D model in proteus as a step file. I then shared this LM317 Astable Multivibrator 3D model in online community.
The following is the updated astable multivibrator using op amp circuit diagram.
In the above circuit diagram, the updated part is the battery B1 and the 2-terminal block J1 at the output of the LM741 op-amp. For previous PCB and schematic design see astable multivibrator using op amp. The updated PCB layout is shown below.
The battery that I have in mind is to use the Lithium Ion batteries with size 18650 and voltage of 3.7V for test. Yes, 3.7+3.7=7.4V only while I have 12V on the simulation but I think it will also work on 7.4V. So this circuit will produce square wave at the terminal block. Where could it be useful?
I quickly tested the circuit with 7.4V input and below is short animation showing this.
And the 3D view of the designed LM741 Astable Multivibrator is shown below.
Actually this circuit is also called square wave generator and any op-amp will do the work. The circuit is easy to build and one can use the Astable Multivibrator Op-Amp Online Calculator to calculate the component values. The automatic oscillation of this astable multivibrator is due to charging and discharging of capacitor and time determined by the capacitor and resistor circuit. The formula for astable multivibrator circuit like this are below.
\(
f_o = \frac{1}{2R_F C ln(\frac{-V_{sat}-V_{LT}}{+V_{sat}-V_{UT}})}
\) ---->(1)
\(
V_{UT}=\frac{R_1 (+V_{sat})}{R_1+R_2}
\) ------->(2)
\(
V_{LT}=\frac{-R_1 (-V_{sat})}{R_1+R_2}
\) ----------->(3)
So the output signal of the op-amp astable multivibrator is a square wave with frequency given by equation 1. The frequency of the Astable multivibrator using op-amp depends upon 3 resistors and one capacitor. The other resistors and capacitors is for the dual power supply. Internally, the LM741 op-amp and most of the other operational amplifier IC are build using BJT differential amplifier. This is very similar to building Astable multivibrator with BJT transistors. See the following video for this.
The astable multivibrator or square generator using operational amplifier generates a square wave while the transistor based astable multivibrator also generates a square wave. The op-amp astable multivibrator circuits requires external capacitors and resistors while the transistor multivibrator circuit is simple. I think the op-amp based multivibrator circuit is more stable and powerful. What about the frequency? Initially I thought that the op-amp based square wave generator will produce signal with frequency higher than the transistor square wave generator. But we know that op-amp can produce frequency only in audio range to some MHz. There are specialized op-amp designed for higher frequency than that maybe 30MHz and higher but most readily available op-amp like LM358, LM741, TLo74 etc have frequency limitation. But the frequency of the transistor based square wave generator only requires two transistors which can very high frequency limitation. So maybe the transistor based astable multivibrator can generate higher frequency square wave than the op-amp astable multivibrator.
On the basis of operation, the transistor square wave generator circuit is based on charge and discharge and the comparator circuit is build into it while in the above LM741 op-amp astable multivibrator also has charge and discharge capacitors but these are located outside and the LM741 is used as comparator circuit. The astable multivibrator is one types of oscillator.
The operation of astable multivibrator with E-MOSFET is exactly the same as the operation explained in astable multivibrator using transistor.
Advantages of Astable Multivibrators with E-MOSFETs
High Efficiency
E-MOSFETs feature low on-resistance and high switching speeds, enhancing efficiency compared to other transistor types.Precise Timing
These circuits deliver precise and stable square wave output frequencies, making them ideal for applications that require accurate timing.Ease of Implementation
Using E-MOSFETs simplifies the design process while enhancing the reliability of astable multivibrators due to their robustness and low susceptibility to temperature variations.Applications of Astable Multivibrators with E-MOSFETs
- Signal Generation: Astable multivibrators are widely utilized in signal generators where stable square wave outputs are necessary.
- Pulse Width Modulation (PWM): They are essential in PWM circuits for controlling power delivery in systems such as motor speed control, voltage regulation, lighting systems etc.
- Clock Generation: These circuits serve as clock generators in digital systems, providing precise timing signals for synchronous operations.
For a visual demonstration of how an astable multivibrator using MOSFET works, check out our video tutorial.
