A common outcome of a direct short is a melting of the die and metal, eventually opening the circuit. For example, a suitably high voltage applied between the gate and source (V GS ) will break down the MOSFET gate oxide. A common transistor I use is the 2N3904. You can easily switch big, like great than 12 volt loads with this transistor’s max 40 volt rating. Its current rating is only 200mA, but that is enough for most relays. 2N3904 from Mouser. MOSFET which has become the most commonly used three terminal devices brings revolution in the world of electronic circuits. Without MOSFET, the design of integrated circuits seems impossible nowadays. These are quite small and their process of manufacturing is very simple. The implementation of both analog and digital circuits integrated.
As discussed under the section on JFETs, the common drain amplifier is also known as the source follower. The prototype amplifier circuit with device model is shown in Figure (PageIndex{1}). As with all voltage followers, we expect a non-inverting voltage gain close to unity with a high (Z_{in}) and a low (Z_{out}).
Figure (PageIndex{1}): Common drain (source follower) prototype.
3 reasons to start a business in retirement. As is usual, the input signal is applied to the gate terminal and the output is taken from the source. Because the output is at the source, biasing schemes that have the source terminal grounded, such as zero bias and voltage divider bias, cannot be used.
13.3.1: Voltage Gain
The voltage gain equation for the common drain follower is developed as follows: We begin with the fundamental definition that voltage gain is the ratio of (v_{out}) to (v_{in}), and proceed by expressing these voltages in terms of their Ohm's law equivalents. The load is now located at the MOSFET's source, and thus can be referred to as either (r_L) or (r_S).
[A_v = frac{v_{out}}{v_{i n}} = frac{v_S}{v_G} = frac{v_L}{v_G} A_v = frac{i_D r_L}{i_D r_L+v_{GS}} A_v = frac{g_m v_{GS} r_L}{g_m v_{GS} r_L+v_{GS}} A_v = frac{g_m r_L}{g_m r_L+1} label{13.5}]
or, if preferred
[A_v = frac{g_m r_S}{g_m r_S+1} label{13.5b}]
If (g_mr_S gg 1), the voltage gain will be very close to unity; a desired outcome.
13.3.2: Input Impedance
The analysis for source follower's input impedance is virtually identical to that for the common source amplifier. The same commentary applies regarding the simplification of gate biasing resistors to arrive at the value of (r_G).
[Z_{in} = r_G || r_{GS} approx r_G label{13.6}]
13.3.3: Output Impedance
In order to determine the output impedance, we modify the circuit of Figure (PageIndex{1}) by separating the load resistance from the source bias resistor. This is shown in Figure (PageIndex{2}).
Figure (PageIndex{2}): Source follower output impedance analysis.
Looking back into the source from the perspective of the load we find that the source biasing resistor, (R_S), is in parallel with the impedance looking back into the source terminal.
[Z_{out} = R_S || Z_{source} nonumber]
To find (Z_{source}), note that the voltage at the source is (v_{GS}) and the current entering this node is (i_D). The ratio of the two will yield the impedance looking back into the source.
[Z_{source} = frac{v_{GS}}{i_D} Z_{source} = frac{v_{GS}}{g_m v_{GS}} Z_{source} = frac{1}{g_m} label{13.7}]
Therefore, the output impedance is
[Z_{out} = R_S || frac{1}{g_m} label{13.8}]
Looking at Equation ref{13.8} it is obvious that the higher the transconductance, the lower the output impedance. As noted earlier, a large transconductance also means that the voltage gain will be close to unity. As a general rule then, a large transconductance is desired for the source follower.
Time for a few illustrative examples.
Example (PageIndex{1})
For the circuit of Figure (PageIndex{3}), determine the voltage gain and input impedance. Assume (V_{GS(off)}) = −0.8 V and (I_{DSS}) = 30 mA.
Figure (PageIndex{3}): Circuit for Example (PageIndex{1}).
This amplifier uses self bias so we need to determine (g_{m0}R_S).
[g_{m0} =− frac{2 I_{DSS}}{V_{GS (off )}} nonumber]
[g_{m0} =− frac{2 times 30mA}{−0.8 V} nonumber]
[g_{m0} = 75mS nonumber]
The DC source resistance is the 270 (Omega) biasing resistor resulting in (g_{m0} R_S) = 16.2. From the self bias equation or graph this produces a drain current of 2.61 mA.
[g_m = g_{m0} sqrt{frac{I_D}{I_{DSS}}} nonumber]
[g_m = 75 mS sqrt{frac{2.61 mA}{30 mA}} nonumber]
[g_m = 22.1mS nonumber]
The voltage gain is
[A_v = frac{g_m r_S}{g_m r_S+1} nonumber]
[A_v = frac{22.1 mS(270Omega || 150 Omega )}{22.1 mS times (270 Omega || 150Omega ) +1} nonumber]
[A_v = 0.68 nonumber]
Finally, for the input impedance we have
[Z_{in} = 1.2 MOmega || Z_{in(gate)} approx 1.2 MOmega nonumber]
Example (PageIndex{2})
For the circuit of Figure (PageIndex{4}), determine the voltage gain and input impedance. Assume (V_{GS(off)}) = −2.5 V and (I_{DSS}) = 80 mA.
Figure (PageIndex{4}): Circuit for Example (PageIndex{2}).
This follower uses a P-channel device with combination bias. Note that the source terminal is toward the top of the schematic. First, determine (g_{m0}R_S) and the bias factor, (k). Then the combination bias equation can be used to determine the drain current.
[g_{m0} =− frac{2 I_{DSS}}{V_{GS (off )}} nonumber]
[g_{m0} =− frac{2 times 80 mA}{−2.5 V} nonumber]
[g_{m0} = 64 mS nonumber]
The DC source resistance is the 1.8 k(Omega) biasing resistor resulting in (g_{m0} R_S) = 115.2. The bias factor is (V_{SS}/V_{GS(off)}), or 4. The combination bias equation (Equation 10.9) yields (I_D) = 6.67 mA.
We can now find the transconductance and voltage gain.
[g_m = g_{m0} sqrt{frac{I_D}{I_{DSS}}} nonumber]
[g_m = 64mS sqrt{frac{6.67 mA}{80mA}} nonumber]
[g_m = 18.5mS nonumber]
The voltage gain is
[A_v = frac{g_m r_S}{g_m r_S+1} nonumber]
[A_v = frac{18.5 mS(1.8k Omega || 800Omega )}{18.5 mS times (1.8 kOmega || 800 Omega ) +1} nonumber]
[A_v = 0.91 nonumber]
Lastly, the input impedance is
[Z_{in} = 560 kOmega || Z_{in(gate)} approx 560 kOmega nonumber]
MOSFET
Common Mosfet Types
The MOSFET is an important element in embedded system design which is used to control the loads as per the requirement. Many of electronic projects developed using MOSFET such as light intensity control, motor control and max generator applications. The MOSFET is a high voltage controlling device provides some key features for circuit designers in terms of their overall performance. This article provides information about different types of MOSFET applications.
MOSFET and Its Applications
The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) transistor is a semiconductor device which is widely used for switching and amplifying electronic signals in the electronic devices.The MOSFET is a three terminal device such as source, gate, and drain. The MOSFET is very far the most common transistor and can be used in both analog and digital ckt.
The MOSFET works by varying the width of a channel along which charge carriers flow (holes and electrons). The charge carriers enter the channel from the source and exits through the drain. The channel width is controlled by the voltage on an electrode is called gate which is located between the source and drain. It is insulated from the channel near an extremely thin layer of metal oxide. There is a different type of MOSFET applications which is used as per the requirement.
Types of MOSFET Devices
The MOSFET is classified into two types such as;
- Depletion mode MOSFET
- Enhancement mode MOSFET
Depletion Mode: When there is zero voltage on the gate terminal, the channel shows its maximum conductance. As the voltage on the gate is negative or positive, then decreases the channel conductivity.
Depletion Mode MOSFET
Enhancement Mode
When there is no voltage on the gate terminal the device does not conduct. More voltage applied on the gate terminal, the device has good conductivity.
Enhance Mode MOSFET
MOSFET Working Principle
The working of MOSFET depends upon the metal oxide capacitor (MOS) that is the main part of the MOSFET. The oxide layer presents among the source and drain terminal. It can be set from p-type to n-type by applying positive or negative gate voltages respectively. When apply the positive gate voltage the holes present under the oxide layer with a repulsive force and holes are pushed downward through the substrate. The deflection region populated by the bound negative charges which are allied with the acceptor atoms.
P- Channel MOSFET
The P-Channel MOSFET consist negative ions so it works with negative voltages. When we apply the negative voltage to gate, the electrons present under the oxide layer through pushed downward into the substrate with a repulsive force. Endless war 3&& try the games. The deflection region populates by the bound positive charges which are allied with the donor atoms. The negative voltage also attracts holes from p+ source and drain region into the channel region.
P-Channel MOSFET
N- Channel MOSFET
When we apply the positive gate voltage the holes present under the oxide layer pushed downward into the substrate with a repulsive force. The deflection region is populated by the bound negative charges which are allied with the acceptor atoms. The positive voltage also attracts electrons from the n+ source and drain regions into the channel. Now, if a voltage is applied among the drain and source the current flows freely between the source and drain and the gate voltage controls the electrons in the channel. In place of positive voltage if we apply a negative voltage (hole) channel will be formed under the oxide layer.
N-Channel MOSFET
MOSFET Applications
The applications of the MOSFET used in various electrical and electronic projects which are designed by using various electrical and electronic components. For better understanding of this concept, here we have explained some projects.
MOSFET Used as a Switch
In this circuit, using enhanced mode, a N-channel MOSFET is being used to switch the lamp for ON and OFF. The positive voltage is applied at the gate of the MOSFET and the lamp is ON (VGS =+v) or at the zero voltage level the device turns off (VGS=0). If the resistive load of the lamp was to be replaced by an inductive load and connected to the relay or diode to protect the load. In the above circuit, it is a very simple circuit for switching a resistive load such as LEDs or lamp. But when using MOSFET to switch either inductive load or capacitive load protection is required to contain the MOSFET applications. If we are not giving the protection, then the MOSFET will be damaged. For the MOSFET to operate as an analog switching device, that needs to be switched between its cutoff region where VGS =0 and saturation region where VGS =+v.
Common Mosfet Circuit
Auto Intensity Control of Street Lights using MOSFET
Now-a-days most of lights placed on the highways are done through High Intensity Discharge lamps (HID), whose energy consumption is high. Earn to diewatermelon gaming. Its intensity cannot be controlled according to the requirement, so there is a need to switch on to an alternative method of lighting system, i.e., to use LEDs. This system is built to overcome the present day drawbacks of HID lamps.
Auto Intensity Control of Street Lights using MOSFET
This project is designed to control the lights automatically on the highways using microprocessor by variants of the clock pulses. In this project, MOSFET plays major role that is used to switch the lamps as per the requirement. The proposed system using a Raspberry Pi board that is a new development board consist a processor to control it. Here we can replace the LEDs in place of HID lamps which are connected to the processor with the help of the MOSFET. The microcontroller release the respective duty cycles, then switch the MOSFET to illuminate the light with bright intensity
Marx Generator Based High Voltage Using MOSFETs
The main concept of this project is to develop a circuit that delivers the output approximately triple to that of the input voltage by Marx generator principle. It is designed to generate high-voltage pulses using a number of capacitors in parallel to charge during the on time, and then connected in series to develop a higher voltage during the off period. If the input voltage applied is around 12v volts DC, then the output voltage is around 36 volts DC.
This system utilizes a 555 timer in astable mode, which delivers the clock pulses to charge the parallel capacitors during on time and the capacitors are brought in a series during the off time through MOSFET switches; and thus, develops a voltage approximately triple to the input voltage but little less, instead of exact 36v due to the voltage drop in the circuit. The output voltage can be measured with the help of the multimeter.
EEPROM based Preset Speed Control of BLDC Motor
The speed control of the BLDC motor is very essential in industries as it is important for many applications such as drilling, spinning and elevator systems. This project is enhanced to control the speed of the BLDC motor by varying the duty cycle.
EEPROM based Preset Speed Control of BLDC Motor
The main intention of this project is to operate a BLDC motor at a particular speed with a predefined voltage . Therefore, the motor remains in an operational state or restarted to operate at the same speed as before by using stored data from an EEPROM.
The speed control of the DC motor is achieved by varying the duty cycles (PWM Pulses) from the microcontroller as per the program. The microcontroller receives the percentage of duty cycles stored in the EEPROM from inbuilt switch commands and delivers the desired output to switch the driver IC in order to control the speed of the DC motor. If the power supply is interrupted, the EEPROM retains that information to operate the motor at the same speed as before while the power supply was available.
LDR Based Power Saver for Intensity Controlled Street Light
In the present system, mostly the lightning-up of highways is done through High Intensity Discharge lamps (HID), whose energy consumption is high and there is no specialized mechanism to turn on the Highway light in the evening and switch off in the morning.
LDR Based Power Saver for Intensity Controlled Street Light
Its intensity cannot be controlled according to the requirement, so there is a need to switch to an alternative method of lighting system, i.e., by using LEDs. This system is built to overcome the present day, drawback of HID lamps.
This system demonstrates the usage of LEDs (light emitting diodes) as light source and its variable intensity control, according to the requirement. LEDs consume less power and its life is more, as compared to conventional HID lamps.
The most important and interesting feature is its intensity that can be controlled according to requirement during non-peak hours, which is not feasible in HID lamps. A light sensing device LDR (Light Dependent Resistance) is used to sense the light. Its resistance reduces drastically according to the daylight, which forms as an input signal to the controller .
A cluster of LEDs is used to form a street light. The microcontroller contains programmable instructions that controls the intensity of lights based on the PWM (Pulse width modulation) signals generated.
The intensity of light is kept high during the peak hours, and as the traffic on the roads tend to decrease in late nights; the intensity also decreases progressively till morning. Finally the lights get completely shut down at morning 6 am, to resume again at 6pm in the evening. The process thus repeats.
SVPWM (Space Vector Pulse Width Modulation)
The Space Vector PWM is a sophisticated technique for controlling AC motors by generating a fundamental sine wave that provides a pure voltage to the motor with lower total harmonic distortion. This method overcomes the old technique SPWM to control an AC motor that has high-harmonic distortion due to the asymmetrical nature of the PWM switching characteristics.
In this system, DC supply is produced from the single-phase AC after rectification, and then is fed to the 3-phase inverter with 6 numbers of MOSFETs. For each phase, a pair of MOSFETare used, and, therefore, three pairs of MOSFETs are switched at certain intervals of time for producing three-phase supply to control the speed of the motor. This circuit also gives light indication of any fault that occurs in the control circuit
Therefore, this is all about types of MOSFET applications, Finally, we will conclude that, the MOSFET requires high voltage whereas transistor requires low voltage and current. As compared to a BJT, the driving requirement for the MOSFET is much better.Furthermore, any queries regarding this article you can comment us by commenting in the comment section below.