Unlike bipolar transistors MOSFETs are voltage operated devices, not current operated. An electrical charge (voltage) on the gate (G) relative to the source (S) will switch on the device. The only purpose of Rg (10K) is to bleed-off any remaining charge on gate terminal to shut the transistor off. MOSFET driver transistors - Aiming to reduce costs and board space requirements Our MOSFET drivers are ideal for power management applications. Offered in three different configurations and a small foot print, they can help you save both costs and board space. DRV8711 Bipolar Stepper Motor Controller gate driver, CSD19534Q5A 100V, N-Channel NexFET Power MOSFET s, CSD17483F4 30V, N-Channel FemtoFET™ MOSFET, MSP430G2553 MCU, and LM5009 12 V buck converter. This design is focused on demonstrating a unique configuration of the DRV8711. Design Resources TIDA-00872 Design Folder DRV8711 Product Folder.

A transistor is a linear semiconductor device that controls current with the application of a lower-power electrical signal. Transistors are usually categorized into two groups: bipolar and field-effect.

A bipolar transistor is a commonly used for amplification. The device can amplify analog or digital signals. It can also switch DC or function as an oscillator. Physically, a bipolar transistor amplifies current, but it can be connected in circuits designed to amplify voltage or power.

Mosfet Bipolar Amplifier

The field–effect transistor (FET) is a type of transistor which uses an electric field to control the flow of current. FETs are devices with three terminals: source, gate, and drain.

JFET/JUGFETTransistor

JFET (Junction field-effect transistor) also referred to as JUGFET, is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically-controlled switches, amplifiers or voltage-controlled resistors.

Typically, field-effect transistor is composed of a siliconsection whose conductance is controlled by an electric field. The section ofsilicon through which current flows is referred to as channel and is made up of one type of silicon either p-type or n-type. The connections ateither end of the device are referred to as the source and drain. The electricfield to control the current is applied to a third electrode referred to as gate, because it is the only electricfield controlling the current flowing in the channel. Therefore, the JFET is avoltage controlled device with a high input impedance usually megohms.

Junction field effect transistor, JFET is a very usefulactive device and in this regard, they are used in many electronic circuitdesigns, due to some of the advantages they are able to display. The advantagesinclude low noise, high input impedance and simple biasing among others. JFETs are used as:

• High impedance wide band amplifiers.
• An electronic switch
• Buffer amplifiers
• Voltage variable resistors (VVR) or voltagedevelopment resistor (VDR).
• Phase shift oscillator
• Constant current source

What You Need ToKnow about JFET Transistors

• JFET stands for Junction Field Effect Transistor.
• It is the simplest type of field effect transistor in which the current can either pass from source to drain or drain to source. JFET uses the voltage applied to the gate terminal to control the current flowing through the channel between the drain and source terminals which results in output current being proportional to the input voltage.
• JFETs can only be operated in the depletion mode because of reverse biasing of its pn-junction.
• JFETs have input impedance of (~10^8 Ω) which is very much lower than that of MOSFETs.
• JFETs are less susceptible to electrostatic discharge because they offer higher input capacitance than MOSFETs.
• The gate leakage current of JFET is of the order of nanoAMPs (10^-9 A).
• Usually, the drain resistance of JFETs is higher than that of MOSFETs, therefore the output characteristics tend to be flatter than the MOSFET.
• JFETs have been in use for a longer period of time and therefore they have slowly been replaced in many of its original use cases by more modern devices like the CMOS OpAmp.
• JFETs are simpler to fabricate and thus they are very much available and cheaper in cost.
• JFETs are perfect for use in low noise applications such as electronic switches, buffer amplifiers etc.

MOSFETTransistor

The metal oxide semiconductorfield-effect transistor (MOSFET), also referred to as the metal oxide silicon transistor, is atype of field effect transistor that has an insulated gate and is fabricated bycontrolled oxidation of a semiconductor, typically silicon. MOSFET is the mostcommon semiconductor device in digital and analog circuits, and the most commonpower device used for switching and amplifying signals in the electronicdevices.

The MOSFET is a four terminal device with sources (S), gates(G), drain (D) and body (B) terminals. The body of the MOSFET is frequentlyconnected to the source terminal thereby making it a three terminal device likefield effect transistor.

MOSFET works by varying the width of a channelelectronically along with charge carriers flow. The charge carriers enter thechannel at source and exit through the drain. The width of the channel iscontrolled by the voltage on an electrode which is located between source anddrain referred to as gate. The gate is insulated from the channel with a thinlayer of metal oxide.

Applications ofMOSFETs

Mosfet Vs Bjt

• High switching speed of MOSFETs makes it anideal choice in designing chopper circuits.
• MOSFET amplifiers are extensively used in radiofrequency applications.
• DC motors can be regulated by power MOSFETs.
• It acts as a passive element like resistor,capacitor and inductor.

What You Need Toknow About MOSFET Transistor

• MOSFET stands for MetalOxide Semiconductor Field Effect Transistor. It is a four-terminal semiconductor field effect transistor fabricated by controlled oxidation of silicon and where the applied voltage determines the electrical conductivity of the device. The gate which is located between the source and drain channels is electrically insulated from the channel by a thin layer of metal oxide. The reason is to control the voltage and current flow between the sources and drain channels.
• MOSFET can be operated in either depletion or in enhancement mode because MOSFETs gate connection is completely isolated from the main current carrying channel.
• Due to metal oxide insulator, MOSFET’s have input impedance of (~10^10 Ω to 10^15 Ω) which is very much higher than that of JFETs.
• MOSFETs are more susceptible to electrostatic discharge damage because of additional metal oxide insulator which reduces the capacitance of the gate making the transistor vulnerable to high voltage damages.
• The gate leakage current of MOSFET is in order of PicoAMPs (10^-12 A).
• Usually, the drain resistance of MOSFETs is lower than that of JFETs and therefore the output characteristics tend to be less flat (curved) than the JFETs.
• MOSFETs are generally popular around the globe and therefore they have an important component in most of the integrated circuits.
• Addition of metal oxide layer to MOSFETs, makes the manufacturing process complex and sophisticated, therefore they are comparatively expensive to JFETs.
• MOSFETs are mainly used for high noise applications such as switching and amplifying analog or digital signals. Also, they are used in embedded systems and motor controlled applications.

What Are Some OfThe Similarities Between MOSFETs And JFETs?

• Both JFETs and MOSFETs have lesstransconductance value when compared to that of bipolar junction transistors(BJTs).
• Both JFETs and MOSFETs are voltage-controlledsemiconductor devices used to amplify weak signals using an electric fieldeffect.
• Both have common attributes corresponding toamplifying and switching.
• Both JFETs and MOSFETs can operate in depletion mode.
• JFET and MOSFET are the most popular fieldeffect transistors commonly used in electronic circuits.
• In both JFET and MOSFET, current never flowsinto the gate.
• Both MOSFETs and JFETs are fabricated withn-channel or p-channel doping.

by Crutschow, Electro-Tech-Online.com community member

Here’s a circuit that uses two back-to-back N-MOSFETs and one PNP BJT to form a low-side bipolar switch for either AC or plus/minus DC sources. This has much lower On voltage drop and power loss than a TRIAC (for example a Triac will typically dissipate >10W at 10Arms current whereas the two MOSFETs will dissipate a total of <1W for low ON resistance MOSFETs at the same current).

The circuit also doesn’t latch on with a DC source as a TRIAC does.
It can be used in place of an SSR when the AC/DC voltage source is already isolated from the Mains by a transformer and thus isolation for the control signal is not required.

Difference Between Transistor And Mosfet

The circuit takes advantage of the fact that a MOSFET conducts equally well in both directions when biased ON. Two back-to-back N-MOSFETs are used to allow blocking for both polarities of the supply source (otherwise the parasitic MOSFET substrate diode would conduct in the reverse direction.)

Mosfet Bipolar Driver

Fet Vs Bjt

The common-base configured PNP allows the positive control voltage to turn on the MOSFETs, but blocks the negative voltages (Vg, purple trace below) that occur at the gate from the gate-source connection when the control signal is 0V (giving Vg,s = 0V) and the supply voltage goes negative.

The input ON control voltage must be equal to the Vgs voltage for which the ON resistance, Rds(on), is specified in the MOSFET spec sheet. This is typically 10V for standard MOSFETs and 5V (or less) for logic-level type MOSFETs.

The maximum allowed peak AC or DC voltage is determined by the voltage ratings of the MOSFETs and the PNP transistor (whichever is lower). For inductive loads, back-to-back zeners or other transient suppressors will need to be used from the drain of M1 to ground, to limit the peak voltage to at least 25% below the ratings of the transistors.

The LTspice simulation below shows the current through the load resistor R_Load, for high (ON) and zero (OFF) input voltages and a 30Vrms AC source voltage. The ON voltage drop equals the load current times twice the ON resistance of the MOSFET type selected.

For more information about this circuit design go to Electro-Tech-Online.