Mutchus

Basic mosfet amplifier This Basic MOSFET amplifier is very simple to build and low cost. Is perfect for Hi-Fi amplifiers and instrument amplifiers (guitar, keyboards …). Output power is + / – 100 Wrms at 8 ohm load or + / – 160 Wrms below 4 ohms. The simplicity of this circuit cause distortion + / -0.1%. Bandwidth at -3 dB is from 4 Hz to 96 Khz and it is limited by C1, R1, R2 and C2. Transistors T1 and T2 makes a first differential stage, current source of +/- 1 mA is set by R3. P1 allows a fine tuning of DC voltage at amplifier’s output. Place P1 at it’s half value for first power up, then turn it slowly for a lowest DC output voltage. It is recommended to use a first quality component. Input sensitivity is 1.2 volts. The gain of 27x is archived by R7/R6. It may be modified by changing R7 value. Transistors T5 and T6 makes the second differential stage. Transistors T3 and T4 works as a current mirror source. They push the second differential stage to drain equal current. Doing so we ...

In most cases where you want to control an AC load, a triac or SCRs will be used. However, it is not easy to drive a triac or SCR. The drive requirement for the triac or SCR makes it sometimes difficult to control it as we want. One thing is that we can not turn the triac or SCR on or off as we desire, because once we turn it on, it latches and stays on until the next zero crossing or until current stops flowing through it. Also, driving a triac with reference to MT1 (or A1) is not as straightforward as we would want. However, a MOSFET can be controlled as we want. Set the gate high (with a sufficient voltage) and current can flow from drain to source. Set the gate low and current can no longer flow. Convenient! However, a MOSFET can only be used to control DC loads since it is a unidirectional switch - current flow can be controlled when it is flowing from drain to source, but can not be controlled from source to drain. So, certainly it can not be used to control AC loads. Right? Well...

If you have an application in which a MOSFET is already used to switch a load, it is relatively easy to add short-circuit or overload protection. Here we make use of the internal resistance R DS(ON), which produces a voltage drop that depends on the amount of current flowing through the MOSFET. The voltage across the internal resistance can be sensed using simple comparator or even a transistor, which switches on at a voltage of around 0.5 V. You can thus avoid the use of a sense resistor (shunt), which usually produces an undesirable extra voltage drop. Short-Circuit Protection Circuit Diagram The comparator can be monitored by a microcontroller. In case of an overload, the software can initiate suitable countermeasures (PWM regulation, alarm, emergency stop etc.). It is also conceivable to connect the comparator out-put directly to the gate of the MOSFET, in order to immediately cut off the transistor in case of a short circuit.

Depletion MOSFET Depletion MOSFET may be fabricated from the basic MOSFET structure. An n-type channel is obtained by diffusion between N + type source and drain in an n-channel MOSFET. The below figure shows the structure of depletion MOSFET.     n-channel Depletion MOSFET Working of Depletion MOSFET When the gate source voltage is zero considerable drain current flows. When the gate is applied with negative voltage. Positive charges are induced in the n-channel through the Si O 2 layer of the gate capacitor. The conduction in n-channel FET is due to electrons i.e. the majority carriers. Therefore the induced positive charges make the n-channel less conductive. The drain current therefore gets reduced with increase in the gate bias voltage. The distribution of charges in the channel results in depletion of majority carriers. That is why this type of FET is called depletion MOSFET. The voltage drop due to the drain current causes the channel region nearer to the drain more dep...