Electronic Circuits and Diagram-Electronics Projects and Design

Unlike BJTs, thermal runaway does not occur with FETs, as already discussed in our blog. However, the wide differences in maximum and minimum transfer characteristics make ID levels unpredictable with simple fixed-gate bias voltage. To obtain reasonable limits on quiescent drain currents ID and drain-source voltage VDS, source resistor and potential divider bias techniques must be used. With few exceptions, MOSFET bias circuits are similar to those used for JFETs. Various FET biasing circuits are discussed below: Fixed Bias. DC bias of a FET device needs setting of gate-source voltage VGS to give desired drain current ID . For a…

From the circuit named “looking into the drain” of the FET it is seen (for small signal operation) an equivalent circuit consisting of two generators in series, one of – u times the gate signal voltage Vin and the second (u + 1) times the source-signal voltage Vs and the resistance rd + ((a + 1) RG. Note that the source-signal voltage Vs and the resistance in the source lead are both multiplied by the same factor, (u + 1).

CHARACTERISTICS OF JFETS There are two types of static characteristics viz (1) Output or drain characteristic and (2) Transfer characteristic. 1. Output or Drain Characteristic. The curve drawn between drain current Ip and drain-source voltage VDS with gate-to source voltage VGS as the parameter is called the drain or output characteristic. This characteristic is analogous to collector characteristic of a BJT: (a) Drain Characteristic With Shorted-Gate. The circuit diagram for determining the drain characteristic with shorted-gate for an N-channel JFET is given in figure. and the drain characteristic with shorted-gate is shown in another figure. Initially when drain-source voltage Vns is zero,…

As already mentioned, JFETs are of two types viz N-channel JFETs and P-channel JFETs.Generally N-channel JFETs are preferred. N-channel and P-channel JFETs are shown in figures.   Basic Construction. Its structure is quite simple. In an N-channel JFET an N-type silicon bar, referred to as the channel, has two smaller pieces of P-type silicon material diffused on the opposite sides of its middle part, forming P-N junctions, as illustrated in fig. The two P-N junctions forming diodes or gates are connected internally and a common terminal, called the gate terminal, is brought out. Ohmic contacts (direct electrical connections) are made at…

Series Inductor Filter. In this arrange­ment a high value inductor or choke L is connected in series with the rectifier element and the load, as illustrated in figure. The filtering action of an inductor filter de­pends upon its property of opposing any change in the current flowing through it. When the output current of the rectifier increases above a certain value, energy is stored in it in the form of magnetic field and this energy is given up when the output current falls below the average value. Thus by placing a choke coil in series with the rectifier output and…

The drawbacks of pi-filters are the comparatively larger cost, more weight, bigger size and external field developed by the series inductor. However, these drawbacks can be overcome by replacing the series inductor by a series resistor R. Such a circuit is called the R-C filter circuit and is given in figure. By deliberate design R is kept much larger than XC1 at the ripple frequency. So the ripples are dropped across series resistor R instead of across the load resistor RL. Typically R is kept at least 10 times greater than XC2; this means that each section reduces the ripples…

INTRODUCTION to FET-Field Effect Transistor So far we have discussed the circuit applications of ordinary transistors, in which both holes and electrons take part. This is the reason that these are sometimes called the bipolar transistors. Such transistors have two main drawbacks namely low input impedance because of forward biased emitter junction and considerable noise level. Both of these drawbacks have been overcome, to a great extent in the field effect transistor (FET), which is an electric field (or voltage) controlled device. FET’s because of possessing all the advantages that tubes and ordinary transistors (BJTs) have, are replacing both the…

A simple series inductor reduces both the peakand effective values of the output current and output voltage. On the other hand a simple shunt capacitor filter reduces the ripple voltage but increases the diode current. The diode may get damaged due to large current and at the same time it causes greater heating of supply transformer resulting in reduced efficiency. In an inductor filter, ripple factor increases with the increase in load resistance RL while in a capacitor filter it varies inversely with load resistance RL. From economical point of view also, neither series inductor nor shunt capacitor type filters…

Shunt Capacitor Filter and types of it: This is the most simple form of the filter circuit and in this arrangement a high value capacitor C is placed directly across the output terminals, as shown in figure. During the conduction period it gets charged and stores up energy to it during non-conduction period. Through this process, the time duration during which Ft is to be noted here that the capacitor C gets charged to the peak because there is no resistance (except the negligible forward resistance of diode) in the charging path. But the discharging time is quite large (roughly 100…

The output from any of the rectifier circuit is not purely dc but also has some ac components called ripples, along it. The ripples are maximum in the single phase half-wave rectifier and being reduced in the full-wave rectifier and being reduced further with the increase in the number of phases. Such supply is not useful for driving sophisticated electronic devices/circuits. Of course, for a circuit such as battery charger the pulsating nature of supply available from a rectifier is no great detriment as long as the dc level provided results in charging of battery. But for supply circuits to…