Unit4. electric electronic

Every radio receiver or transmitter contains one or more oscillatory circuits round which is built. Such as a circuit consists of an inductance (a coil) and a capacitance (a capacitor) connected in series to from a closed LC circuit.

When a LC circuit is connected to a source of an alternating current an emf of self-inductance is generated in the coil. A LC circuit works on the principle that : if an alternating current flowing in a conductor will produce a alternating magnetic field around it, then a change in the number of lines of magnetic force which cut the conductor induce an efm of self-induction in it. The magnitude of this emf depends on the rate of change of magnetic flux linking with the conductor, i.e., on the rate of change of circuit, current and also on a property known as inductance.

The unit of inductance is henry (H). a circuit is said to have an inductance on henry when a current change one ampere per second induces an emf of self-induction of one volt. One thousandth of a henry is a millihenry (mH)

A straight conductor has its own self-inductance when it carries a varying current, but it is very small in magnitude. If this same conductor is wound into a coil, a magnetic flux produced by the current flowing in it will then link with the entire coil turns and lead to a significant rise in the inductance. To obtain large values of inductance, a core of steel or some other highly magnetic material is placed within the coil. The core makes it possible to effectively increase the flux leakages with the coil and therefore make it have a greater inductance. If it is necessary to obtain low inductance, the coil is wound without cores or wound on powdered-iron cores which have a low permeability. Coil are wound on powdered-iron cores when they have to operate at high frequencies, solid or laminated steel cores are impermissible in such cases because of the large energy loss in the core due to alternating magnetization.

Powdered-iron cores consist of fine grains of magnetic material, such as carbonyl iron, alsifer, etc, mixed with a suitable bonding material, (resins or waterglass) and hot pressed into core shapes at elevated temperatures

The simplest form of inductance coil is one wound in a single cylindrical layer on a round bobbin of pressboard. For higher inductance, the coil is wound with several layers of wire in a special manner known as "universal"

The second component of an LC oscillatory circuit is the capacitor or condenser. In its simplest form a capacitor consists of a two metal plates, or electrodes, separated by a layer of insulation, called the dielectric (which may be air). When a capacitor is connected to the terminals of a battery, it is charged, the plate connected to the positive pole accumulates a positive charge, while the left one connected to negative pole accumulates a negative charge. The potential difference equal to a battery emf is produced across the plates and an electric field is set up in the dielectric. When the charged capacitor is disconnected from the battery, the energy stored in its electric field is retained and the capacitor is become a sour of electric energy. The larger the area of the plates, the larger the charge it will take before its potential difference is equal to the emf of the source. The capacity of capacitance therefore depends on the plate area.

The unit of capacitance is farad (F), or the capacitance of a capacitor in which a charge of one coulomb of electricity produces a change of one volt in the potential difference between its plates. The farad is, however, much too large for the practical work. Capacitance is usually measured in microfarads; it is one millionth of a farad.

When a capacitor is connected in a circuit with an unvarying EMF (a dc circuit ), the current changing it will only flow around the circuit until the capacitor is charged to the full EMF of the source. In the circuit where the EMF is continuously varying in magnitude and reversing in direction (an ac circuit), on the other hand, the capacitor will be charged and discharged at the frequency of the applied EMF, and an capacitor will pass an alternating current and block the way for a direct current.

In accordance with the dielectric employed in them, the capacitors are classed as air, mica, paper, or ceramic capacitors. As to design, capacitor may be fixed or variable. In the first group are those the capacitance of which can not be changed, while the second group includes those in which the capacitance can be changed at over a large range. Fixed capacitors are most frequently made in the form of two long, metal-foil ribbon edges; the capacitor is completed by encasing it in a paper sleeve.