In this note, we are discuss about Oscillatory Circuit or Tank Circuit. Waveform of this Circuit and Frequency of response and types of oscillator circuit. Welcome to Poly Notes Hub, a leading destination for polytechnic notes.
Author Name: Arun Paul.
What is Oscillatory Circuit or Tank Circuit ?
A circuit that produces electrical oscillations of any desired frequency is known as an Oscillatory Circuit or Tank Circuit.
A simple oscillatory circuit consists of a capacitor (C) and inductance coil (L) in parallel as shown in Fig below. This electrical system can produce electrical oscillations of frequency determined by the values of L and C. To understand how this occurs, suppose the capacitor is charged from a D.C. source with a polarity as shown in Fig (i).
Circuit Details and Working
Below we explain the circuit of tank circuit and also told about the working of oscillatory circuit –
- In the position shown in Fig (i), the upper plate of the capacitor has a deficit of electronic, and the lower plate has an excess of electrons. Therefore, there is a voltage across the capacitor and the capacitor has electrostatic energy.
- When switch S is closed as shown in Fig (ii), the capacitor will discharge through inductance and the electrons flow will be in the direction indicated by the arrow. This current flow sets up a magnetic field around the coil. Due to the inductive effect, the current builds up slowly towards a maximum value. The circuit current will be maximum when the capacitor is fully discharged. At this instant, electrostatic energy is zero but because electron motion is greatest (i.e. maximum current), the magnetic field energy around the coil is maximum. This is shown in Fig (ii). Obviously, the electrostatic energy across the capacitor is completely converted into magnetic field energy around the coil.
- Once the capacitor is discharged, the magnetic field will begin to collapse and produce a counter e.g. According to Lenz’s law, the counter e.m.f. will keep the current flowing in the same direction. The result is that the capacitor is now charged with opposite polarity, making the upper plate of the capacitor negative and the lower plate positive as shown in Fig (iii).
- After the collapsing field has recharged the capacitor, the capacitor now begins to discharge; the current now flowing in the opposite direction. Fig (iv) shows the capacitor fully discharged and the maximum current flow.
The sequence of charge and discharge results in the alternating motion of electrons or an oscillating current. The energy is alternately stored in the electric field of the capacitor and the magnetic field of the inductance coil. This interchange of energy between L and C is repeated over and again resulting in the production of oscillations.
Waveform of Oscillatory Circuit
If there were no losses in the tank circuit to consume the energy, the interchange of energy between L and C would continue indefinitely. In a practical tank circuit, there are resistive and radiation losses in the coil and dielectric losses in the capacitor. During each cycle, a small part of the originally imparted energy is used up to overcome these losses. The result is that the amplitude of the oscillating current decreases gradually and eventually it becomes zero when all the energy is consumed as losses. Therefore, the tank circuit by itself will produce damped oscillation as shown in the figure below.
Frequency of Oscillation
The frequency of oscillation in the tank circuit is determined by the constants of the circuit ( L and C ). The actual frequency of oscillation of the tank circuit is given by –
fₓ = 1⁄(2π√LC)
where,
f = Frequency in Hz
L = Inductance in Henry
C = Capacitance in Farad
Types of Oscillator Circuit
The following are the transistor oscillators commonly use at various places in the electronic circuit –
- Tuned Collector
- Hartley
- Wien Bridge
- Colpitt’s
- Phase Shift
- Crystal
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