Types of Transistor Oscillators:
A transistor can be operated as an oscillator for producing continuous undamped oscillations of any desired frequency if tank (or oscillatory) and feedback circuits are properly connected to it. All types of transistor oscillators under different names have similar function i.e. they generate continuous undamped output. However, they differ in methods of supplying energy to the tank or oscillatory circuit to meet the losses and the frequency ranges over which they are used.
The frequency spectrum over which oscillators are employed to produce sinusoidal signals is extremely wide (from less than 1 Hz to many GHz). However, no single oscillator design is practical for generating signals over this entire range. Instead, a variety of designs are employed, each of which generates sinusoidal outputs most advantageously over various portions of the frequency spectrum.
Oscillators, which use inductance-capacitance (L-C) circuits as their tank or oscillatory circuits, are very popular for generating high frequency (e.g. 10 kHz to 100 MHz) outputs. The most widely used LC oscillators are the Hartley and Colpitt’s oscillators. Although they slightly differ from one another in their electronic circuitry but they have virtually identical frequency ranges and frequency-stability characteristics.
However, such types of transistor oscillators are not suitable for generating low- frequency sinusoidal outputs. This is due to the fact that the components required in construction of low-frequency L-C resonant circuits are too bulky and heavy. So resistor-capacitor (R-C) oscillators are generally employed for generating low-frequency (from 1 Hz to about 1 MHz) sinusoidal signals. Two most common R-C oscillators are the Wien bridge and phase shift types.
Other less frequently used oscillators are the crystal oscillators and the negative resistance oscillators. The operating frequency ranges of various types of most commonly used oscillators are given below:
|Type of Oscillator||Approximate Frequency Ranges|
|Wien bridge oscillator||1 Hz — 1 MHz|
|Phase shift oscillator||1 Hz — 10 MHz|
|Hartley oscillator||10 kHz — 100 MHz|
|Colpitt’s oscillator||10 kHz — 100 MHz|
|Negative resistance oscillator||> 100 MHz|
|Crystal oscillator||Fixed frequency|