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Colpitts Oscillator Circuits and Other Linear/Nonlinear Oscillator Functions

Damped LC Oscillator graphs

Working in the field of electronics is an adventure that continually changes and always challenges you to think about what is possible. However, like all things in life, there is a beginning and an end at some point. Hopefully, my opportunity to learn and grow within this incredible field is nowhere in sight, but I still remember my beginnings.

In this case, it is my first electronics kit complete with a small soldering station. The kit, of course, included an assortment of electronic components as well as some ideas for simple builds. So, with that in mind, I decided to build a basic radio for my first project. Overall, it functioned, and I was relatively satisfied with the outcome, but this also leads me to want to see what else I could build. Now fast forward to my electronics training at Naval Guided Missile Command, and our first hands-on quiz requires me to create a simple radio.

The Electronic Oscillator

The one thing the irony of that situation taught me is that everything has its purpose. Furthermore, this certainly applies to electronic components. Take, for example, the electronic oscillator. As you may know, an electronic oscillator is an electronic circuit that generates an intermittent (periodic), oscillating (electronic) signal, which is typically a square wave or a sine wave. The overall functionality of an Oscillator is the conversion of DC (direct current) from a power supply to an AC (alternating current) signal.

The Functionality and Application of Oscillators

In general, oscillators provide functionality for a wide array of electronic devices. These devices include calculators, clock generators, and PCs, to name a few. Furthermore, oscillator generated signals are in use in transmitters (TV), radio broadcasts, and even quartz clocks. With the various signals that an oscillator can produce, it is understandable that we can also characterize oscillators by the frequency of their output signal. Such as:

Note: Inverters are oscillators that generate a high-power AC output from a DC supply by design.

Types of Electronic Oscillators

Overall, the two major types of electronic oscillators are nonlinear and linear. With linear oscillators, energy always flows from the active components to the passive components within the circuit. The feedback path of linear oscillators determines the oscillation frequency. 

With a nonlinear oscillator, the active and passive components exchange energy. The charging and discharging time-constants involved in that process determines the oscillation frequency.

Linear oscillators generate sine wave outputs (low-distortion), and nonlinear oscillators produce non-sinusoidal outputs, i.e., triangular, square, or sawtooth waveforms.

Additionally, there are various types of Oscillators, and they include:

  • Wien Bridge Oscillator
  • RC Phase Shift Oscillator
  • Hartley Oscillator
  • Voltage Controlled Oscillator
  • Colpitts Oscillator
  • Clapp Oscillators
  • Crystal Oscillators

This, of course, is only a partial list of the types of oscillators, and now that we have briefly covered the basics, the focus of the article will shift to Colpitts Oscillator.

The Colpitts Oscillator

The Colpitts Oscillator falls into the category of a linear oscillator. Also, the Colpitts Oscillator is a type of LC oscillator, and the credit for its invention goes to Edwin Colpitts in 1918. Since it is a harmonic or linear oscillator, its frequency of oscillation is a derivative of the positive feedback rather than its input signal. Furthermore, one of the distinctive features of the Colpitts oscillator is that it takes the active device feedback from a voltage divider made from two capacitors in series across its inductor. Upon closer analysis, you will notice that the Colpitts oscillator is the electrical equivalent of a dual Hartley oscillator.

Colpitts oscillator diagram

Moreover, like other LC oscillators, the Colpitts oscillator utilizes a gain device that connects its output to its input via a feedback loop. Also, the gain devices in use in Colpitts oscillators include field-effect transistors, operational amplifiers, vacuum tubes, and even bipolar junction transistors. Furthermore, the feedback loop itself contains a parallel (tuned) LC circuit that acts as a bandpass filter and sets the frequency of oscillation.

Circuit diagram of colpitts oscillator

Now, as an example, the above circuit diagram shows a Colpitts oscillator with a common-base circuit design. In this configuration, the inductor (L) and the series combination of capacitor 1 (C1) and capacitor 2 (C2) form what we call the parallel resonant tank circuit. As you may know, the parallel resonant tank circuit determines the frequency of the oscillator. Furthermore, the voltage across C is applied to the base-emitter junction of the transistor, as feedback to create oscillations.

The Functionality of the Colpitts Oscillator

For the Colpitts oscillator in the above circuit diagram, the frequency of oscillation is more or less the resonant frequency of the series combination of C1 and C2 in parallel with the inductor (LC circuit). The following formula represents this:

I am sure you noticed that I stated that the frequency of oscillation is more or less the resonant frequency of the LC circuit. Well, this is because the actual frequency of oscillation is a little lower because of the resistive loading and junction capacitances of the transistor.

Also, like all oscillators, the active component’s amplification needs to be slightly larger than the diminution of the capacitive voltage divider, to achieve stable operation. For example, if a Colpitts oscillator is in use as a VFO (variable-frequency oscillator), it will perform best when tuning is via variable inductance. The performance of the Colpitts oscillator will also be in contrast if tuning is via one of the two capacitors. However, if such tuning is a requirement, do so by adding a third capacitor in parallel to the inductor.

The Colpitts oscillator is distinctive since it derives its feedback from what we call a center-tapped capacitance. However, it is actually a voltage divider composed of two series capacitors. Furthermore, a Colpitts oscillator’s functionality permits its use as a VFO, such as in a superheterodyne receiver or even a spectrum analyzer if utilizing a variable inductor.

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