Oscillator Basics
The basic condition of oscillation is positive feedback, shown in the accompanying figure. Here (A) represents the feed-forward amplifier or loop gain; (β) represents the feedback network; and (Σ) represents the means of generating the feedback signal. With the right circuit configuration and conditions, there will be a sustained periodic oscillation with a well-defined frequency at the output. For oscillation to start, the voltage gain around the loop needs to be greater than 1. To maintain sustained oscillation, the closed-loop needs to be reduced to 1. With an op-amp's high-performance parameters, it is a good candidate to be used in constructing the oscillator circuit. Note that although op-amp oscillators offer decent temperature coefficients, power efficiency, and a wide range of operating frequencies, nonetheless quartz, ceramic resonators, and RC relaxation-type clock oscillators are common choices for providing microcontroller clock signals. In summary, there are two conditions that must be met to sustain oscillation:
- The phase shift around the feedback loop must be effectively 0°.
- The closed loop gain (Acl) must equal 1 (unity).
Feedback oscillators can be used to generate sinusoidal waves. Each internal stage of the oscillator affects the frequency response and adds to the overall phase lag between the input and the output.
The figure above is an op-amp used in a positive feedback configuration as a comparator, as discussed in the comparator page. The transfer function of the oscillator, i.e. Vout / Vin is shown below.
With the right amount of feedback attenuation and phase shift, a well-defined oscillation frequency can be achieved. The figure below demonstrates the concept of producing oscillation with positive feedback and phase shift. A sustained oscillation is initiated by any noise picked up in the system, including the power supply transients.
If the product of the loop gain (A) and feedback (β) equals 1, then the closed loop gain is 1, and the phase shift around the loop is 0 degrees. The figure below shows the conditions for oscillation.
