Phase Shift Oscillator

There are many things in life I do-not understand, but one of the simpler ones is the phase shift oscillator implemen ted using an opamp. Don´t get me wrong - the circuit I-understand perfectly. The bit I don´t understand is how come.

In its heyday, this circuit was used almost anywhere a simple sine wave-oscillator was needed, and I have seen it made with-valves, transistors and even FETs. What I had not seen until I designed one was a phase shift oscillator using an opamp. As it transpires, although I-had not seen this done, my trusty editor in the UK has.

He has seen it in a number of publications including John Linsley-Hood´s "The Art of Linear Electronics". JLH also supplies the equation for frequency calculation ...

Fo = √ 6 / (2 * ? * C * R)

opamp Phase Shift Oscillator Circuit
Phase Shift Oscillator

Loop gain- must be 29.25 dB , and lacking further information I must assume that the formula only, applies if all resistors and capacitors are equal, and gain would be the minimum required for the circuit to oscillate. 

The frequency stability of this circuit is quite good, but as with all phase shift oscillators the amplitude varies when the_frequency is changed. any resistor can be varied tochange the frequency, and the use of a pot allows continuous variation over a 5:1 range (or more if you experiment with the component values).

This is a perfect example of an opamp being unable to obey Rule #1, and its operation is governed by Rule #2. The circuit is essentially unstable, and the opamp is always trying to play catch-up (without success, or the circuit would stop oscillating).

The frequency is a cow to determine if different_values are used for R or C, and although I believe there is a formula, it is apparently a very tedious process (I´ve not seen it myself). The circuit shown above will run at about 420Hz, with a sinewave output of around 500mV (with +/-15V supplies) - although JLH´s formula seems to indicate that it should oscillate at 390Hz If you really want to know, you will have to build one. Changing the value of any resistor or capacitor will change both frequency and amplitude. The square wave at the output is at almost the full +/-15V supply voltage (limited only by the output circuit of the opamp).

The sinewave shown on the oscilloscope trace is obtained from the Vout terminal, and the square wave is obtained from the opamp´s output (these are not to scale). The string of resistors and caps acts as a phase shift network, and oscillation takes place at that frequency where there is an exact 180 degree shift, converting negative feedback into positive feedback. The circuit is stable at DC, since it has negative feedback through the string of resistors.

Let´s have a look at how it works. Remember Rule #2? Now have a look at the signal at the inverting input. As you can see, the output takes the polarity of the most positive input, so when the -in terminal is positive, the output is negative. Over a period of time based on the resistance and capacitance, the voltage on the -in terminal will fall towards zero volts, and will eventually become negative - the output promptly swings positive, and the cycle repeats. Like all filter circuits, the resistor / capacitor (R/C) network introduces a time delay, and it is this (plus the simple low-pass filter formed) that produces a sinewave with less than 1% distortion. By no means wonderful, but quite adequate for a number of simple applications.

The sinewave output is at relatively high impedance, and should be buffered with an opamp before use. Any loading will _alter_both amplitude and frequency.

Keywords : Opamp, Phase, Shift, Oscillator Op-amp, Amplifiers, Amplifier, Amplificator, Amplification, Info, Tutorial
Writer : delon  |
13 Mar 2006 Mon   
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