Question

a)Explain the statement 'The external RC network makes the system oscilliate' means.

b)Draw an oscilliating waveform

Answer 1

In an RC Oscillator circuit, the input is shifted 180^o through the amplifier stage and 180^oagain through a second inverting stage giving us “180^o + 180^o = 360^o” of phase shift which is effectively the same as 0^o thereby giving us the required positive feedback. In other words, the phase shift of the feedback loop should be “0”.

In a Resistance-Capacitance Oscillator or simply an RC Oscillator, we make use of the fact that a phase shift occurs between the input to a RC network and the output from the same network by using RC elements in the feedback branch, for example.

The circuit on the left shows a single resistor-capacitor network whose output voltage “leads” the input voltage by some angle less than 90^o. An ideal single-pole RC circuit would produce a phase shift of exactly 90^o, and because 180^o of phase shift is required for oscillation, at least two single-poles must be used in an RC oscillator design.

However, in reality it is difficult to obtain exactly 90^o of phase shift so more stages are used. The amount of actual phase shift in the circuit depends upon the values of the resistor and the capacitor, and the chosen frequency of oscillations with the phase angle (?) being given as:

Where: X_C is the Capacitive Reactance of the capacitor, R is the Resistance of the resistor, and ƒ is the Frequency.