IN tuned oscillator it uses an LC (inductor-capacitor) tank circuit, a frequency-selective RC (resistor-capacitor) circuit, or a quartz crystal circuit in its feedback path. In general the output waveform of a tuned oscillator circuit is sinusoidal and to make this happen positive feedback is used around an amplifying device such as a transistor or op amp.

If negative feedback is applied to an amplifier, the amplifier’s gain decreases but stability increases. However, with positive feedback, the gain increases but the stability decreases. This increase in gain produces a situation where an AC sinusoidal output is obtained with no signal input. The amplifier has now been converted to an oscillator providing an AC output and the power required to maintain this oscillation is drawn from the DC supply.

A tank circuit consisting of either a parallel tuned LC circuit or an RC circuit is used as a frequency determination unit that is “tuned” to give oscillations around its resonant frequency, hence the name tuned oscillator. The output of this device is feedback to its own input in such a way that the feedback signal helps the change in the input signal. No input signal is required because the frequency determination unit provides its own signal through the feedback network in such a way that the circuit is self-exciting. So this type of circuit is generally known as a Feedback Oscillator (positive feedback) and the oscillators that use this technique are:

LC Oscillators: As the name implies, LC oscillators consist of a parallel tuned inductor-capacitor tank circuit as their frequency determination unit. The capacitor is constantly charging and discharging through the inductor coil at its selected resonant frequency, but due to large losses in the coil resistive element, the capacitor dielectric, and in circuit radiation. So, in a practical LC circuit, the amplitude of the oscillating voltage decreases every half cycle and these oscillations would eventually reduce to zero. If enough power is applied at the right time from a DC power source in the cycle to overcome these losses, the oscillations will continue at a constant frequency and amplitude indefinitely. The resonant frequency occurs when the inductive reactance of the coil (XL) is equal to that of the capacitive reactance (XC). Oscillations are controlled by varying the value of the capacitor (varactor).

Tuned oscillator circuits that use an LC (inductor/capacitor) tank circuit include:

  • Hartley Oscillator
  • Clapp Oscillator
  • Colpitts Oscillator
  • tuned collector oscillator
  • piercing oscillator
  • miller oscillator

RC Oscillators: RC oscillators are also known as “phase shift oscillators” because their oscillating elements are made up of resistor-capacitor circuits that produce a phase shift circuit that corresponds to positive feedback. RC networks are not naturally oscillating circuits, but become oscillating elements when wired around transistors or op-amps.

RC oscillators do not use inductors, but instead produce oscillations at a frequency where the RC network rotates 180 degrees. phase change. A single stage amp will produce 180 degrees. phase shift between its input and output and that can be used as a stage to produce the required positive feedback. The output of the amplifier is fed back through the RC network to its input. The input is shifted 180 degrees. through the amplifier and 180 degrees. through the RC network and 180 degrees. + 180 degrees = 360 degrees or zero phase shift.

A useful property of the RC oscillator is that the output frequency is inversely proportional to the capacitance, which means that a change in capacitance produces a much higher frequency compared to the LC oscillator. However, the disadvantages are that the output power of the RC oscillator is low due to dissipation in the resistive elements and for positive feedback to occur the amplifier gain must be greater than 29.

Tuned oscillator circuits that use an RC (resistor/capacitor) phase shift circuit include:

  • Ladder Phase Shift Oscillator
  • relaxation oscillator
  • quadrature oscillator
  • Wein Bridge Oscillator
  • Switched Capacitor Oscillator
  • digitally switched oscillators
  • Phase advance oscillator (current transfer)
  • Phase Delay Oscillator (Voltage Transfer)

Crystal Oscillators: Quartz and some other crystalline substances exhibit the “piezoelectric” effect. When mechanical stress or physical strain is applied to two surfaces of a properly cut crystal, a voltage will be produced between the surfaces. Also, when a voltage is applied to the crystal, it causes a small physical deformation in the actual shape of the crystal.

So if the voltage produced by mechanical deformation is fed back in some way, it will produce mechanical distortion in the crystal which will produce a voltage that… will go on forever. This forms the basis of various crystal oscillators, because this feedback occurs only at the natural frequency of vibration of the crystal, and this natural frequency value is determined by the “cutoff” of the crystal. So the crystal actually behaves like a resonant circuit with a very narrow bandwidth.

There is a limit to the stability and frequency that can be obtained from normal LC or RC tuned oscillators. Quartz crystal oscillators operate at very high frequencies of up to 10 Mhz when operating in parallel mode. They also have very high stability and a resonant frequency with a very high Q factor, making them ideal for use in CPU, microcontroller, and video applications.

“Untuned Oscillators”

Unlike previous tuned oscillators, an untuned oscillator has no LC tank circuit, frequency selective RC, or crystal circuit in its feedback path. In contrast, a non-tuned oscillator uses non-linear feedback, and typically the output waveform of a non-tuned oscillator is non-sinusoidal, such as a square wave, triangle wave, or pulse, and is characterized by a sudden transition from one steady state or condition to the next. Untuned oscillators are more commonly known as relaxation oscillators. Non-tuned oscillator types include:

Ring Oscillator: Ring oscillators consist of an “odd” number of logic gates or amplifiers connected together in a series chain, so that the output of the latter is connected to the input of the former, producing a ring-type circuit. The oscillation frequency depends on the ratio delay of the components used and the number of odd “stages” found within the ring. The oscillation frequency is very high in terms of power consumption. Ring oscillators are more of a novelty as their high frequency and use of components make them impractical as an oscillator.

Relaxation oscillators: Relaxation oscillators are more commonly known as multivibrators. They are a class of oscillator in which the active devices (usually a transistor) in the circuit are driven well past their cutoff and saturation regions for a period of time. Relaxation oscillators are cheap and easy to build with the three main types of multivibrators.

  • Astable multivibrator: – does not have a stable state.
  • Monostable Multivibrator: – has a stable state.
  • Bistable Multivibrator: – It has two stable states.

555 and Timer Chips: In addition to our old favourite, the NE555 timer and its variations, there are a host of different chips available in both TTL and CMOS that can be used to generate a variety of different waveforms and signals, some of the most popular being the 74LS121, 74LS123, 74LS221 and their variants.


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