What’s the Difference Between a Tuned and an Untuned Oscillator Circuit?mbedewy
What’s the Difference Between a Tuned and an Untuned Oscillator Circuit?
A tuned oscillator uses a LC (inductor-capacitor) tank-circuit, a frequency-selective RC (resistor-capacitor) circuit or a quartz crystal circuit in its feedback path. Generally, the output waveform shape from a tuned oscillator circuit is sinusoidal and for this to 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 gain of the amplifier is decreased but the stability is increased. With positive feedback however, the gain is increased but the stability is decreased. This increase in gain produces a situation where an alternating sinusoidal output is obtained without a signal input. The amplifier has now become an oscillator giving an alternating output with the energy required to maintain this oscillation is obtained from the d.c. supply.
A tank-circuit consisting of a parallel-tuned LC circuit or RC circuit is used as the frequency determining unit which is “tuned” to give oscillations around its resonant frequency, hence the name tuned oscillator. The output from this device is feedback to its own input in such a way that the feedback signal aids the change in input signal. No input signal is required because the frequency determining unit provides its own signal via the feedback network in such a way that the circuit is self-exciting. Then this type of circuit is known generally as a Feedback Oscillator (positive feedback) and oscillators which use this technique are:
LC Oscillators: As their name implies, LC oscillators consist of a parallel tuned inductor-capacitor tank circuit as their frequency determining unit. The capacitor is constantly charging and discharging through the inductor coil at its selected resonant frequency but due to the heavy losses in the resistive element of the coil, the dielectric of the capacitor, and in radiation from the circuit. So in a practicle LC circuit the amplitude of the oscillatory voltage decreases at each half cycle and these oscillations would eventually die away to zero. If sufficient energy is applied at the appropriate time from a d.c. power supply in the cycle to overcome these losses then oscillations will continue at a constant frequency and amplitude indefinitely. Resonant frequency occurs when the coils inductive reactance (XL) equals that of the capacitive reactance (XC). Oscillations are controlled by varying the value of the capacitor (varactor).
Tuned oscillator circuits which use an LC (Inductor/Capacitor) tank circuit include:
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