The type of oscillator we are discussing in this tutorial largely depends on a resonant network that consists of 2 capacitors (series capacitance is equal C in total) and an inductor (L) connected in parallel with them. This L-C network resonates at a frequency, f = 1/2 π(LC)-1/2. The op amp is wired in the circuit as an inverting amplifier with a gain of about 30. Its non-inverting (1) input is kept at half the supply voltage (1V/2) by the two 22 kΩ resistors that are acting as a potential divider. The LC network is placed in the +ve feedback loop of the op amp. At the resonant frequency level the output coming from the op amp makes the network to resonate. The tapped point between the capacitors exists at 1V/2, but the part of the oscillating signal across C2 is fed to the inverting amplifier. It is then amplified and maintains the network oscillating strongly.
Oscillator in Action
An animated diagram of a Colpitts oscillator is present on
the Companion Site.
When we turn the circuit on first, there is no signal, but small random currents that are present in the circuit are amplified. The signals at the resonant frequency are amplified more than the others and steadily build up in amplitude until the circuit is in state of strong oscillation. If someone is using this circuit, the capacitors can be 100 nF each. For the inductor, wind up 2 or 3 turns of ordinary single-stranded connecting wire having 10 mm diameter. Neither core nor former is required.
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