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Colpitts oscillator
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==Overview== {| align="left" | valign="top" | [[Image:Cb colp.svg|thumb|130px|Figure 1: Simple [[common-base]] Colpitts oscillator (with simplified [[biasing]])]] | valign="top" | [[Image:Cc colp2.svg|thumb|130px|Figure 2: Simple [[common-collector]] Colpitts oscillator (with simplified [[biasing]])]] |} The Colpitts circuit, like other LC oscillators, consists of a gain device (such as a bipolar junction [[transistor]], field-effect transistor, operational amplifier, or [[vacuum tube]]) with its output connected to its input in a [[feedback loop]] containing a parallel [[LC circuit]] ([[tuned circuit]]), which functions as a [[bandpass filter]] to set the frequency of oscillation. The amplifier will have differing input and output impedances, and these need to be coupled into the LC circuit without overly damping it. A Colpitts oscillator uses a pair of capacitors to provide voltage division to couple the energy in and out of the tuned circuit. (It can be considered as the electrical dual of a [[Hartley oscillator]], where the feedback signal is taken from an "inductive" voltage divider consisting of two coils in series (or a tapped coil).) Fig. 1 shows the common-base Colpitts circuit. The inductor ''L'' and the series combination of ''C''<sub>1</sub> and ''C''<sub>2</sub> form the resonant [[tank circuit]], which determines the frequency of the oscillator. The voltage across ''C''<sub>2</sub> is applied to the base-emitter junction of the transistor, as feedback to create oscillations. Fig. 2 shows the common-collector version. Here the voltage across ''C''<sub>1</sub> provides feedback. The frequency of oscillation is approximately the resonant frequency of the LC circuit, which is the series combination of the two capacitors in parallel with the inductor: :<math>f_0 = \frac{1}{2\pi \sqrt{L \frac{C_1 C_2}{C_1 + C_2}}}.</math> The actual frequency of oscillation will be slightly lower due to junction capacitances and resistive loading of the transistor. As with any oscillator, the amplification of the active component should be marginally larger than the attenuation of the resonator losses and its voltage division, to obtain stable operation. Thus, a Colpitts oscillator used as a [[variable-frequency oscillator]] (VFO) performs best when a variable inductance is used for tuning, as opposed to tuning just one of the two capacitors. If tuning by variable capacitor is needed, it should be done with a third capacitor connected in parallel to the inductor (or in series as in the [[Clapp oscillator]]). ===Practical example=== [[File:Colpitts Osz JFET Gate.gif|frame|Figure 3: Practical common-gate Colpitts oscillator with an oscillation frequency of ~10 MHz]] Fig. 3 shows an example with component values.<ref>{{cite book | last = Hayward | first = Wes | title = Introduction to Radio Frequency Design | chapter = Figure 7.12 Colpitts oscillators using a JFET | publisher = ARRL | year = 2004 | location = US | page = 281 | url = https://archive.org/details/isbn_9780872594920 | doi = | id = | isbn = 0-87259-492-0}}</ref> Instead of [[field-effect transistors]], other active components such as [[bipolar junction transistor]]s or [[vacuum tube]]s, capable of producing gain at the desired frequency, could be used. The [[common-gate|common gate amplifier]] has a low input impedance and a high output impedance. Therefore the amplifier input, the source, is connected to the low impedance tap of the LC circuit L1, C1, C2, C3 and the amplifier output, the drain, is connected to the high impedance top of the LC circuit. The resistor R1 sets the [[operating point]] to 0.5mA drain current with no oscillating. The output is at the low impedance tap and can drive some load. Still, this circuit has low [[Distortion#Harmonic_distortion|harmonic distortion]]. An additional [[variable capacitor]] between drain of J1 and ground allows to change the frequency of the circuit. The load resistor RL is part of the simulation, not part of the circuit.
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