Oscillatori Colpitts: Yê çi ye? (Wergarên Circut û Jêrvekeriya Çalakîya Oscillatori Colpitts)

Colpitts Osîlatoreya Çi Ye?

Colpitts Osîlatoreya dike bir LC osîlatoreyê ye. Colpitts osîlatoreyan 1918an de ameryakî mühendis Edwin H. Colpittsê çêkirin. Di navbera yekîtiyan LC osîlatoreyan de, Colpitts osîlatoreyan induktoran (L) û kapasitordan (C) bikaranîn da ku bi bêyên din xwe dengîna herî tevahî bibikin. Colpitts osîlatoreyan li ser vîstina spesifikê ya wan dike ku feedbacki ji bo devayê aktîf hatine têkildar kapasitordan du derbaran ser inductor.

Ev dikare... pêche bêrîve ne.

Bêje lê Colpitts osîlatoreya circuitê bibînin da ku bixebitîn êke weşê ve were kerd.

Colpitts Osîlatoreya Circuit

Şekil 1 Colpitts osîlatoreya bi tank circuitê nîşan dide. Induktor L bi kombinasyonê serialê kapasitoran C1 û C2 (ji roja sor ên şermînan) paralel e.

Di circuitan de komponentên din yên din yên common-emitter (CE) û voltage divider network an jî hene, R_C collector resistor e, R_E emitter resistor e ku ji stabîl kirina circuitê bikiyar e, û resistoran R1 û R2 voltage divider bias network form dike.

Herî, kapasitoran Ci û Co input û output decoupling kapasitoran ne û emitter capacitor CE bypass capacitor e ku AC signals amplified bipejde.

Li vir, dema power supply ON bike, transistor bixebite da ku collector current I_C zêdetir bike, ku ji ber vê kapasitoran C1 û C2 bişike bike. Dema maqulîna bişikan pir bibe, wan dest pê ku bişikan bişike bike le inductor L.

Li virûdî vê prosesa, energy electrostatic ê ku di kapasitor de wergerandin da ku electromagnetic energy di inductor de wergerandin.

Dema inductor bipejde, kapasitoran werdigere, ev cycle davam dike, ku dengîna di tank circuit de bigire.

Herî, şekil nîşan dide ku outputi amplifier across C1 dike û ji ber vê in-phase bi voltageya tank circuit û energy lost re-supply bikin.

Li virûdî vê, feedbacki voltage transistor across capacitor C2 dike, ji ber vê feedback signal 180° out-of-phase bi voltageya transistor e.

Ji ber vê, voltages developed across the capacitors C1 û C2 opposite in polarity ne ji ber vê point where they join is grounded.

Herî, signal additional phase-shift of 180° by the transistor bikin, ji ber vê net phase-shift of 360° around the loop, satisfying the phase-shift criterion of Barkhausen principle.

Colpitts Osîlatoreya Frekanse

Li virûdî vê, circuit can effectively act as an osîlatore producing sustained oscillations by carefully monitoring the feedback ratio given by (C1 / C2). The frequency of such a Colpitts Osîlatore depends on the components in its tank circuit and is given by

Where the C_eff is the effective capacitance of the capacitors expressed as

As a result, these oscillators can be tuned either by varying their inductance or the capacitance. However, the variation of L does not yield a smooth variation.

Hence they are usually tuned by varying the capacitances which are generally ganged, due to which a change in any one of them changes both of them. Nevertheless, the process is tedious and requires a special large-valued capacitor.

Thus, the Colpitts oscillators are seldom preferred in applications wherein the frequency varies but are more popular as fixed frequency oscillators due to their simple design.

Further, they offer better stability than the Hartley Oscillators as they are exempted from the mutual inductance effect present in-between the two inductors of the latter case.

Apart from the BJT-based Colpitts Oscillator shown, they are also realizable using valves or FET (Field Effect Transistor) or Op-Amp.

Figure 2 shows such a Colpitts oscillator which uses an Op-Amp in inverting configuration in its amplifier section. At the same time, the tank circuit remains similar to that in the case of Figure 1.

This kind of circuit functions almost analogous to that of the one explained earlier. However, here the gain of the oscillator can be adjusted individually just by using the feedback resistor R_f, as the gain of the inverting amplifier is given as -R_f / R1.

From this, it can be noted that, in this case, the gain of the circuit is less dependent on the circuit elements of the tank circuit.

Typically, the operating frequency of the Colpitts oscillators ranges from 20 kHz to 300 MHz. However, they can even be used for microwave applications as their capacitors provide a low reactance path for high-frequency signals.

This results in better frequency stability as well as a better sinusoidal output waveform. Moreover, they are also extensively used as surface acoustical wave (SAW) resonators, sensors and in mobile and communication systems.