Colpitts oscillator (English: Colpitts oscillator), also known as Kopitz oscillator, capacitance three-point typeoscillator, capacitanceFeedback oscillator, by the United StatesElectrical engineerOne invented by Edwin Kopitz in 1918lc oscillator (An electronic oscillator that uses a combination of capacitance and inductance to determine the oscillation frequency) design.The characteristic of Colpitts oscillator is that the feedback of active components comes from aseries connectionA voltage divider consisting of two capacitors.
Like otherslc oscillator Similarly, Colpitts circuit consists of a gain device (such asBipolar junction transistor、Field effect transistor、Operational amplifierorVacuum tube）The output of is connected to its input, and the feedback loop contains a LC parallel circuit (tuning circuit) as a bandpasswave filterFixed oscillation frequency.Colpitts oscillator can be regarded as the dual of Hartley oscillatorHartley oscillatorThe middle feedback signal comes from the "inductive" voltage divider composed of two coils in series (or a tap coil).Figure 1 shows the common base Colpitts circuit.The frequency of the oscillator is determined by the parallel resonant circuit composed of L and C1 and C2 in series.The voltage at both ends of C2 is applied to the base emitter junction of the transistor as feedback to generate oscillation.Figure 2 shows the common collector version.Here, the voltage at both ends of C1 provides feedback.The oscillation frequency is about the resonance frequency of the LC circuit (that is, the series combination of two capacitors and inductors),
Due to the junction capacitance andtransistorThe actual frequency of oscillation will be slightly reduced due to resistive load.
Like any oscillator, in order to work stably, the amplification of active components should be slightly greater thanCapacitive voltage dividerAttenuation of.Therefore, variableinductanceWhen the variable frequency oscillator is tuned, compared with adjusting one of the two capacitors, the Colpitts oscillator can achieve the best performance.If requiredVariable capacitorFor tuning, the third capacitor should be connected in parallel with the inductor (or connected in series as in the Clapper oscillator).
example
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Figure 3 shows a working example marked with device parameters.exceptBipolarTransistors can also be used to generate active devices such as field effect tubes or vacuum tubes at the required frequency.
The base capacitance provides an AC path to ground for parasitic inductance that may produce unwanted frequencies.The selection of base bias resistor is not simple.The periodic oscillation starts when the critical bias current is reached, and will reach a higher value with the change of the bias current, which will be observedchaososcillation[1]。
analysisoscillatorOne of the methods of is to calculate the input impedance corresponding to one of the input terminals when the feedback effect is ignored. If the calculated input impedance is negative, oscillation may occur.The following uses this method to determine the oscillation conditions and oscillation frequency.
On the right is an ideal model.This model uses the common collector amplifier mentioned in the previous section.At first, the influence of parasitic capacitance or other nonlinear components is ignored, and then these items are replaced for more accurate calculation after the analysis is completed.Although it seems that a lot of things have been ignored, the calculated solution is still acceptable after compared with the experimental results.
Ignore inductance, so inputimpedanceIt can be written as:
The expression of can be obtained by substituting:
The input impedance looks like the impedance of two capacitors in series with a wonderful term.because
It is proportional to the impedance product of two capacitors:
if
And
Is the plural number with the same number,
It will be negative resistance.if
And
with
and
Substitution
：
If the inductance is connected to the input, the circuit will start to oscillate when the absolute value of the negative impedance is greater than the impedance of the inductance.The oscillation frequency can be seen in the expression in the previous section.
Taking the previous oscillator as an example, the emitter current is about 1 mA.The transconductance is about 40 milliSiemens. By substituting the above expression, the input impedance is about:
The absolute value of negative impedance in the formula is enough to exceed any resistance in the circuit.During checking calculation, it will be found that oscillation is more likely to occur under larger transconductance and smaller capacitance.A more complex analysis of the common base oscillator shows that a low-frequency amplifier can oscillate only when its voltage gain is at least four.Low frequency gain is:
If these two capacitors are replaced by inductors and the effect of magnetic coupling between inductors is ignored, the circuit becomes a Hartley oscillator.Thus, the input impedance is the sum of two inductance values, and the negative impedance can be written as:
In the circuit of Hartley oscillator, oscillation is more likely to occur under larger transconductance and larger inductance.
Interestingly, the above analysis can also describePierce oscillatoract.Pierce oscillator, with two capacitors and one inductance, equivalent to Colpitts oscillator[2]。It can be proved by setting the grounding point between two capacitors.The electrical duality of a standard Pierce oscillator with two inductors and one capacitorHartley oscillatorEquivalence.
Oscillation amplitude
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The amplitude of oscillation is generally difficult to predict, but it can often be accurately estimated using the description function method.
For the common reference oscillator in Figure 1, this method can be applied to a simplified model to predict the output (collector) voltage amplitude:
among
Is the bias current, while
Is the load resistance of the collector.
Here it is assumed that the transistor is unsaturated, the collector current flows in a narrow pulse, and the output voltage is sinusoidal (low distortion).
This approximation also applies to oscillators with different active devices, such as MOSFETs andVacuum tube。
