A Colpitts oscillator, invented in 1918 by American engineer Edwin H. Colpitts,[1] is one of a number of designs for LC oscillators, electronic oscillators that use a combination of inductors (L) and capacitors (C) to produce an oscillation at a certain frequency. The distinguishing feature of the Colpitts oscillator is that the feedback for the active device is taken from a voltage divider made of two capacitors in series across the inductor.[2][3][4][5]
The Clapp oscillator is actually a modified version of the Colpitts oscillator. The Clapp oscillator is a Colpitts oscillator that has an additional capacitor placed in series with the inductor. It was published by James Kilton Clapp in 1948. The Oscillator DesignGuide contains templates that can be used in the ADS software environment. It consists of generic colpitts, clapp, modified colpitts, modified clapp, and hartley oscillator design examples, and a library of components and component characterization tools. The Pierce oscillator (Figure 6) is a series resonant tuned circuit. Capacitors C2 and C3 are used to stabi-lize the amount of feedback preventing overdrive to the transistor amplifier. The Pierce oscillator has many desirable characteris-tics. It will operate over a large range of frequencies and has very good short-term stability 6. Sep 04, 2018 Clapp oscillator is a variation of Colpitts oscillator in which an additional capacitor (C 3) is added into the tank circuit to be in series with the inductor in it, as shown by Figure 1. Apart from the presence of an extra capacitor, all other components and their connections remain similar to that in the case of Colpitts oscillator. Tuned base oscillator is a kind of LC transistor oscillator where the tuned circuit is placed between the base and ground of the transistor. The primary coil of a transformer and a capacitor forms the tuned circuit.
Overview[edit]
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.
A Colpitts oscillator is 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. L and the series combination of C1 and C2 form the parallel resonant tank circuit, which determines the frequency of the oscillator. The voltage across C2 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 C1 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:
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 capacitive voltage divider, 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 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[edit]
Figure 3: Practical[dubious] common-base Colpitts oscillator with an oscillation frequency of ~50 MHz
Fig. 3 shows a working example with component values. Instead of bipolar junction transistors, other active components such as field-effect transistors or vacuum tubes, capable of producing gain at the desired frequency, could be used.
The capacitor at the base provides an AC path to ground for parasitic inductances that could lead to unwanted resonance at undesired frequencies.[6] Selection of the base's biasing resistors is not trivial. Periodic oscillation starts for a critical bias current and with the variation of the bias current to a higher value chaotic oscillations are observed.[7]
Theory[edit]
Ideal Colpitts oscillator model (common-collector configuration)
One method of oscillator analysis is to determine the input impedance of an input port neglecting any reactive components. If the impedance yields a negative resistance term, oscillation is possible. This method will be used here to determine conditions of oscillation and the frequency of oscillation.
An ideal model is shown to the right. This configuration models the common collector circuit in the section above. For initial analysis, parasitic elements and device non-linearities will be ignored. These terms can be included later in a more rigorous analysis. Even with these approximations, acceptable comparison with experimental results is possible.
Ignoring the inductor, the input impedance at the base can be written as
where v1{displaystyle v_{1}} is the input voltage, and i1{displaystyle i_{1}} is the input current. The voltage v2{displaystyle v_{2}} is given by
where Z2{displaystyle Z_{2}} is the impedance of C2{displaystyle C_{2}}. The current flowing into C2{displaystyle C_{2}} is i2{displaystyle i_{2}}, which is the sum of two currents:
where is{displaystyle i_{s}} is the current supplied by the transistor. is{displaystyle i_{s}} is a dependent current source given by
where gm{displaystyle g_{m}} is the transconductance of the transistor. The input current i1{displaystyle i_{1}} is given by
where Z1{displaystyle Z_{1}} is the impedance of C1{displaystyle C_{1}}. Solving for v2{displaystyle v_{2}} and substituting above yields
The input impedance appears as the two capacitors in series with the term Rin{displaystyle R_{text{in}}}, which is proportional to the product of the two impedances:
If Z1{displaystyle Z_{1}} and Z2{displaystyle Z_{2}} are complex and of the same sign, then Rin{displaystyle R_{text{in}}} will be a negative resistance. If the impedances for Z1{displaystyle Z_{1}} and Z2{displaystyle Z_{2}} are substituted, Rin{displaystyle R_{text{in}}} is
If an inductor is connected to the input, then the circuit will oscillate if the magnitude of the negative resistance is greater than the resistance of the inductor and any stray elements. The frequency of oscillation is as given in the previous section.
For the example oscillator above, the emitter current is roughly 1 mA. The transconductance is roughly 40 mS. Given all other values, the input resistance is roughly
This value should be sufficient to overcome any positive resistance in the circuit. By inspection, oscillation is more likely for larger values of transconductance and smaller values of capacitance. A more complicated analysis of the common-base oscillator reveals that a low-frequency amplifier voltage gain must be at least 4 to achieve oscillation.[8] The low-frequency gain is given by
Comparison of Hartley and Colpitts oscillators
If the two capacitors are replaced by inductors, and magnetic coupling is ignored, the circuit becomes a Hartley oscillator. In that case, the input impedance is the sum of the two inductors and a negative resistance given by
In the Hartley circuit, oscillation is more likely for larger values of transconductance and larger values of inductance.
The above analysis also describes the behavior of the Pierce oscillator. The Pierce oscillator, with two capacitors and one inductor, is equivalent to the Colpitts oscillator.[9] Equivalence can be shown by choosing the junction of the two capacitors as the ground point. An electrical dual of the standard Pierce oscillator using two inductors and one capacitor is equivalent to the Hartley oscillator.
Oscillation amplitude[edit]
The amplitude of oscillation is generally difficult to predict, but it can often be accurately estimated using the describing function method.
For the common-base oscillator in Figure 1, this approach applied to a simplified model predicts an output (collector) voltage amplitude given by[10]
where IC{displaystyle I_{C}} is the bias current, and RL{displaystyle R_{L}} is the load resistance at the collector.
This assumes that the transistor does not saturate, the collector current flows in narrow pulses, and that the output voltage is sinusoidal (low distortion).
This approximate result also applies to oscillators employing different active device, such as MOSFETs and vacuum tubes.
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References[edit]
Further reading[edit]
Retrieved from 'https://en.wikipedia.org/w/index.php?title=Colpitts_oscillator&oldid=917405022'
The Clapp oscillator or Gouriet oscillator is an LC electronic oscillator that uses a particular combination of an inductor and three capacitors to set the oscillator's frequency. LC oscillators use a transistor (or vacuum tube or other gain element) and a positive feedback network. The oscillator has good frequency stability.
History[edit]
The Clapp oscillator design was published by James Kilton Clapp in 1948 while he worked at General Radio.[1] According to VaÄkáÅ, oscillators of this kind were independently developed by several inventors, and one developed by Gouriet had been in operation at the BBC since 1938.[2]
Circuit[edit]
Clapp oscillator (direct-current biasing network not shown)
The Clapp oscillator uses a single inductor and three capacitors to set its frequency. The Clapp oscillator is often drawn as a Colpitts oscillator that has an additional capacitor (C0) placed in series with the inductor.[3]
The oscillation frequency in Hertz (cycles per second) for the circuit in the figure, which uses a field-effect transistor (FET), is
The capacitors C1 and C2 are usually much larger than C0, so the 1/C0 term dominates the other capacitances, and the frequency is near the series resonance of L and C0. Clapp's paper gives an example where C1 and C2 are 40 times larger than C0; the change makes the Clapp circuit about 400 times more stable than the Colpitts oscillator for capacitance changes of C2.[4]
Capacitors C0, C1 and C2 form a voltage divider that determines the amount of feedback voltage applied to the transistor input.
Although, the Clapp circuit is used as a variable frequency oscillator (VFO) by making C0 a variable capacitor, VaÄkáŠstates that the Clapp oscillator 'can only be used for operation on fixed frequencies or at the most over narrow bands (max. about 1:1.2).'[5] The problem is that under typical conditions, the Clapp oscillator's loop gain varies as f â3, so wide ranges will overdrive the amplifier. For VFOs, VaÄkáŠrecommends other circuits. See VaÄkáŠoscillator.
References[edit]
Further reading[edit]
External links[edit]
Delay Line Oscillator
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