Clapp Oscillator: Frequency Formula And Circuit Diagram

What is a Clapp Oscillator?

Clapp Oscillator is actually a modified version of Colpitt’s oscillator. The modification is made in the feedback network of Colpitt’s Oscillator. The below figures show the modification and original circuit of Colpitt’s and Claspp Oscillator.

Clapp Oscillator

How does C3 improve the Frequency Stability?

  • Referring to the above figure, we can write that the effective capacitance C is given by,

\mathbf{\frac{1}{C} = \frac{1}{C_1}+ \frac{1}{C_2}+\frac{1}{C_3}}

  • The oscillator frequency is given by,

\mathbf{f = \frac{1}{2\pi \sqrt{CL_3}}}

  • The value of C is approximately equal to C3 because the value of C3 is much smaller than C1 and C2.

∴ C ≈ C3

  • As the capacitor C3 is not dependent on any of the transistor parameters, it remains constant. This will keep the frequency of oscillations remain constant.

Also Read: Crystal Oscillator: Circuit, Frequency & Working Principle

Clapp Oscillator Using Transistor

The Clapp oscillator using a transistor is shown in the figure below.

Clapp Oscillator Using Transistor

R1, R2, and RE are used for biasing the transistor. Note the addition of C3 in the feedback network of the oscillator. The oscillator frequency is given by,

\mathbf{f = \frac{1}{2\pi \sqrt{CL_3}} \approx \frac{1}{2\pi \sqrt{L_3C_3}} }

Where, \mathbf{C = \frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3} \approx C_3}

It is possible to vary the frequency of oscillations by keeping C3 variable. The biggest advantage of the Clapp oscillator over the other LC oscillators is its frequency stability. For further improvement in the frequency stability, we can use another oscillator called ”Crystal oscillator”.

Also Read: What is Oscillator? Types of Oscillators

Clapp Oscillator Using FET

The circuit diagram of a Clapp oscillator using FET is shown in the below figure.

Clapp Oscillator Using FET

The Clapp oscillator is a variation of Colpitt’s oscillator.

The basic difference is that an additional capacitor C3 is in series with the inductor in the resonant feedback circuit as shown in the figure.

The type of FET amplifier used here is a common source amplifier with bypassed source resistance R3 and self-biasing (resistance R1).

C3 appears in series with C1 with C2, hence total capacitance in the tank circuit is given by,

\mathbf{C_T = \frac{1}{\frac{1}{C_1}+\frac{1}{C_2}+\frac{1}{C_3}}}

If Q of the tank circuit (resonant circuit) is high (typically greater than 10) then the frequency of oscillations is given by,

\mathbf{f_r \approx \frac{1}{2\pi \sqrt{LC_T}}}

C3 is normally very small as compared to C1 and C2. Therefore the value of CT is almost equal to C3. Hence the frequency of oscillations of the Clapp oscillator is almost entirely controlled by C3. Hence,

\mathbf{f_r \approx \frac{1}{2\pi \sqrt{LC_3}} …. [since C_T \approx C_3]]}

Why does the Clapp Oscillator have better stability?

  • One end of C1 and C2 is connected to the ground. Hence the junction capacitance of the transistor and other stray capacitances appear in parallel with C1 and C2 to ground.
  • So values of C1 and C2 will get altered due to the junction and stray capacitance.
  • But the value of C3 is not affected in this manner and the frequency of oscillations depends on the capacitor C3.
  • Hence Clapp oscillators have better frequency stability.


What is a Clapp oscillator?

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.

What are the disadvantages of Clapp oscillators?

Because of inductor L circuit becomes bulky and the cost of the circuit is more.
Poor frequency stability.
Difficult to adjust feedback as capacitor values have to be changed.
Poor Isolation (Load impedance v/s frequency).
Hard to design

What are the advantages of Clapp oscillators?

The Clapp Oscillator possesses higher frequency stability than other oscillators.

Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

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