Clapp versus Colpitts

Edwin Henry Colpitts (January 19, 1872 – March 6, 1949)
James Kilton Clapp (December 03, 1897 – 1965)

The two persons above are the geniuses who gave us two classic oscillator circuits as shown in Figure 1.

Figure 1 The two classic oscillators circuits: Colpitts (left) and Clapp (right).

We’ve looked at these two oscillators individually before in “The Colpitts oscillator” and “Clapp oscillator”.

However, a side-by-side examination of the two oscillators is additional time well spent.

The Clapp oscillator was devised as an improvement over the Colpitts oscillator by virtue of adding one capacitor, C3, in the above image.

The amplifier “A” is nominally at a gain value of unity, but as a matter of practicality, the gain value is slightly lower than that because the amplifier is really a “follower”. If made with a vacuum tube, then “A” is a cathode follower. If made with a bipolar transistor, then “A” is an emitter follower. If made with a field effect transistor, then “A” is a source follower. The concept itself remains the same.

Each oscillator works because the RLC network develops a voltage step-up at the frequency of oscillation. The “R” is not an incorporated component though. The “R” (R1 or R2) simply represents an output impedance of the follower. The 10 ohms that we see here is purely an arbitrary value guess on my part. The other components are also of arbitrary value choices, but they are convenient values for illustrating just how these little beasties work.

We use SPICE simulations to examine the transfer functions of the two RLC networks as shown in Figure 2.

Figure 2 Colpitts versus Clapp spice simulations using the transfer functions of the two RLC networks.

Each RLC network has a peak in its frequency response which will result in oscillation at that peak frequency. However, the peak of the Clapp circuit is much sharper and narrower than that of the Colpitts circuit. This narrowing has the beneficial effect of suppressing spectral noise centered around the oscillation frequency.

Note in the examples above that the oscillation peaks differ by 0.16% and that the reactance of the L1 inductor and the reactance of the L2 C3 pair differ by 1.12%. That’s just a matter of my having chosen some convenient numbers with the intent of having the two curves match in that regard at the same peak frequency. (I almost succeeded.)

The Clapp oscillator has several advantages over the Colpitts oscillator. The transfer function peak of the Clapp circuit is narrower than that of the Colpitts which tends to yield an oscillator output with less spurious off-frequency energy meaning a “cleaner” signal.

Another advantage of the Clapp circuit is that capacitors C4 and C5 can be made very large as the L2 C3 combination is made to look like a very small inductance value at the oscillation frequency. The larger C4 and C5 values mean that any variations of those capacitance values brought about by variations of the input capacitance of the “A” stage have a minimal effect on the oscillation frequency.

That’s because frequency control of the Clapp circuit is primarily set by the series resonance of the L2 C3 pair rather than the parallel resonance of L1 versus the C1 C2 pair in the Colpitts circuit. If the “A” input capacitance tends to vary for this reason or that, the Clapp circuit is far less prone to an unwanted frequency shift as shown in Figure 3.

Figure 3 A Clapp versus Colpitts frequency shift comparison showing how the Clapp circuit (right) is far less prone to this unwanted shift in frequency.

John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).

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