IEEE Circuits and Systems Magazine - Q4 2020 - 9

We review different types of Colpitts oscillators, including singleended, differential, complementary, gm-boosted and introduce a
timeline for the development of these circuits.

I. Introduction
n February 1918, the Canadian American engineer Edwin H. Colpitts, filed a patent titled OSCILLATION GENERATOR, on a simple oscillator circuit [1] today known
as the Colpitts oscillator. Colpitts proposed three different
configurations of the oscillator in his patent, all realized
using a single vacuum tube [2]-[4], the most popular one
is shown in Fig. 1. The circuit from the patent redrawn
using modern circuit representation is shown in Fig. 2,
where ground is used as reference node and chokes
used instead of resistors. He proposed the circuit probably as a modification of the Hartley oscillator patented
three years earlier [5] by his colleague Ralph Hartley at

9
8
6

American Bell Telephone Company in Boston, Massachusetts [6]. The oscillators developed by Colpitts and Hartley were among the first single vacuum tube oscillators,
which made the systems more compact, simpler in design
and less power consuming than earlier oscillators, as
vacuum tubes are bulky and power-hungry components.
Both oscillators are feedback type, made of a gain stage A
and a feedback LC tank b, as shown in Fig. 3. The main difference between these two is in the way the LC tank in the
feedback is configured [7]. The LC tank in Hartley uses a
split inductor and the Colpitts uses a split capacitor, as
shown in Fig. 4. Depending on the amplifier configuration
and the direction of the feedback, various configuration
of these oscillators with different properties can be designed as presented in following Sections.
Colpitts and Hartley oscillators became very popular
both in industry and academia due to their simplicity,
ease of operation and superior stability. In early literature these circuits were already referred to as Colpitts
and Hartley oscillators [2] and were widely used in audio and communication applications [8]-[12]. Colpitts
however, became the preferred choice when used in

17
A

16
20
21

R
22

-A

Figure 1. Reprint from the patent of the Colpitts oscillator [1].

Z3
Z1

Z2
Î²

Choke 2

Figure 3. A generalized model of a Colpitts or Hartley oscillator, the gain stage can be either non-inverting or inverting
depending on the configuration of the feedback.
+
C

L
C1

Choke 1

Figure 2. Original Colpitts oscillator in Fig. 1 redrawn using
a modern circuit representation method and added chokes.
Inductor L, Capacitors C1 and C2 form the tank. Cb is a large
coupling capacitor.

(a)

(b)

Figure 4. Feedback in (a) Colpitts configuration, (b) Hartley
configuration.

Mehdi Azadmehr and Luca Marchetti are with the department of Microsystems, university of South-Eastern Norway, Horten, 3184 Norway, e-mail:mehdi
.azadmehr@usn.no. and Igor Paprotny is with department of Electrical and Computer Engineering, University of Illinois at Chicago.
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