[Editor's Note] Antonio Miclaus, systems applications engineer at Analog Devices, Inc., discusses a hands-on Colpitts oscillator demonstration using the ADALM2000 educational kit.


Oscillators can be implemented in many different forms. This article introduces how to build a Colpitts oscillator circuit using a tapped capacitor divider to provide a feedback path using the ADALM2000 educational kit from Analog Devices (ADI).

The Colpitts oscillator is a particularly good circuit for generating sine wave signals with low distortion in the RF range of 30 kHz to 30 MHz. The Colpitts configuration can be easily identified by the use of a tapped capacitor divider (C1 and C2 in Figure 1). The oscillation frequency can be calculated in the same way as for a typical parallel resonant circuit, using Equation 1.

Since the values of two capacitors connected in series are selected, the total series capacitance (CTOT) can be obtained by Equation 2.

Figure 1 shows a typical Colpitts oscillator. The frequency-determining parallel resonant tuning circuit consists of C1, C2, and L1, and is used as the collector load impedance of the common-base amplifier Q1. This provides high gain to the amplifier only at the resonant frequency. This configuration of the Colpitts oscillator uses a common-base amplifier. The base of Q1 is biased to the appropriate DC level by the resistor divider R1 and R2, but is connected directly to AC ground by C3.

In common-base mode, the output voltage waveform at the collector and the input signal at the emitter are in phase. This allows a portion of the output signal from the node between C1 and C2 to be fed back to the emitter from the tuned collector load, providing the required positive feedback. It is important to note that this feedback only works for alternating current (AC), there is no direct current (DC) path from collector to emitter.

The combined capacitance of C1 and C2, together with the emitter resistor R3, forms a low-frequency time constant that provides an average DC voltage level proportional to the amplitude of the feedback signal applied to the emitter of Q1. This automatically controls the gain of the amplifier to adjust the closed-loop gain of the oscillator.

As with all other oscillators, the Barkhausen criteria require that the overall gain be 1 and the phase difference from input to output be 0 degrees to sustain oscillation. Since the emitter node is used as the common-base amplifier input, the emitter resistor R3 is not isolated. The base is connected to AC ground by C3, which provides very low reactance at the oscillator frequency.

■ Simulation before experiment

First, create a simulation circuit diagram of the Colpitts oscillator shown in Figure 1. Calculate the values of the bias resistors R1 and R2 so that the collector current of the NPN transistor Q1 is approximately 1 mA when the emitter resistor R3 is set to 1 kΩ.

The circuit is assumed to be powered by a 10 V supply voltage. To keep the standing current flowing through the resistor divider as low as possible, keep the sum of R1 and R2 as large as possible (total resistance greater than 10kΩ).

Remember that C3 provides AC ground to the base of Q1.

Set the base decoupling capacitor C3 and the output AC coupling capacitor C4 to 0.1 μF.

Set L1 to 100 μH and calculate the values of C1 and C2 so that the resonant frequency approaches 500 kHz.

Perform transient response simulations. Save the results for comparison with measurements taken on the actual circuit and include them in the experimental report.

■ Materials List

- ADALM2000 Active Learning Module
- Solderless breadboard and jumper wire kit
- 1 x 2N3904 NPN transistor
- 2 x 10μH inductors
- 2 x 100μH inductors
- 1 x 1nF capacitor (marked 102)
- 1 x 4.7nF capacitor (marked 472)
- 2 x 0.1μF capacitors (marked 104)
- 1 x 10kΩ resistor
- Other resistors, capacitors, inductors, etc. (add as needed)

■ Instructions

The Colpitts oscillator circuit shown in Figure 2 is built using a solderless breadboard. If you configure bias resistors R1 and R2 by selecting standard values from the parts kit and set the emitter resistor R3 to 1 kΩ, the collector current of NPN transistor Q1 should be about 1 mA.

The frequency of this oscillator can be adjusted from about 500 kHz to 2 MHz depending on the values selected for C1, C2, and L1. The initial settings start with L1 = 100 μH, C1 = 4.7 nF, and C2 = 1 nF. This oscillator circuit can produce a sine wave output greater than 10 Vp-p at approximately the frequency set by the value selected for L1.

In Figure 2, the rectangles indicate where the arbitrary waveform generator (AWG), oscilloscope channels, and power supply of the ADALM2000 module should be connected. Before turning on the power, be sure to double-check the wiring connections.

■ Hardware Settings

Please refer to Figure 3 for the breadboard circuit.

■ Procedure

Once you have completed the Colpitts oscillator circuit, turn on both the +5V and -5V power supplies and connect one of the oscilloscope channels to the output terminal to check if the circuit is oscillating properly. You will notice that the value of the R3 resistor is very important. If it is not the right value, a large, distorted waveform may be output, or the output may be weak or intermittent.

To find the optimum value of R3, you can replace R3 with a variable resistor of 1 kΩ or 5 kΩ to find the value that gives the best waveform and reliable amplitude in your experiment. The optimum value of R3 may vary depending on the resonant frequency.

Figure 4 shows an example of the waveform when R1 = 10kΩ, R2 = 1kΩ, C1 = 4.7nF, and C2 = 1nF are used.

■ Question

1. What are the main functions of the Colpitts oscillator?
2. In what applications can Colpitts oscillators be used?

Answers to the above questions and further explanations can be found on the Analog Devices website StudentZone (https://ez.analog.com/studentzone/).

※ About the author
Antoniou Miclaus is a Software Engineer at Analog Devices, working on Linux and non-OS drivers, ADI academic programs, Quality Assurance (QA) automation, and process management. He joined ADI in Cluj-Napoca, Romania, in February 2017. He holds an M.Sc. in Software Engineering from Babes-Bolyai University and a B.Eng. in Electronics and Communication Engineering from the Technical University of Cluj-Napoca.