Objective
Oscillators come in many forms. This lab activity explores the Colpitts configuration, which uses a tapped capacitor divider to provide the feedback path.
Background
The Colpitts oscillator is a particularly good circuit for producing low distortion sine wave signals in the RF range of 30 kHz to 30 MHz. The Colpitts configuration can be recognized by its use of a tapped capacitor divider (C1 and C2 in Figure 1). The frequency of oscillation can be calculated in the same way as any parallel resonant circuit, using Equation 1.
The values of the two capacitors (connected in series) are chosen so their total capacitance in series (CTOT) is given by Equation 2.
Figure 1 shows a typical Colpitts oscillator. The frequency determining parallel resonant tuned circuit is formed by C1, C2, and L1 and is used as the collector load impedance of the common base amplifier Q1. This gives the amplifier a high gain only at the resonant frequency. This configuration of the Colpitts oscillator uses a common base amplifier. The base of Q1 is biased to an appropriate DC level by resistor dividers R1 and R2 but is connected directly to an AC ground by C3. In the common base mode, the output voltage waveform at the collector and the input signal at the emitter are in phase. This ensures that the fraction of the output signal from the node between C1 and C2, fed back from the tuned collector load to the emitter, provides the required positive feedback. Note that this feedback is AC only and there is no DC path from the collector to the emitter.
The combined capacitance of C1 and C2 also forms a low frequency time constant with emitter resistor R3 to provide an average DC voltage level proportional to the amplitude of the feedback signal at the emitter of Q1. This provides automatic control of the gain of the amplifier to regulate the closed-loop gain of the oscillator. As with all oscillators, the Barkhausen criteria must be adhered to by requiring a total gain of one and a phase shift of zero degrees from input to output. Emitter resistor R3 is not decoupled because the emitter node is used as the common base amplifier input. The base is connected to AC ground by C3, which will provide a very low reactance at the oscillator frequency.
Pre-Lab Simulations
Build a simulation schematic of the Colpitts oscillator as shown in Figure 1. Calculate values for bias resistors R1 and R2 such that, with emitter resistor R3 set to 1 kΩ, the collector current in NPN transistor Q1 is approximately 1 mA. Assume the circuit is powered from a 10 V power supply. Be sure to keep the sum of R1 and R2 (total resistance greater than 10 kΩ) as high as practical to keep the standing current in the resistor divider as low as practical. Remember that C3 provides an AC ground at the base of Q1. Set base decoupling capacitor C3 and output AC coupling capacitor C4 to 0.1 μF. Calculate values for C1 and C2 such that the resonant frequency, with L1 set equal to 100 μH, will be close to 500 kHz. Perform a transient simulation. Save these results to compare with the measurements you take on the actual circuit and to include with your lab report.
Materials
- ADALM2000 active learning module
- Solderless breadboard and jumper wire kit
- One 2N3904 NPN transistor
- Two 10 μH inductors
- Two 100 μH inductors
- One 1 nF capacitor (marked 102)
- One 4.7 nF capacitor (marked 472)
- Two 0.1 μF capacitors (marked 104)
- One 1 kΩ resistor
- Other resistors, capacitors, and inductors as needed
Directions
Build the Colpitts oscillator shown in Figure 2 using your solderless breadboard. Choose standard values from your parts kit for bias resistors R1 and R2 such that with emitter resistor R3 set to 1 kΩ, the collector current in NPN transistor Q1 is approximately 1 mA. The frequency of the oscillator can be from around 500 kHz to 2 MHz depending on the values chosen for C1, C2, and L1. Start with L1 = 100 μH, C1 = 4.7 nF, and C2 = 1 nF. This oscillator circuit can produce a sine wave output in excess of 10 V p-p at an approximate frequency set by the value chosen for L1.
Hardware Setup
See Figure 3 for the breadboard circuit.
The squares in Figure 2 indicate where to connect the ADALM2000 module AWG, scope channels, and power supplies. Be sure to only turn on the power supplies after you double check your wiring.
Procedure
Having finished construction of the Colpitts oscillator, check that the circuit is oscillating correctly by turning on both the +5 V and –5 V power supplies and connecting one of the oscilloscope channels to the output terminal. It may be found that the value of R3 is fairly critical, producing either a large, distorted waveform or an intermittent low or no output. To find the best value for R3, it could be replaced by a 1 kΩ or 5 kΩ potentiometer for experimentation to find the value that gives the best wave shape and reliable amplitude. The optimal value for R3 may change depending on the resonant frequency.
A plot example using R1 = 10 kΩ, R2 = 1 kΩ, C1 = 4.7 nF, C2 = 1 nF is presented in Figure 4.
Questions
- What is the main function of a Colpitts oscillator?
- What practical applications use the Colpitts oscillator?
