Objective
Oscillators come in many forms. This lab activity explores the Hartley configuration, which uses a tapped inductor divider to provide the feedback path.
Background
The Hartley oscillator is a particularly good circuit for producing fairly low distortion sine wave signals in the RF range of 30 kHz to 30 MHz. The Hartley configuration can be recognized by its use of a tapped inductor divider (L1 and L2 in Figure 1). The frequency of oscillation can be calculated in the same way as any parallel resonant circuit by using Equation 1:
Where L = L1 + L2
Figure 1 shows a typical Hartley oscillator. The frequency determining parallel resonant tuned circuit is formed by L1, L2, and C1 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 Hartley oscillator uses a common base amplifier. The base of Q1 is biased to an appropriate DC level by resistor divider 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 L1 and L2, fed back from the tuned collector load to the emitter via the coupling capacitor C2, provides the required positive feedback.
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 give the closed-loop gain of 1 required by the oscillator. 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 Hartley oscillator 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 a value for C1 such that the resonate frequency, with L1 set equal to 1 μH and L2 set to 10 μH, will be close to 750 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
- One 1 µH inductor
- One 10 µH inductor
- One 100 µH inductor
- One 1 nF capacitor (C1 optional values as listed below)
- Two 0.1 µF capacitors (marked 104)
- Two 0.01 µF capacitors (marked 103)
- One 1 kΩ resistor
- Other resistors, capacitors, and inductors as needed
Directions
Build the Hartley oscillator shown in Figure 2 using your solderless breadboard. Select standard values from your parts kit for bias resistors R1 and R2 so that emitter resistor R3 is 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 L1, L2, and C1. Start with L1 = 10 µH and L2 = 100 µH. 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 C1. After experimenting with various values for C1, change L1 = 1 µH and L2 = 10 µH.
Hardware Setup
The green squares 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. See the breadboard circuit in Figure 3.
Procedure
Having finished construction of the Hartley oscillator, check that the circuit is oscillating correctly by turning on both the +5 V and –5 power supplies and connecting one of the oscilloscope channels to the output terminal. The value of R3 may be fairly critical, producing either a large and distorted waveform or an intermittent low or no output. To find the best value for R3, it can be replaced by a 1 kΩ potentiometer for experimentation to find the value that gives the best wave shape and reliable amplitude. A plot example using R1 = 10 kΩ, R2 = 1 kΩ, R3 = 100 Ω, and C1 = 4.7 nF is presented in Figure 4.
Questions
- What is the main function of a Hartley oscillator?
- What practical applications use the Hartley oscillator?
