Analog Devices Wiki

This version (15 Feb 2018 21:29) was approved by Doug Mercer.

Activity: The Hartley Oscillator

Objective:

Oscillators come in many forms. In this lab activity we will explore the Hartley configuration which uses a taped 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 range, 10kHz to 100KHz. 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, using:

Where L = total inductance of L1 and 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 Q1is 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.

Figure 1, Basic Hartley Oscillator

C2 also forms a low frequency time constant with the 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. The 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 as shown in figure 1. Calculate values for bias resistors R1 and R2 such that with emitter resistor R3set to 4.7 KΩ, the collector current in NPN transistor Q1 is approximately 250 uA. Assume the circuit is powered from a +5V 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 0.1uF. Calculate a value for C1 such that the resonate frequency, with L1 connected as 1 winding of the HPH1-1400L and L2 connected as 5 windings of the HPH1-1400L, will be close to 10 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:

Solder-less breadboard, and jumper wire kit
1 - 2N3904 NPN transistor
1 - HPH1-1400L inductor
2 - 0.1 uF capacitors (marked 104)
2 - 0.047 uF capacitor (marked 473) (C1 values as listed below )
2 - 0.01 uF capacitor (marked 103)
2 - 4.7 KΩ resistors
1 - 10 KΩ resistor
2 - 20 KΩ resistor
1 - 1 Meg Ω resistor

Directions:

Build The Hartley Oscillator shown in figure 2 using your solder-less breadboard. Start with the standard values as shown in the schematic for bias resistors R1 and R2 such that with emitter resistor R3set to 4.7 KΩ, the collector current in NPN transistor Q1 will be approximately 250 uA. The HPH1-1400L 6 winding coupled inductor is used for L1 and L2. The frequency of the oscillator should be approximately 10 KHz depending on the total number windings used and the tap connections chosen for L1, L2 and C1. Start with L1 as 1 winding and L2 as the remaining 5 windings connected in series. This oscillator circuit can produce a sine wave output beyond the 0 to 5 V range of the ADALM1000 input at the resonate frequency set by the value chosen for C1. The AC coupling capacitor C4 and the DC bias resistor R4 should center to output on +2.5 V.

After experimenting with various values for C1 change tap position and total number of windings used on the HPH1-1400L for the feedback.

Figure 2 Hartley Oscillator

Hardware Setup:

The green squares indicate where to connect the ADALM1000 module, scope channels and power supplies. Be sure to only connect on the power supplies after you double check your wiring.

Procedure:

Having finished construction the Hartley oscillator check that the circuit is oscillating correctly by connecting the +5 V power supply and connecting one of the oscilloscope channels to the AC coupled 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 10 KΩ potentiometer for experimentation to find the value that gives the best wave shape and reliable amplitude.

Questions:

Measure the peak to peak output voltage of the output. Measure the DC ( average ) level of the output waveform at the collector of Q1 and on the other (output) side of AC coupling capacitor C4. Measure the period (time T) of the output waveform and its frequency (1/T). Compare this measured frequency to what you calculated b:

Fill in the table below with the measured frequency for other C1 values. Use the values in the table as suggested options but try to include as many different values as possible using series and parallel combinations of the capacitors supplied in your parts kit. Any of the C1 optional values shown below should give reliable oscillation.

C1 OptionsL1 1Wind L2 5WindL1 2Wind L2 4Wind
ValueFrequencyFrequency
10 nF
20 nF
23.5nF
47nF