This article presents a hands-on process that examines the characteristics of a Peltz oscillator configuration using the ADALM2000 educational kit from Analog Devices.

■ Peltz oscillator using two transistors

Unlike the Clapp, Colpitts, and Hartley oscillators, which use one transistor, the Peltz oscillator uses two transistors.

Looking at Figure 1, transistor Q1 is configured as a common-base amplifier stage.

The resonant tank consisting of L1 and C1 provides the collector load. The output from the collector is transmitted to the base of transistor Q2, which is configured as an emitter follower (common collector) stage.

The positive feedback required for oscillation is formed by connecting the output of the emitter follower (the emitter of Q2) back to the emitter of Q1, which is the input of the common-base stage.

The voltage gain of the common-base amplifier stage is maximum at the parallel resonant frequency of the LC tank, at which time the impedance approaches infinity.

The gain of an emitter follower is always slightly less than 1. At resonance, the combined gain of the entire loop becomes much greater than 1, allowing oscillation to continue.


The resonant frequency of the LC tank can be obtained using Formula 1.


In this oscillator configuration, the peak-to-peak swing across the LC tank is limited.

When the base voltage of Q2 swings more positive than ground, the collector-base junction of Q2 becomes forward biased, limiting the maximum positive swing to approximately one forward diode voltage drop.

Likewise, at maximum negative swing, if the collector voltage of Q1 swings sufficiently negative, the collector-base junction of Q1 becomes forward biased.

When the collector-base junction of a BJT transistor is forward biased, the base current increases rapidly.

This increased base current can be used to increase the peak-to-peak swing observed across the LC tank.

If resistors are inserted in series at the bases of Q1 and Q2, as shown in Figure 2, the base voltages of Q1 and Q2 will be lowered due to the additional current flowing through the resistors at the extreme values of the LC tank voltage.


■ Simulation before experiment

Create a simulation circuit diagram of the Peltz oscillator as shown in Figure 1 and Figure 2.

Calculate the value of bias resistor R1 so that the collector current of each transistor Q1 and Q2 is greater than 200 μA.

Assume the circuit is powered by a -5V supply.

After calculating the values of C1 and L1 so that the resonant frequency is at least 1 MHz, a transient response simulation is performed.

The peak-to-peak output swing across the LC tank must be limited to less than the forward diode voltage drop (approximately ±0.6 V).

Calculate and simulate the values of R2 = R3 so that the output swing increases to at least ±1.25 V. Save these results, compare them with the values measured in the actual circuit, and include them in your experimental report.

■ List of experimental materials

· ADALM2000 active learning module
· Solderless breadboard
· Jumper wires
· Two small-signal NPN transistors (2N3904)
· 1 x 10kΩ resistor
· 2 x 4.7kΩ resistors
· 1 x 100μH inductor
· 1 x 100pF capacitor

■ Guidelines

Build the Feltz oscillator circuit shown in Figure 3 using a solderless breadboard. The square marks indicate where to connect the oscilloscope channels and power supply of the ADALM2000 module. Be sure to double check the wiring before turning on the power supply.


■ Hardware settings

Set both oscilloscope inputs to 200 mV/div and the time base to 1 μs/div. Set the trigger on the rising edge of channel 1. Refer to Figure 4 for the breadboard circuit.


■ Procedure

After turning on the -5V power supply, observe the output waveform across the LC tank on oscilloscope channel 1. This waveform can also be observed at the emitters of Q1 and Q2 using oscilloscope channel 2.


■ Question

1. What are the main functions of the Peltz oscillator?

2. What type of oscillator is the Peltz oscillator?

3. What components distinguish the Peltz oscillator from the Colpitts and Clapp oscillators?

4. When is the Feltz oscillator preferred over other LC oscillators, such as the Colpitts or Clapp?

Answers to the above questions and further explanations can be found on the Analog Devices StudentZ·ne blog at https://ez.anal·gc·m/studentz·ne/.


※ About the author
Antoniou Miclaus is a Software Engineer at Analog Devices, where he is responsible for embedded software development for Linux and non-OS drivers, ADI's 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.