Objective
The objective of this lab activity is to investigate the characteristics of the Peltz oscillator configuration.
Background
Unlike Clapp, Colpitts, and Hartley oscillators, which have a single transistor, the Peltz configuration uses two transistors. Looking at Figure 1, note that transistor Q1 is configured as a common base amplifier stage. The resonate tank consisting of L1 and C1 provides the collector load. Its output at the collector feeds the base of transistor Q2, which is configured as an emitter follower (common collector) stage. The positive feedback that is required for oscillation is formed when the output of the emitter follower (emitter Q2) is connected back to the input of the common base stage at the emitter of Q1. The voltage gain of the common base amplifier stage is a maximum at the parallel resonate frequency of the LC tank where its impedance approaches infinity. The gain of the emitter follower is always slightly less than one. The combined gain around the loop will be much greater than 1 at resonance to sustain oscillation.
The resonate frequency of the LC tank is given by Equation 1.
The peak-to-peak swing across the LC tank is limited in this oscillator configuration. As the voltage on the base of Q2 swings more positive than ground, the collector of Q2, the collector base junction will forward bias, limiting the maximum positive swing to around one forward diode drop. This is also the case for the peak negative swing, when the collector of Q1 swings negative enough to forward bias the collector base junction of Q1. When the collector base junction of a BJT transistor is forward biased, the base current increases dramatically. We can make use of this increased base current to increase the peak-to-peak swing seen across the LC tank. If we insert resistors in series with the base of both Q1 and Q2, as shown in Figure 2, the additional current through the resistors will lower the base voltage of Q1 and Q2 at the extremes of the LC tank voltage.
Pre-Lab Simulations
Build a simulation schematic of the Peltz oscillator as shown in figures 1 and 2. Calculate a value for bias resistor R1 such that the collector current in transistors Q1 and Q2 is greater than 200 μA each. Assume the circuit is powered from a –5 V power supply. Calculate a value for C1 and L1 such that the resonate frequency will be at least 1 MHz. Perform a transient simulation. The peak-to-peak output swing across the LC tank should be limited to less than ±1 forward diode drop (~±0.6 V). Calculate and simulate values for R2 = R3 such that the output swing increases to at least ±1.25 V. Save these results to compare with the measurements taken on the actual circuit and to include with the lab report.
Materials
- ADALM2000 Active Learning Module
- Solderless breadboard
- Jumper wires
- Two small signal NPN transistors (2N3904)
- One 10 kΩ resistor
- Two 4.7 kΩ resistors
- One 100 μH inductor
- One 100 pF capacitor
Directions
Build the Peltz oscillator circuit shown in Figure 3 on your solderless breadboard. The squares indicate where to connect the ADALM2000 module scope channels and power supply. Be sure to only turn on the power supply after double checking the wiring.
Hardware Setup
Set both scope inputs to 200 mV/div and the time base to 1 μs/div. Set the trigger on the rising edge of Channel 1. See the breadboard circuit in Figure 4.
Procedure
Turn on the –5 V power supply. Observe the output waveform across the LC tank on Scope Channel 1. The waveform can also be seen at the emitters of Q1 and Q2 using Scope Channel 2.
Questions
- What is the main function of a Peltz oscillator?
- The Peltz oscillator is a variation of which oscillator?
- Which component configuration differentiates the Peltz oscillator from the Colpitts and Clapp oscillators?
- When is a Peltz oscillator preferred over other LC oscillators like Colpitts or Clapp?
You can find the answers at the StudentZone blog.
