Collector-Coupled BJT Oscillator: Design, Simulation, and Results, Lab Reports of Electrical and Electronics Engineering

Download Collector-Coupled BJT Oscillator: Design, Simulation, and Results and more Lab Reports Electrical and Electronics Engineering in PDF only on Docsity! Formal Lab Report: Analysis of a Collector- Coupled BJT Oscillator ECE 3042 Roshni Goel gtg914p Section: L01 21st November 2006 Abstract Oscillation, the periodic movement between two particular states, is a fundamental form of motion. Oscillators, devices that generate oscillations, are used to generate sinusoidal waveforms in function generators. In this experiment, a collector-coupled BJT oscillator was analyzed to determine if it could provide accurate waveforms as that required by various electronic applications. The design specifications required the circuit output to be a 4.2 kHz frequency sine wave with 4.00 V peak-to-peak voltage and Total Harmonic Distortion less than 5%. The circuit was designed, simulated, assembled and evaluated to determine if it met the design criteria. The simulation results gave a maximum error of 3.09% and the experimental results gave a maximum error of 1.97% for the frequency. It was concluded that the collector-coupled oscillator circuit combined with a band-pass filter provided an accurate method of generating waveforms that could be utilized different electronic instruments. Another important advantage determined was that the output of the oscillator circuit had no added error due to input signals, because there was no input signal. Oscillators allow for the generation of standard signals with error margins dependant only on the different component error margins. 2 Infinite-Gain Multi-Feedback Band-Pass Filter with the critical frequency as fo. A typical Infinite-Gain Multi-Feedback Band-Pass Filter consisting of a 741 operational amplifier and resistors and capacitors can be seen in the next figure: Figure 2. Infinite-gain multi-feedback band-pass filter. Simulation Using the above values for R and C, the collector-coupled BJT oscillator was simulated in MULTISIM. The following figure shows the circuit schematic used: A transient analysis was performed on the above circuit to observe the output waveform at the collector of either BJT. The output of the oscillator was cascaded with a filter as per the design specifications to remove unwanted harmonics: 5 Figure 3. Col ector- coupled BJT scillator. Figure 4. Oscillator and filter circuit schematic. A transient analysis was performed at the output of the 741 operational amplifier to observe the output waveform. A Fourier analysis was also done to calculate the Total Harmonic Distortion (THD). A Fourier analysis reveals the frequency components of the output waveform. For the purpose of this experiment, it was desired to have only one frequency component: a sine wave of frequency 4.2 kHz. The THD indicates the deformation of a signal with respect to a pure sine wave. As the desired output was a sine wave, the THD should ideally be 0, or as per the design specifications it should be less than 5%. Protoboard Assembly The following picture shows the schematic in Figure 4 built on the protoboard: 6 Figure 5. Circuit built on protoboard. The oscillator circuit can be seen at the bottom of the board, and the filter is assembled on the top half of the board. The HP E3630A power supply was used to supply ±15 V to the 741 op-amp and 3.5 V to the oscillator circuit. The Tektronix TDS 3012B oscilloscope was used to observe the outputs of the oscillator and the filter. 7 The output of the collector-coupled oscillator can be seen in the following figure: Figure 9. Oscillator output. As seen in the screenshot, the output was a 4.38 V peak-to-peak square wave with a frequency of 4.138 kHz. With the square wave input, the band-pass filter gave a sine wave output: Figure 10. Band-pass filter output. 10 It was observed that the sine wave had a frequency of 4.21 kHz. The voltage on the input signal was adjusted to give a final 4.00 V peak-to-peak sine signal with a frequency of 4.238 kHz. This signal can be observed in the next figure. Figure 11. Sine wave output. The next figure displays the Fourier analysis of the sine wave. As expected, the fundamental frequency was at 4.283 kHz. There were very few distinguishable harmonic frequencies. The THD was calculated by dividing the summation of the voltages at the harmonics by the voltage at the fundamental. The THD was calculated to be 4.57%. Figure 12. Fourier analysis of sine wave. 11 A table with the simulation and experimental results is provided: Table 1. Simulation and Experimental Results While the simulation results have approximately 3.09% error for the frequency and 1.75% error for the amplitude, the experimental results have proved to be within 2% of the frequency specification and exactly equal to the voltage specification. Also, both the simulation and experimental THD met the design specifications as each THD was less than 5%. ParameterDesign SpecificationSimulation ResultExperimental ResultSquare wave frequency4.2 kHz4.07 kHz4.283 kHzSquare wave voltage (peak-to-peak)4.00 V4.07 V4.00 VSine wave frequency4.2 kHz4.07 kHz4.283 kHzSine wave voltage (peak-to-peak)4.00 V4.07 V4.00 VTotal Harmonic Distortion5% or less4.774% 4.57% 12