Laboratory Sheet 10 Principles of Electrical Engineering UFMFJT-30-1 [Total Marks: 30] The Colpitts Oscillator Oscillators are repetitive waveform generators consisting generally of amplifiers with positive feedback, via a frequency selective network, (or circuits that exhibit negative resistance characteristics). In order to satisfy the conditions for oscillation there must be a voltage gain of greater than that of the attenuation in the feedback network. In the case of positive feedback, it can be Ao shown that Voltage Gain A'= 1– Bao where A' is the circuit gain, Ao is the Open Loop gain of the transistor amplifier and ẞ is the Feedback fraction. Assuming that the transistor gain is high, the greater the feedback, or the closer BAo is to unity the greater the gain, therefore, with positive feedback the gain is reinforced, which results in oscillation. Most oscillators start up by virtue of internal noise, created at all frequencies. This means that there will be signals at one frequency where they will result in positive feedback and full oscillation at that tuned frequency. This particular sinusoidal oscillator is commonly used in Radio Frequency (RF) equipment. It is often crystal controlled and used as the core oscillator for transmitters. The tuned element used here is the L & C parallel tuned circuit in the collector of the amplifier, with feedback to the emitter via the 'tapped' capacitors. C1 0.1μF V1 -5V R110MQ C2 0.1μF L1 C4 10mH Key=A 100pF 10.7 % Q1 BC108BP C3 = 0.47μF R2 1.5kQ XSC1 Ext Trig Ο Construct the circuit using Multisim and confirm that it oscillates with a sinusoidal waveform at the collector. You may need to adjust the value of R1 for a pure sine waveform. [4 Marks] Laboratory Sheet 10 1. Investigation: Principles of Electrical Engineering UFMFJT-30-1 Take snapshots of Oscilloscope waveforms and Spectra with measurements displayed. Don't forget to add a title to anything pasted into your log book. [2 Marks] 1. Record the waveforms at Collector output and Emitter input, and using the Digital Oscilloscope, determine the resonant frequency by measurement. Note the phase relationship between the input and output of the transistor. [3 Marks] Q. Where is the input? [1 Mark] 1 2. Calculate the Frequency F == Note that C1 & C2 are in series. [1 Mark] 211√LC Q. Why is the frequency not quite what you calculated? [1 Mark] 3. Measure and record the DC bias voltages at Emitter, Base & Collector [1 Mark] Q. What bias configuration is the transistor in? Q. What is the function of C3? Q. Why is the feedback point from the junction of different value capacitors? [1 Mark] [2 Marks] [2 Marks] 4. Use the FFT maths function of the oscilloscope to have a look at the spectrum of the output signal and check that there are no spurious harmonics (unobservable in the time domain). [4 Marks] 5. Try to capture the oscillator start up signals (Time domain) at the instant of supplying the DC supply and examine/identify the various parts of the waveform. [2 Marks] 2. Discussion on Results and Observations: What objectives do you think can be achieved in carrying out this experimental work? Reflect on these objectives. Summarise the whole lab session in your own words and conclude what did you gain from this exercise? Explain your experience of analysing electronic circuits that use passive components such as resistor, capacitors and inductors. What are the main applications of these types of circuits? What are the drawbacks of these circuits? Comment on the skills you gained in this lab session - how do you feel about the session in terms of understanding the concepts given in the lecture? [6 Marks]