Oscillator Manual

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CIRCUIT #1 Variable Resistor CIRCUIT #2 Resistor



Variable Resistor Capacitor

Transistor Inductor

R1 R2 R3 R4 VR2 C1 C2 C3 C4 C5 C6 TR1 L1 L2

10k 1k 10k 1k 5k 1F, 50V 10nF 10nF 2.2nF 1F, 50V 1F, 50V 2N3904 1mH 10mH

CIRCUIT #3 Variable Resistor Resistor



VR3 R5 R6 R7 R8 C7 C8 C9 C10 TR2

1k 1.2k 4.7k 68k 4.7k 47nF 47nF 47nF 22nF 2N3906


MODULAR ANALOG ELECTRONICS TRAINER LC OSCILLATORS OBJECTIVES : Describe how the required phasing for positive feedback is achieved in Hartley and Colpitts Oscillators. Calculate the resonant frequency of a Hartley tuned circuit. Calculate the resonant frequency of a Colpitts tuned circuit. Discuss the Class of bias used in an efficient oscillator circuit. State the range of frequencies over which the Hartley and Colpitts Oscillators will operate.

INTRODUCTION : Oscillations in a Tuned CircuitThe resonant frequency of a tuned (oscillatory) circuit is determined by the inductive reactance being the same as the capacitive reactance. XL = XC 2fL = 1 / 2fC f2 = 1/42LC fo = 1/ [ 2 (LC)1/2 ]

In this condition the two currents have the same magnitudes and cancel out (as far as external circuit is concerned), resulting in large internal currents, which circulate around the (parallel) tuned circuit. The energy is transfer between the magnetic field of the coil (maximum current) and the electrical field of the capacitor (maximum voltage).

Fig. 1.1 The only losses in the circuit will be largely due to the resistance of the wire of the coil. A small signal injection of energy at the peak of the cycle is all that is necessary to maintain the oscillation. In electronic terms this corresponds to the highly efficient Class C mode of operation of the amplifier.


MODULAR ANALOG ELECTRONICS TRAINERThis is similar to the effect when a weight is suspended from a pivot and set in swinging motion.

Suspended Weight in Motion

Fig. 1.2

Discrete Amplifier OscillatorsMost forms of oscillator circuits were derived for the thermionic tubes (valves), the normal form of which provided an inverting amplifier. These were readily adapted for transistors (BJT) when they came along, the common emitter amplifier mode being naturally adapted. The exercises in the Chapter are designed to investigate two of the most common of these, the Hartley and the Colpitts Oscillators. Each of these employs an inverting amplifier and a feedback () network with a built-in inversion to give the positive feedback which is the primary requirement of an oscillator.

Hartley and Colpitts Tank CircuitsThe basic difference between them is in the method used to provide a reference ground point for the inversion of the signal.

Fig. 1.3


MODULAR ANALOG ELECTRONICS TRAINERIn the case of the Hartley Oscillator a center-tapped inductors is used in the oscillatory circuit (Fig. 1.3 (a)). The Colpitts Oscillator uses center-tapped capacitors (Fig. 1.3 (b)). When calculating the resonant frequency, the Hartley tuned circuit contains an inductor consisting of two coils in series. L = L1 + L2. In the case of the Colpitts Oscillator the series components are capacitors. C = [ C1 x C2 ] / [ C1 + C2 ] In either case the alternating signal voltage at the ends of the inductor A & B will be opposite polarity. The amount of the feedback will be determined by the ratio of the two inductors in the case of the Hartley or the capacitors (inversely) in the Colpitts. The necessary timing of the injection of energy to maintain oscillations can be achieved by Class C bias of the amplifier, which is very efficient. Class A bias can be used, and has been for the circuits investigated in earlier chapters, but it is unnecessary.


MODULAR ANALOG ELECTRONICS TRAINER MATERIALS & APPARATUS1. 2. 3. 4. 5. EDUTECH Oscillator-1 Module Board Circuit #1 & #2. EDUTECH EDU-9100 Analog Base Unit. Oscilloscope. Digital Multimeters (DMM). Signal Generator.

Experiment 1A : Hartley Oscillator The feedback ratio is that of the inductors provided, which is 10:1 (10mH, 1mH). This gives an inductance of 11mH. A range of capacitors are provided to satisfy the needs of the Colpitts Oscillator. With a little ingenuity and multipatching using jumper wires, as many as twelve different values of capacitance are practically possible, each with its own resonant frequency. This circuit would normally be chosen for ratio frequencies between about 30kHz 30MHz. Some of the frequencies obtainable with the components provided will be lower for experimental convenience. The small signal gain of the amplifier is adjustable by the degree of decoupling of the emitter resistor. This allows the bias to be varied all the way from Class A through Class C. Objectives : Describe how the required phasing for positive feedback is achieved in Hartley and Colpitts Oscillators. Calculate the resonant frequency of a Hartley tuned circuit. Calculate the resonant frequency of a Colpitts tuned circuit. Discuss the Class of bias used in an efficient oscillator circuit. State the range of frequencies over which the Hartley and Colpitts Oscillators will operate.

Procedures : 1. Place the EDUTECH Oscillator-1 Module Board on the EDU-9100 Analog Base Unit. 2. Connect the Circuit #1 & #2 as shown in fig. 1.4. Connecting sockets 2.10 & 2.11 for Hartley circuit. Connecting sockets 2.12 & 2.15 to select C3 = 10nF. Connecting sockets 2.7 & 2.9 to to short out the 100nF series capacitor and simplify the resonant frequency calculations. Switch on the base unit. Adjust the variable 0 15V DC power supply on the base unit to maximum to get 15V. 3. Adjust the VR1 on the Module Board to get V1 = 12V. Set the signal gain control VR2 fully clockwise for minimum gain. (maximum resistance of VR2).


MODULAR ANALOG ELECTRONICS TRAINER4. Connect socket 2.19 to Ch.1 of oscilloscope and socket 2.20 to Ch.2 of oscilloscope to monitor the emitter current. Note that the oscilloscope is displaying the quiescent DC bias conditions of the transistor amplifier by the position of the traces.

5. Increase the gain by turning VR2 counterclockwise. Oscillations should commence with VR2 set just below half travel. When first established some instability (hunting) of amplitude will occur while the output is less than about 5Vp-p. Note that a small signal voltage appears at the emitter (Ch.2). 6. Continue to increase gain. The output signal will increase in magnitude until the negative tip meets the positive excursion of the emitter waveform, at which time the transistor will saturate (bottom) and distort the output waveform. Back off the setting of VR2 for maximum undistorted output waveform. The tips of the two waveforms should be just about touching. Note that the gain setting is easy to achieved and that stability is good. This is an excellent practical oscillator circuit. 7. Note the maximum undistorted peak-to-peak value of the output signal = ___________ Vp-p. 8. Change the Ch.2 Y amplifier sensitivity to 0.2V/div and move the Ch.1 trace up the screen out of the way above the Ch.2 trace.


MODULAR ANALOG ELECTRONICS TRAINER9. Turn the gain setting (VR2) fully clockwise again to stop the oscillation and then reduce again, watching the oscilloscope display as you do. Initially, as oscillations recommence, you will see that the emitter waveform contains a full sinewave AC component as the transistor operates in Class A. As the gain is further increased the transistor current bottoms out into Class AB, then to Class B and finally to Class C as condition occurs for less than 180. 10. For each of the values of capacitor given in Table 1.1, calculate the theoretical value of frequency of oscillation from the formula and record in Table 1.1 L = 11mH Theoretical Frequency Measured Frequency C = 10nF C = 2.2nF C = 12.2nF

Table 1.1 11. With the circuit as connected, ensure that the oscilloscope timebase is in the calibrated setting and measure the time for one cycle. Take the reciprocal to obtain the frequency and record the result in Table 1.1. 12. Transfer the jumper wires from between sockets 2.12 & 2.15 to 2.13 & 2.16, and repeat the measurement and calculation for C4 = 22nF and record in Table 1.1. 13. Add a further jumper wires between sockets 2.12 & 2.15 and repeat for C = 12.2nF. 14. Switch off the base unit.


MODULAR ANALOG ELECTRONICS TRAINERExperiment 1B : Colpitts Oscillator The center tap is transfer from the inductors to the capacitors. The available components again give a range of possible frequencies, but not quite so many this time. Only a single inductor is necessary but both will be used initially to compare the operation of the Colpitts and Hartley Oscillators at similar frequencies. The series capacitors necessary for the Colpitts Oscillator result in lower tuned circuit capacitance and therefore higher frequency. Also the Hartley Oscillator requires series inductors which increase the total impedance and reduce frequency. The Colpitts Oscillator will therefore operate at higher frequencies than the Hartley, being practically used at up to 300MHz, although at the very high end capacitances are made up of the stray and inter-electrode capacitances of the transistor. Procedures : 1. Connect the Circuit #1 & #2 as shown in fig. 1.5. Connecting sockets 2.9 & 2.10 for Colpitts circuit. Connecting sockets 2.11 & 2.14 to short out L3. Connecting sockets 2.12 & 2.15 to select C3 = 10nF.Switch on