# oscillators 2. lc oscillators. oscillators oscillators with lc feedback circuits for frequencies...

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• Slide 1
• Oscillators 2. LC Oscillators
• Slide 2
• Oscillators Oscillators With LC Feedback Circuits For frequencies above 1 MHz, LC feedback oscillators are used. We will discuss the Colpitts, Hartley and crystal-controlled oscillators. Transistors are used as the active device in these types.
• Slide 3
• Oscillators Oscillators With LC Feedback Circuits
• Slide 4
• Oscillators Colpitts The Colpitts oscillator utilizes a tank circuit (LC) in the feedback loop as shown in the figure.
• Slide 5
• LC feedback oscillators use resonant circuits in the feedback path. A popular LC oscillator is the Colpitts oscillator. It uses two series capacitors in the resonant circuit. The feedback voltage is developed across C 1. The effect is that the tank circuit is tapped. Usually C 1 is the larger capacitor because it develops the smaller voltage. Oscillators Colpitts
• Slide 6
• Oscillators Oscillators With LC Feedback Circuits If Z T = 0; Colpitts
• Slide 7
• Oscillators Colpitts The resonant frequency can be determined by the formula below. Total capacitance (C T ) is ;
• Slide 8
• Oscillators Colpitts Conditions for oscillation and start up
• Slide 9
• If Q > 10, this formula gives good results. Recall that the total capacitance of two series capacitors is the product-over-sum of the individual capacitors. Therefore, For Q < 10, a correction for Q is Oscillators Colpitts
• Slide 10
• Oscillators Hartley The Hartley oscillator is similar to the Colpitts. The tank circuit has two inductors and one capacitor
• Slide 11
• The Hartley oscillator is similar to the Colpitts oscillator, except the resonant circuit consists of two series inductors (or a single tapped inductor) and a parallel capacitor. The frequency for Q > 10 is One advantage of a Hartley oscillator is that it can be tuned by using a variable capacitor in the resonant circuit. Oscillators Hartley
• Slide 12
• If Z T = 0; Oscillators Oscillators With LC Feedback Circuits Hartley
• Slide 13
• Oscillators Hartley The calculation of the resonant frequency is the same.
• Slide 14
• Oscillators Crystal The crystal-controlled oscillator is the most stable and accurate of all oscillators. A crystal has a natural frequency of resonance. Quartz material can be cut or shaped to have a certain frequency.
• Slide 15
• Since crystal has natural resonant frequencies of 20 MHz or less, generation of higher frequencies is attained by operating the crystal in what is called the overtone mode Oscillators Crystal
• Slide 16
• Oscillators 3. Relaxation Oscillators
• Slide 17
• Oscillators Relaxation Relaxation oscillators make use of an RC timing and a device that changes states to generate a periodic waveform (non-sinusoidal). 1. Triangular-wave 2. Square-wave 3. Sawtooth
• Slide 18
• Oscillators Relaxation Triangular-wave oscillator Triangular-wave oscillator circuit is a combination of a comparator and integrator circuit. A square wave can be taken as an output here.
• Slide 19
• Oscillators Relaxation Triangular-wave oscillator
• Slide 20
• Oscillators Relaxation Triangular-wave oscillator
• Slide 21
• For the triangular wave generator, the frequency is found from: What is the frequency of the circuit shown here? 10 nF 82 k 22 k 10 k = 671 Hz Oscillators Relaxation
• Slide 22
• Normally, the triangle wave generator uses fast comparators to avoid slew rate problems. For non-critical applications, a 741 will work nicely for low frequencies (
• Slide 23
• Oscillators Square-wave A square wave relaxation oscillator is like the Schmitt trigger or Comparator circuit. The charging and discharging of the capacitor cause the op-amp to switch states rapidly and produce a square wave. The RC time constant determines the frequency.
• Slide 24
• Oscillators Square-wave
• Slide 25
• Slide 26
• Oscillators Sawtooth voltage controlled oscillator (VCO) Sawtooth VCO circuit is a combination of a Programmable Unijunction Transistor (PUT) and integrator circuit.
• Slide 27
• Oscillators Sawtooth VCO OPERATION Initially, dc input = V IN V out = 0V, V anode < V G The circuit is like an integrator. Capacitor is charging. Output is increasing positive going ramp.
• Slide 28
• Oscillators Sawtooth VCO OPERATION
• Slide 29
• Oscillators Sawtooth VCO OPERATION When V out = V P V anode > V G, PUT turns ON The capacitor rapidly discharges. V out drop until V out = V F. V anode < V G, PUT turns OFF V P maximum peak value V F minimum peak value
• Slide 30
• Oscillators Sawtooth VCO OPERATION Oscillation frequency
• Slide 31
• Oscillators Sawtooth VCO EXAMPLE In the following circuit, let V F = 1V. a) Find; (i) amplitude; (ii) frequency; b) Sketch the output waveform
• Slide 32
• Oscillators Sawtooth VCO EXAMPLE (contd)
• Slide 33
• Oscillators Sawtooth VCO EXAMPLE Solution a) (i) Amplitude and So, the peak-to-peak amplitude is;
• Slide 34
• Oscillators Sawtooth VCO EXAMPLE Solution a) (ii) Frequency
• Slide 35
• Oscillators Sawtooth VCO EXAMPLE Solution a) (ii) Frequency
• Slide 36
• Oscillators Sawtooth VCO EXAMPLE Solution b) Output waveform
• Slide 37
• Oscillators The 555 timer as an oscillator
• Slide 38
• Oscillators The 555 Timer As An Oscillator The 555 timer is an integrated circuit that can be used in many applications. The frequency of output is determined by the external components R 1, R 2, and C. The formula below shows the relationship.
• Slide 39
• Oscillators The 555 Timer As An Oscillator Duty cycles can be adjusted by values of R 1 and R 2. The duty cycle is limited to 50% with this arrangement. To have duty cycles less than 50%, a diode is placed across R 2. The two formulas show the relationship; Duty Cycle > 50 %
• Slide 40
• Oscillators The 555 Timer As An Oscillator Duty Cycle < 50 %
• Slide 41
• Oscillators The 555 Timer As An Oscillator
• Slide 42
• Oscillators The 555 Timer As An Oscillator The 555 timer may be operated as a VCO with a control voltage applied to the CONT input (pin 5).
• Slide 43
• Oscillators Summary Sinusoidal oscillators operate with positive feedback. Two conditions for oscillation are 0 feedback phase shift and feedback loop gain of 1. The initial startup requires the gain to be momentarily greater than 1. RC oscillators include the Wien-bridge, phase shift, and twin-T. LC oscillators include the Colpitts, Clapp, Hartley, Armstrong, and crystal.
• Slide 44
• Oscillators Summary (contd) The crystal actually uses a crystal as the LC tank circuit and is very stable and accurate. A voltage controlled oscillators (VCO) frequency is controlled by a dc control voltage. A 555 timer is a versatile integrated circuit that can be used as a square wave oscillator or pulse generator.
• Slide 45
• END CHAPTER 5