Oscillators Oscillators with LC Feedback Circuits

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  • Slide 1
  • Oscillators Oscillators with LC Feedback Circuits
  • Slide 2
  • Oscillators Oscillators With LC Feedback Circuits For frequencies above 1 MHz, LC feedback oscillators are used. LC feedback oscillators use resonant circuits in the feedback path. We will discuss the Colpitts, Hartley and crystal-controlled oscillators. Transistors are used as the active device in these types.
  • Slide 3
  • LC Oscillators Colpitts The Colpitts oscillator utilizes a tank circuit (LC) in the feedback loop to provide the necessary phase shift and to act as a resonant filter that passes only the desired frequency of oscillation.
  • Slide 4
  • 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. LC Oscillators Colpitts
  • Slide 5
  • The resonant frequency can be determined by the formula below.
  • Slide 6
  • Figure below shows the input impedance of the amplifier acts as a load on the resonant feedback circuit and reduces the Q of the circuit. The resonant frequency of a parallel resonant circuit depends on the Q, according to the formula below: LC Oscillators Colpitts
  • Slide 7
  • LC Oscillators Hartley The Hartley oscillator is similar to the Colpitts except that the feedback circuit consists of two series inductors and a parallel capacitor.
  • Slide 8
  • The frequency of oscillation 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. LC Oscillators Hartley where L T = L 1 + L 2
  • Slide 9
  • LC Oscillators Crystal-Controlled 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 10
  • 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 LC Oscillators Crystal-Controlled
  • Slide 11
  • Oscillators Relaxation Oscillators
  • Slide 12
  • 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 13
  • 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 14
  • Oscillators Relaxation Triangular-wave oscillator Assume that the output voltage of the comparator is at its maximum negative level. This output is connected to the inverting input of the integrator through R 1, producing a positive-going ramp on the output of the integrator. When the ramp voltage reaches the UTP, the comparator switches to its maximum positive level. This positive level causes the integrator ramp to change to a negative-going direction. The ramp continues in this direction until the LTP of the comparator is reached and the cycle repeats.
  • Slide 15
  • Oscillators Relaxation Triangular-wave oscillator
  • Slide 16
  • Oscillators Relaxation Triangular-wave oscillator Amplitude of the triangular output is set by establishing the UTP and LTP voltages according to the following formulas: The frequency of both waveforms depends on the R 1 C time constant as well as the amplitude-setting resistors, R 2 and R 3. By varying R 1, the f r can be adjusted without changing the output amplitude.
  • Slide 17
  • Determine the frequency of oscillation of the circuit in figure below. To what value must R 1 be changed to make the frequency 20 kHz? Oscillators Relaxation - EXAMPLE Answer: f r = 8.25 kHz, R1 = 4.13 kOhm
  • Slide 18
  • 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 19
  • Oscillators Square-wave
  • Slide 20
  • Slide 21
  • Oscillators Sawtooth voltage controlled oscillator (VCO) Sawtooth VCO circuit is a combination of a Programmable Unijunction Transistor (PUT) and integrator circuit.
  • Slide 22
  • 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 23
  • Oscillators Sawtooth VCO OPERATION
  • Slide 24
  • 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 25
  • Oscillators Sawtooth VCO OPERATION Oscillation frequency
  • Slide 26
  • Oscillators Sawtooth VCO EXAMPLE In the following circuit, let V F = 1V. a) Find; (i) amplitude; (ii) frequency; b) Sketch the output waveform
  • Slide 27
  • Oscillators Sawtooth VCO EXAMPLE (contd)
  • Slide 28
  • Oscillators Sawtooth VCO EXAMPLE Solution a) (i) Amplitude and So, the peak-to-peak amplitude is;
  • Slide 29
  • Oscillators Sawtooth VCO EXAMPLE Solution a) (ii) Frequency
  • Slide 30
  • Oscillators Sawtooth VCO EXAMPLE Solution a) (ii) Frequency
  • Slide 31
  • Oscillators Sawtooth VCO EXAMPLE Solution b) Output waveform
  • Slide 32
  • Oscillators The 555 timer as an oscillator
  • Slide 33
  • 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 34
  • 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 35
  • Oscillators The 555 Timer As An Oscillator Duty Cycle < 50 %
  • Slide 36
  • Oscillators The 555 Timer As An Oscillator
  • Slide 37
  • 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 38
  • END CHAPTER 5

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