V out A v. Feedback Circuit

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1 Oscillators V out A v Feedback Circuit Figure.: Positive Feed Back The feedback network in an oscillator an input to the amplifier, which in turn an input to the feedback network. Since positive feedback produces this circuit action, it is often referred to a. State Barkhausen Criterion What is an oscillator? 009 Richard Lokken 4/0/009

2 What happens when α v A V >? What happens when α v A V <? List three requirements for proper oscillator operation.... Phase Shift Oscillator 009 Richard Lokken 4/0/009

3 Rf 7k U C C C Vout 0 0.uF 0.uF 4 0.uF 5 R k R k R k 0 0 Figure.: Phase Shift Oscillator. is a measure of the oscillators ability to maintain an output that is constant in and. A phase shift oscillator is a simple sine wave electronic oscillator. It contains an inverting amplifier, and a feedback filter which 'shifts' the phase by 80 degrees at the oscillation frequency. The filter must be designed so that at frequencies above and below the oscillation frequency the signal is shifted by either more or less than 80 degrees. This results in constructive superposition for signals at the oscillation frequencies, and destructive superposition for all other frequencies. The most common way of achieving this kind of filter is using three cascaded resistor capacitor filters, which produce no phase shift at one end of the frequency scale, and a phase shift of 70 degrees at the other end. At the oscillation frequency each filter produces a phase shift of 60 degrees and the whole filter circuit produces a phase shift of 80 degrees. 009 Richard Lokken 4/0/009

4 One of the simplest implementations for this type of oscillator uses an operational amplifier (opamp), three capacitors and four resistors. The mathematics for calculating the oscillation frequency and oscillation criterion for this circuit are surprisingly complex, due to each R C stage loading the previous ones. The calculations are greatly simplified by setting all the resistors (except the negative feedback resistor) and all the capacitors to the same values. In the diagram, if R = R = R = R, and C = C = C = C, then: B 9 (.) f r 6RC (.) A CL Rf 9 R (.) Wien Bridge Oscillator Vin R C Vout k uf C uf R k 0 Figure.: Wien Bridge Feedback Network. 009 Richard Lokken 4/0/009 4

5 R 4 R 5 +V R D D -V C R C R Figure.4: Wien Bridge Oscillator R C forms a R C forms a A series combination of a and a forms a The resonant frequency of this high pass filter determines the. A typical Wien Bridge Oscillator is designed so that R C = R C. The frequency of oscillation is then: f r V V out in RC (.4) (.5) 009 Richard Lokken 4/0/009 5

6 Colpitts In the Colpitts circuit, two capacitors and one inductor determine the frequency of oscillation. The Feedback Network L Vout Vin C 0 C Figure.5: Colpitts Oscillator Feedback Network. The operation of the feedback is based on the following key points: Richard Lokken 4/0/009 6

7 f r LC T (.6) CC CT C C (.7) C B C (.8) C A C (.9) Clapp oscillator L 4 C Vout Vin C 0 C Figure.6: Clapp Oscillator Feedback Network. A Clapp oscillator is a Colpitts oscillator with an added capacitor (in series with the feedback inductor) used to reduce the effects. 009 Richard Lokken 4/0/009 7

8 f r LC T (.0) C T C C C (.) Effect of C If C is much smaller than C and C then C almost entirely controls the resonant frequency. Since C and C are both connected to ground at one end, the junction capacitance of the transistor and other stray capacitances appear in parallel with C and C to ground. C is not affected, and thus provides a more stable frequency of oscillation. HARTLEY OSCILLATOR The tank circuit consists of the tapped coil (L and L ) and capacitor C. C Vout Vin L 0 L Figure.7: Hartley Oscillator f r LC T (.) 009 Richard Lokken 4/0/009 8

9 L B L (.) L A L (.4) Crystal Controlled Oscillators Most communications and digital systems require the use of oscillators with extremely stable outputs. The key to the operation of a crystal is the. This means: A quartz crystal is made of silicon dioxide, SiO. Figure.8: Crystal Symbol 009 Richard Lokken 4/0/009 9

10 C C L R C M Figure.9: Equivalent Circuit Figure.0: Pierce Oscillator Series and Parallel Resonance There is no such thing as a series cut crystal as opposed to a parallel cut crystal. The same crystal can be made to oscillate in series resonance mode or parallel resonance mode. The frequency of oscillation of a crystal is usually specified by the manufacturer as either the series resonance frequency or the parallel resonance frequency. A crystal can oscillate in series resonance, meaning that Ls is resonating with C s, and the resonance frequency is then simply. f series (.5) LC S S 009 Richard Lokken 4/0/009 0

11 Some oscillator circuits are designed for series resonance and the oscillation frequency shall equal the specified series resonance value. These series mode oscillators, however, are more sensitive to temperature and component variations. In fact, most crystals oscillators in today's ICs are of the parallel resonance type. The oscillation frequency of a parallel mode oscillator is always higher than f series. The actual oscillation frequency of a parallel mode oscillator is dependent on the equivalent capacitance seen by the crystal. f parallel f series C S C equ (.6) C Cequ Co C C C (.7) Summary: An is an ac signal generator. Barkhausen criterion states: is a measure of the circuits ability to maintain constant output amplitude and frequency. Op amp oscillators are limited to frequencies below because of the of the op amp. are mot stable enough for many communication and digital applications. A uses a quartz crystal to produce an extreme,ely stablwe output frequency. The is the simplest CCO. 009 Richard Lokken 4/0/009

Figure 1: Closed Loop System

Figure 1: Closed Loop System SIGNAL GENERATORS 3. Introduction Signal sources have a variety of applications including checking stage gain, frequency response, and alignment in receivers and in a wide range of other electronics equipment.