Oscillator Analogue Electronics Oscillator 1 0 Objectives u
Oscillator Analogue Electronics
Oscillator 1. 0 Objectives u Describe the basic concept of an oscillator u Discuss the basic principles of operation of an oscillator u Analyze the operation of RC and LC oscillators
Oscillator 1. 1 Introduction u Oscillators – produce a continuous signal of some type without the need of an input. u It is used to convert DC to AC. u. Applications – communications systems, digital systems (including computers), and test equipment.
Oscillator Introduction cont… u Oscillator – produces a repetitive signal from a DC voltage. u The feedback oscillator relies on a positive feedback of the output to maintain the oscillations. u The feedback gain must be kept to unity to keep the output from distorting.
Oscillator 1. 2 Basic principles for oscillation u Oscillator is an amplifier with positive feedback.
Oscillator Basic principles for oscillation – cont…
Oscillator Basic principles for oscillation – cont… The closed loop gain is:
Oscillator Basic principles for oscillation – cont… u In general A and are functions of frequency and thus may be written as; is a loop gain
Oscillator Basic principles for oscillation – cont… u Writing u Replacing s with j the loop gain becomes;
Oscillator Basic principles for oscillation – cont… u At a specific frequency f 0 u At this frequency, the closed loop gain will be infinite i. e. the circuit will have finite output for zero input signal – oscillation.
Oscillator Basic principles for oscillation – cont… Thus, the condition for sinusoidal oscillation of frequency f 0 is; This is known as Barkhausen criterion. The frequency of oscillation is solely determined by the phase characteristic of the feedback loop – the loop oscillates at the frequency for which the phase is zero.
Oscillator Basic principles for oscillation – cont…
Oscillator 1. 3 Design Criteria for Oscillators 1. The magnitude of the loop gain must be unity or slightly larger – Barkhaussen criterion 2. Total phase shift, of the loop gain must be where N=0, 1, 2, …
Oscillator 2. 0 Types of oscillators 1. RC oscillators n n Wien Bridge Phase-Shift 2. LC oscillators n n n Hartley Colpitts Crystal
2. 1 RC Oscillators
Oscillator 2. 1 RC Oscillators u RC feedback oscillators are generally limited to frequencies of 1 MHz or less. u The types of RC oscillators that will be discuss : Wien-bridge and Phase-shift
Oscillator 2. 1. 1 Wien-bridge Oscillator Criteria: § low frequency oscillator. § ranges from a few k. Hz to 1 MHz.
Oscillator Wien-bridge Oscillator cont… The lead-lag circuit is in the positive feedback loop of Wien-bridge oscillator. The voltage divider limits the gain. The lead lag circuit is basically a band-pass with a narrow bandwidth.
Oscillator Wien-bridge Oscillator cont… Lead-lag circuit
Oscillator Wien-bridge Oscillator cont… The loop gain for the oscillator is; where; and
Oscillator Wien-bridge Oscillator cont… Hence; Substitute s=jω,
Oscillator Wien-bridge Oscillator cont… For oscillation frequency, f 0;
Oscillator Wien-bridge Oscillator cont… u Since at the frequency of oscillation, T(j ) must be real (for zero phase condition) The imaginary component must be zero; Thus; o where the frequency of resonance:
Oscillator Wien-bridge Oscillator cont… Insert ωo function into the previous T ( jωo ) equation;
Oscillator Wien-bridge Oscillator cont… The magnitude condition is, T = 1; To start oscillation, the ratio R 2/R 1 must be slightly greater than 2.
Oscillator Wien-bridge Oscillator cont… With the ratio; then; K = 3 ensures the loop gain of unity – oscillation n n K > 3 : growing oscillations K < 3 : decreasing oscillations
Oscillator Wien-bridge Oscillator cont… The lead-lag circuit of a Wien-bridge oscillator reduces the input signal by 1/3 and yields a response curve as shown.
Oscillator Wien-bridge Oscillator cont… Since there is a loss of about 1/3 of the signal in the positive feedback loop, the voltage-divider ratio must be adjusted so that a positive feedback loop gain of 1 is produced. This requires a closed-loop gain of 3. The ratio of R 1 and R 2 can be set to achieve this.
Oscillator Wien-bridge Oscillator cont… The gain around the loop must equal unity (1).
Oscillator 2. 1. 2 Phase-Shift Oscillator u The phase shift oscillator utilizes three (3) RC circuits to provide 180° phase shift. u When coupled with an inverting amplifier with -180°, it provides the necessary feedback to sustain oscillations.
Oscillator Phase-Shift Oscillator cont… Vi C _ A 1 + V 1 C _ A 2 + V 2 V 1 C V 2 R R R V 3 R 2 _ A 3 + Vo
Oscillator Phase-Shift Oscillator cont… Vi C _ A 1 + V 1 R C _ A 2 + V 2 V 1 C V 2 R R V 3 R 2 _ A 3 + Vo
Oscillator Phase-Shift Oscillator cont… Loop gain, T(s):
Oscillator Phase-Shift Oscillator cont… Substitute s=jω,
Oscillator Phase-Shift Oscillator cont… u To satisfy condition T(jωo)=1, real component must be zero since the numerator is purely imaginary. u To get the oscillation frequency: u Substitute ωo in equation:
Oscillator Phase-Shift Oscillator cont… u. To satisfy condition |T(jωo)|=1, To start oscillation, the ratio R 2/R must be slightly greater than 8.
Oscillator Phase-Shift Oscillator cont… u. The gain must be at least 29 to maintain the oscillations. u The frequency of resonance for the this type is similar to any RC circuit oscillator:
Oscillator 2. 2 LC Oscillators
Oscillator 2. 2 LC Oscillators u Use transistors and LC tuned circuits or crystals in their feedback network. u LC feedback oscillators frequency range about hundreds of k. Hz to hundreds of MHz. u The types of RC oscillators that will be discuss: Colpitts, Hartley and Crystal oscillator.
Oscillator 2. 2. 1 Colpitts Oscillator u The Colpitts oscillator – uses an LC circuit in the feedback loop. u The feedback network is made up of a pair of tapped capacitors (C 1 and C 2) and an inductor L to produce a feedback necessary for oscillations.
Oscillator Colpitts Oscillator cont… u The output voltage is developed across C 1 , while the feedback voltage is developed across C 2.
Oscillator Colpitts Oscillator cont… u KCL at the output node: u Using voltage divider:
Oscillator Colpitts Oscillator cont… Substitute (2) into (1): Assume that oscillation has started, then Vo≠ 0, Therefore,
Oscillator Colpitts Oscillator cont… Substitute s=jω, Both real & imaginary component must be zero n Imaginary component:
Oscillator Colpitts Oscillator cont… Both real & imaginary component must be zero n Real component: Substitute (3) in (4):
Oscillator Colpitts Oscillator cont… Important criteria: u To initiate oscillations spontaneously:
Oscillator 2. 2. 2 Hartley Oscillator u The Hartley oscillator is almost identical to the Colpitts oscillator. u The primary difference is the feedback network of the Hartley oscillator uses tapped inductors (L 1 and L 2) and a single capacitor C.
Oscillator Hartley Oscillator cont… u The analysis of Hartley oscillator is identical to that Colpitts oscillator. u The frequency of oscillation:
Oscillator 2. 2. 3 Crystal Oscillator u Most communications and digital applications require the use of oscillators with extremely stable output u Crystal oscillators are invented to overcome the output fluctuation experienced by conventional oscillators. u Crystals used in electronic applications consist of a quartz wafer held between two metal plates and housed in a a package as shown in Fig. 9 (a) and (b).
Oscillator Crystal Oscillator cont… u Piezoelectric Effect n n n The quartz crystal is made of silicon oxide (Si. O 2) and exhibits a property called the piezoelectric. The frequency of the applied ac voltage is equal to the natural resonant frequency of the crystal. The thinner the crystal, higher its frequency of vibration. This phenomenon is called piezoelectric effect.
Oscillator Crystal Oscillator cont… u Characteristic of Quartz Crystal n n n The crystal can have two resonant frequencies; One is the series resonance frequency f 1 which occurs when XL = XC. At this frequency, crystal offers a very low impedance to the external circuit where Z = R. R L C CM
Oscillator Crystal Oscillator cont… u Characteristic of Quartz Crystal n n n The other is the parallel resonance (or antiresonance) frequency, f 2 occurs when reactance of the series leg equals the reactance of CM. At this frequency, crystal offers a very high impedance to the external circuit. R L C CM
Crystal Oscillator cont… u The crystal is connected as a series element in the feedback path from collector to the base so that it is excited in the series-resonance mode. u The oscillation frequency equals the series-resonance frequency of the crystal and is given by: BJT FET
End of 5 th session Quiz Date? ? ?
- Slides: 59