Oscillator Circuits 1

Transcription

1 Oscillator Circuits

2 II. Oscillator Operation For self-sustaining oscillations: the feedback signal must positive the overall gain must be equal to one (unity gain)

3 If the feedback signal is not positive or the gain is less than one, then the oscillations will dampen out. If the overall gain is greater than one, then the oscillator will eventually saturate. 3

4 Types of Oscillator Circuits A. Phase-Shift Oscillator B. Wien Bridge Oscillator C. Tuned Oscillator Circuits D. Crystal Oscillators E. Unijunction Oscillator 4

5 A. Phase-Shift Oscillator Frequency of the oscillator: f 0 (the frequency where the phase shift is 80º) πc 6 Feedback gain β /[ 5α j (6α α 3 ) ] where α /(πfc) Feedback gain at the frequency of the oscillator β / 9 The amplifier must supply enough gain to compensate for losses. The overall gain must be unity. Thus the gain of the amplifier stage must be greater than /β, i.e. A > 9 The C networks provide the necessary phase shift for a positive feedback. They also determine the frequency of oscillation. 5

6 FET Phase-Shift Oscillator Example of a Phase-Shift Oscillator 6

7 Example 7

8 BJT Phase-Shift Oscillator h ie C h fe > C 8

9 Phase-shift oscillator using op-amp 9

10 0 B. Wien Bridge Oscillator So frequency of oscillation is ( ) 0 C C π f Z Z Z Z Z V V V V V β a b d o i and, then and When πc f C C C should have zero phase at the oscillation frequency i.e.,, Z Z Z Z Z C C + β

11 Example Calculate the resonant frequency of the Wien bridge oscillator shown above f πc 3 π(5 0 )( ) 30.7 Hz

12 C. Tuned Oscillator Circuits Tuned Oscillators use a parallel LC resonant circuit (LC tank) to provide the oscillations. There are two common types: Colpitts The resonant circuit is an inductor and two capacitors. Hartley The resonant circuit is a tapped inductor or two inductors and one capacitor.

13 Colpitts Tuned Oscillator Circuit FET based Colpitts Oscillator Frequency of oscillations: f 0 π LC C C, where Ceq eq C + C 3

14 Transistor Colpitts oscillator. 4

15 Op-amp Colpitts oscillator. 5

16 FET Hartley Oscillator Hartley Tuned Oscillator Circuit Frequency of oscillations: f 0, where Leq L + L + L π LeqC 6

17 BJT based Hartley Oscillator 7

18 D. Crystal Oscillators The crystal appears to the rest of the circuit as a resonant circuit. 8

19 The crystal has two resonant frequencies: Crystal esonant Frequencies Series resonant: LC determine the resonant frequency. The crystal has a low impedance. Parallel resonant: L and C M determine the resonant frequency. The crystal has a high impedance. The series and parallel resonant frequencies are very close, within % of each other. 9

20 Series esonant Crystal Oscillator FET Crystal Oscillator 0

21 Parallel esonant Crystal Oscillator BJT Crystal Oscillator

22 Crystal Oscillator using Op-amp

23 E. Unijunction Oscillator Frequency of oscillations: f0 T CT ln where η 0.4 to 0.6 η is a rating of the unijunction transistor. [ ( η) ] 3

24 Unijunction Oscillator Waveforms V V BB P V ηv B BB V B + V B + V D The unijunction oscillator (or relaxation oscillator) produces a sawtooth waveform. 4

25 5

26 Unijunction transistor (UJT): basic construction. 6

27 The UJT is also basically a switching device. Schematic Symbol: UJT Unijunction Transistor 7

28 UJT Basic Operation Even though the UJT is a switching device it works very differently from the SC variety of devices. The equivalent circuit indicates that the UJT is like a diode and a resistive voltage divider circuit. The resistance exhibited by B is variable; it is dependent on the value of current I E. 8

29 UJT Characteristic Curve A voltage is applied across the UJT (V BB ) and to the Emitter input (V E ). Once V E reaches a peak value (V p ) the UJT begins to conduct. At the point where V E V p, the current I E is at minimum. This is the threshold value of V E that puts the UJT into conduction. Once conducting, I E increases and V E decreases. This phenomenon occurs because the internal resistance labeled B in the equivalent circuit decreases as the UJT conducts more and more. This is called negative resistance. 9