ELG4139: Oscillator Circuits

Transcription

1 ELG439: Oscillator ircuits Positive Feedback Amplifiers (Oscillators) L and rystal Oscillators JBT; FET; and I Based Oscillators The Active-Filter-Tuned Oscillator Multivibrators

2 Introduction There are two different approaches for the generation of sinusoids, most commonly used for the standard waveforms: Employing a positive-feedback loop that consists an amplifier and an or L frequency-selective network. It generates sine waves utilizing resonance phenomena, are known as linear oscillators (circuits that generate square, triangular, pulse waveforms are called non-linear oscillators or function generators.) A sine wave is obtained by appropriate shaping a triangular waveform.

3 The Oscillator Feedback Loop A basic structure of a sinusoidal oscillator consists of an amplifier and a frequencyselective network connected in a positive-feedback loop. The condition for the feedback loop to provide sinusoidal oscillations of frequency w 0 is Barkhausen riterion: At w 0 the phase of the loop gain should be zero. At w 0 the magnitude of the loop gain should be unity.

4 L and rystal Oscillators For higher frequencies (> MHz) wo L( ) wo ( L L) (a) olpitts and (b) Hartley.

5 Hartley Oscillator Used in radio receivers and transmitters More stable than Armstrong oscillators adio frequency choke (F) L L f 0 L eq where L M Mutual coupling between L eq L L M & L

6 olpitts Oscillators BJT; FET; and I Based f i - f 0 L eq L network where eq L network F is an impedance which is high (open) at high F frequencies and low (short) to dc voltages

7 Equivalent ircuit of the olpitts Oscillator wo L( ) omplete ircuit for a olpitts Oscillator

8 rystal Oscillators rystal is a piezo-electric device which converts mechanical pressure to electrical voltage or vice-vasa Series frequency f S S L Parallel frequency f P S P S P L adio communications, broadcasting stations Piezoelectric effect Why are crystal oscillators used in many commercial transmitters? 8

9 An Application of rystal Oscillator rystals are fabricated by cutting the crude quartz in a very exacting fashion. The type of cut determines the crystal s natural resonant frequency as well as it s temperature coefficient. rystal are available at frequencies about 5kHz and up providing the best frequency stability. However above 00MHz, they become so small that handling becomes a problem. Two crystals producing two different frequencies for measuring temperature Timing devices 9

10 Op-Amp rystal Oscillator Op-amp voltage gain is controlled by the negative feedback circuit formed by f and. More NFB will damp the oscillation, critical NFB will have a sine wave output and less NFB will have a square wave output. It is very flexible to construct the Op. Amp. crystal oscillator due to high amplifier gain and differential input facility of the Op. Amp. - f Op-amp The two Zener diodes connected face to face is to limit the peak to peak output voltage equal to twice of Zener voltage. + s V z The crystal is fed in series to the positive feedback which is required for oscillation. Therefore the oscillation frequency will be crystal series resonant frequency f s. 0

11 Example rystal used instead of inductor in the tank circuit of olpitts oscillator

12 The Phase Shifter Oscillator The phase-shifter consists of a negative gain amplifier (-K) with a third order ladder network in the feedback. The circuit will oscillate at the frequency for which the phase shift of the network is 80 o. Only at the frequency will the total phase shift around the loop be 0 o or 360 o. The minimum number of sections is 3 because it is capable of producing a 80 o phase shift at a finite frequency.

13 Phase-shift Oscillator bav i V i A b bav i = V i (or) Ab = Frequency of oscillation f 6 ondition of oscillation b 9 Ab A 9 Ab But L AVi // r // 40k 8k V DD D =? 8k 40k 40k - 8k 0k =? f 6. nf 6 f 6 0k k Let A 40 9 A g 40 8k m L L g 5000S D d D A f = khz D FET Phase-shift Oscillator r d = 40k g m = 5000mS =0k Example: Determine the value of capacitance and the value of D of the Phase-shift oscillator shown, if the output frequency is khz. Take r d = 40k and g m =5000mS, for the FET and = 0kW. m b

14 BJT Phase-Shift Oscillator V DD =? Example: Determine the value of capacitance and the value of h fe of the Phase-shift oscillator shown, if the output frequency is khz. Take =0 k. = k. Frequency of oscillation f 6 4 / ondition of oscillation b Ab A 9 9 for BJT h 3 9 fe 4 f 6 4 0k k for BJT h 0k 3 9 k / khz 6 4k /0k fe 3 9 k 4 0k 0k 6 4k /0k 0.006F 6nF

15 I Phase-shift Oscillator Frequency of oscillation f i Example: Determine the value of capacitance and the value of f of the I Phase-shift oscillator shown, if the output frequency is khz. Take =0kW. i =kw. f 6. nf 6 f 6 0k k 6 5 A for I inverting amplifier, f i 6 ondition of oscillation Ab A 9 for I inverting amplifier, A f i 9 b 9 f 9 9 i 9k - + f A b 5

16 Wien Bridge Oscillator f Frequency of oscillation 3 4 f ondition of oscillation 3 4 if if Example: Determine the value of capacitance and if =0kW = 0.mF 3 =0k 4 =kw in the Wien bridge oscillator shown has an output frequency of khz. f f 4 Frequency of oscillation 4 f ms 0.05ms k 0k k 99.96k 00k 0.05ms 00k 0k k 0.F 0k 0.05ms pf 0k ondition of oscillation

17 Tuned Oscillators (adio Frequency Oscillators) Tuned oscillator is a circuit that generates a radio frequency output by using L tuned (resonant) circuit. Because of high frequencies, small inductance can be used for the radio frequency of oscillation. Tuned-input and tuned-output Oscillator tuned-output feedback coupling ci L F output co tuned-input L f 0 L L 7

18 The Active-Filter-Tuned Oscillator Assume the oscillations have already started. The output of the band-pass filter will be a sine wave whose frequency is equal to the center frequency of the filter. The sine-wave signal is fed to the limiter and then produces a square wave.

19 Practical implementation of the active-filter-tuned oscillator

20 Bistable Multivibrators Another type of waveform generating circuits is the nonlinear oscillators or function generators which uses multivibrators. A bistable multivibrator has stable states. The circuit can remain in either state indefinitely and changes to the other one only when triggered. Metastable state: v + =0 and v O =0. The circuit cannot exist in the mestastable state for any length of time since any disturbance causes it to switch to either stable state.

21

22 Bistable ircuit with Inverting Transfer haracteristics Assume that v O is at one of its two possible levels, say L +, and thus v + = βl +. As v I increases from 0 and then exceeds βl +, a negative voltage developes between input terminals of the op amp. This voltage is amplified and v O goes negative. The voltage divider causes v + to go negative, increasing the net negative input and keeping the regenerative process going. This process culminates in the op amp saturating, that is, v O = L -. The circuit is said to be inverting Trigger signal

23 Bistable ircuit with Noninverting Transfer haracteristics

24 Application of the Bistable ircuit as a omparator To design a circuit that detects and counts the zero crossings of an arbitrary waveform, a comparator whose threshold is set to 0 can be used. The comparator provides a step change at its output every time zero crossing occurs.

25 Bistable ircuit with More Precise Output Level Limiter circuits are used to obtain more precise output levels for the bistable circuit. L+ = V Z + V D and L = (V Z + V D ), where V D is the forward diode drop. L+ = V Z + V D + V D and L = (V Z + V D3 + V D4 ).

26 Operation of the Astable Multivibrator onnecting a bistable multivibrator with inverting transfer characteristics in a feedback loop with an circuit results in a square-wave generator.

27 Operation of the Astable Multivibrator

28 Generation of Triangular Waveforms Triangular waveforms can be obtained by replacing the low-pass circuit with an integrator. Since the integrator is inverting, the inverting characteristics of the bistable circuit is required.

29 Generation of a Standard Pulse In the stable state, V A =L+ (why?), V B =V D, V =βl+ (D: ON and 4>>). When a negative-going step applies at the trigger input: D conducts heavily and pulls node down (lower than V B ). The output of the op amp switch to L- and cause V to go toward βl-. D OFF and isolates the circuit from changes at the trigger input. D OFF and begins to discharge toward L-. When V B < V, the output of the op amp switch to L+.

30 Generation of a Standard Pulse

31 The 555 ircuit ommercially available integrated-circuit package such as 555 timer exists that contain the bulk of the circuitry needed to implement monostable and astable multivibrator. /3 V /3 V