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Published byFelipe Caris Modified over 2 years ago

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Oscillators Fixed Frequency, Controlled Power Transmitters Receivers/Tuners Clock Circuits AvAv B(j ) + + V out V in System poles @ A v B(s) = 1

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Barkhausen Criteria System poles @ A v B(s) = 1 Pole in RHP: Oscillation amplitude grows until amplifier starts to clip, then A v ’ < A v. 00 Root locus for A v ’ < A v

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Oscillator Design AvAv B(j ) + + V out V in Common Base: r in = r e A v = R c ’/r e Common Emitter: r in = ( +1)r e A v = -R c ’/r e Frequency Selective Feedback Network LC Network Quartz Crystal

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Amplifier Gain A v = + R c ’/r e r e = 26 mV/I E (from DC Analysis) R c ’ : Parallel Combination of 1.Transformed Load: R L ’ = R L / v 2 2.Parasitic Loading (finite Q): R coil = Q u X T 3.Feedback Network: R e ’ =..... 4.Collector Dynamic (AC) Resistance: r c ~ 20 k 5.Any other resistances in Collector Circuit

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Hartley Oscillator (common base, autotransformer feedback) E C

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R1R1 R2R2 R3R3 R4R4 RLRL n1n1 n2n2 n3n3 CTCT LTLT, Q u BP V CC, r c + - R2R2 R3R3 IBIB V CC R1R1 I C I B R3R3 rere I E I B ieie rcrc R4R4 n2n2 n1n1 + v e - B C E B C E rere Hartley Oscillator DC Equivalent AC Equivalent

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R3R3 rere ieie rcrc RPRP R4R4 n2n2 n1n1 + v e - B C E + v c - B1vcB1vc v e = B 1 B 2 v c R3R3 rere ieie rcrc RPRP + v e - B C E + v c - B=B 1 B 2 v e = Bv c Ideal

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rere ieie rcrc RPRP + v e - B C E + v c - B v e = Bv c rere ieie + v e - B C E + v c - B v e = Bv c Note: any additional resistance placed across the tank circuit must be included as an additional parallel contribution.

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B AVAV Barkhausen: is A V B > 1 ? Book Example : f = 1 Mhz I C ~ I E = 1 ma n 1 = 10, n 2 = 100, n 3 = 5 (subscripts changed) L T = 53 uH, Q u = 50, X T = 333 R L = 50 R 3 = 1 k R 4 = 0 therefore, B 2 = 1 r c = 50 k R1R1 R2R2 R3R3 R4R4 RLRL n1n1 n2n2 n3n3 CTCT LTLT, Q u BP V CC, r c rere

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Maximum Power Considerations The maximum 0-pk collector AC voltage is: The maximum power to the load is: V CC was not given for this problem, but we can determine a minimum value required to prevent saturation at max power by recalling that...

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Colpitts Oscillator (common base, capacitive feedback) E C

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