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Oscillators overview

Oscillators overview. This chapter will highlight the following concepts Conditions for oscillation, basically Nyquist and Barkhausen criteria for oscillation Detailed circuit analysis for oscillator The Colpits and Hartely oscillator The concept of negative resistance The pierce oscillator

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Oscillators overview

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  1. Oscillators overview This chapter will highlight the following concepts Conditions for oscillation, basically Nyquist and Barkhausen criteria for oscillation Detailed circuit analysis for oscillator The Colpits and Hartely oscillator The concept of negative resistance The pierce oscillator Crystal oscillators Voltage controlled oscillators Relaxation oscillators 1

  2. Oscillators conditions for oscillation Electronic oscillators are feedback networks Both negative and positive feedback oscillators will be considered A negative feedback oscillator normally can be built from a common emitter circuit A positive feedback oscillator can be built from a common base oscillator 2

  3. Oscillators Nyquist Criteria Consider the block diagram shown below The G(jω) is the transfer function of the gain block H(jω) is the transfer function of the feedback block 3

  4. Oscillators Nyquist Criteria If we write an expression of the output voltage in terms of the input, then we have For oscillation, the output voltage V0 is nonzero even If the input is zero This condition can be met if the forward loop gain is infinite (which is impractical) or If the denominator is 4

  5. Oscillators Nyquist Criteria This leads to the well-known condition for oscillation, Nyquist criterion, at some frequency ω0 This means that the magnitude of the open loop transfer function is equal to 1, or And the phase shift is 180º, or 5

  6. Oscillators Barkhausen Criteria If a positive feedback is used the loop phase shift must be zero, The condition for stability are also known as the Barkhausen criteria, Barkhausen states that if the closed-loop transfer function is Then the system will oscillate, provided that 6

  7. Oscillators Barkhausen Criteria This is equivalent to the Nyquist criterion The difference being the transfer function is written for loop with positive feedback Both Nyquist and Barkhausen criterions states that the total phase shift around the loop must be 360º at the frequency of oscillation Also the magnitude of the open-loop gain must be unity at that frequency 7

  8. CE amplifier-oscillator If a single stage common emitter amplifier is used with feedback from collector to base, Then the feedback network must supply 180º phase shift, since there is 180º between the base and the collector. The total phase shift around the loop must be 360º 8

  9. CB amplifier-oscillator On the other hand if a common base amplifier is used, the feedback loop must not provide any phase shift, since there is no phase shift between the emitter and the collector in the common base amplifier 9

  10. Analysis of CB amplifier oscillator Consider the following the common base amplifier oscillator, It is desired to analyze this oscillator and finds an expression for the oscillation frequency 10

  11. Analysis of CB amplifier oscillator The small signal AC equivalent circuit for this amplifier is shown below. The value of the bias resistor RB and the transistor output ro are considered large enough so that they can be ignored 11

  12. Analysis of CB amplifier oscillator Since the amplifier is in common base configuration, positive feedback must be used The conditions for oscillation are The loop gain is calculated by openning the feed back loop, applying a signal 12

  13. Analysis of CB amplifier oscillator 13

  14. Analysis of CB amplifier oscillator 14

  15. Analysis of CB amplifier oscillator 15

  16. Analysis of CB amplifier oscillator 16

  17. Analysis of CB amplifier oscillator 17

  18. Analysis of CB amplifier oscillator example Example: Design a 20- MHz common base sinusoidal oscillator using a transistor with a minimum β of 100. Solution: The design starts by using the magnitude criteria of the amplifier according to the equation below Req can be determined from 18

  19. Analysis of CB amplifier oscillator Since the amplifier is in common base configuration, positive feedback must be used The conditions for oscillation are The loop gain is calculated by openning the feed back loop, applying a signal 19

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