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EE 330 Lecture 31

EE 330 Lecture 31. Current Source Biasing Current Sources and Mirrors. Basic Amplifier Gain Table. Review from Last Lecture. CE/CS. CC/CD. CB/CG. CEwRE/CSwRS. BJT. MOS. BJT. MOS. BJT. MOS. BJT. MOS. A V. R in. R out.

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EE 330 Lecture 31

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  1. EE 330Lecture 31 Current Source Biasing Current Sources and Mirrors

  2. Basic Amplifier Gain Table Review from Last Lecture CE/CS CC/CD CB/CG CEwRE/CSwRS BJT MOS BJT MOS BJT MOS BJT MOS AV Rin Rout Can use these equations only when circuit is EXACTLY like that shown above !!

  3. Review from Last Lecture Basic Amplifier Characteristics Summary • Large noninverting gain • Low input impedance • Moderate (or high) output impedance • Used more as current amplifier or, in conjunction with CD/CS to form • two-stage cascode CE/CS • Gain very close to +1 (little less) • High input impedance for BJT (high for MOS) • Low output impedance • Widely used as a buffer CC/CD • Large noninverting gain • Low input impedance • Moderate (or high) output impedance • Used more as current amplifier or, in conjunction with CD/CS to form • two-stage cascode CB/CG • Reasonably accurate but somewhat small gain (resistor ratio) • High input impedance • Moderate output impedance • Used when more accurate gain is required CEwRE/ CSwRS

  4. Review from Last Lecture High-gain BJT amplifier To make the gain large, it appears that all one needs to do is make RC large ! But Vt is fixed at approx 25mV and for good signal swing, ICQRC<(VDD-VEE)/2 If VDD-VEE=5V, Gain is practically limited with this supply voltage to around 100

  5. Review from Last Lecture High-gain MOS amplifier To make the gain large, it appears that all one needs to do is make RD large ! But VEB is practically limited to around 100mV and for good signal swing, IDQRD<(VDD-VSS)/2 If VDD -VSS=5V and VEB=100mV, Gain is practically limited with this supply voltage to around 100 Are these fundamental limits on the gain of the BJT and MOS Amplifiers?

  6. Review from Last Lecture High-gain amplifier This gain is very large ! Too good to be true ! Need better model of MOS device!

  7. Review from Last Lecture High-gain amplifier This gain is very large (but realistic) ! But how can we make a current source?

  8. High-gain amplifier How can we build the ideal current source? What is the small-signal model of an actual current source?

  9. Current Sources/Mirrors If the base currents are neglected

  10. Current Sources/Mirrors If the base currents are neglected since VBE1=VBE2 Behaves as a current source ! Actually termed a “sink” current since coming out of load

  11. Current Sources/Mirrors Current Sink • Multiple Outputs Possible • Can be built at sourcing or sinking currents • Also useful as a current amplifier • MOS counterparts work very well and are not plagued by base current

  12. Current Sources/Mirrors Multiple-Output Bipolar Current Sink

  13. Current Sources/Mirrors Multiple-Output Bipolar Current Source

  14. Current Sources/Mirrors Multiple-Output Bipolar Current Source and Sink

  15. Current Sources/Mirrors Multiple-Output Bipolar Current Source and Sink

  16. Current Sources/Mirrors npn Current Mirror • Termed a “current mirror” • Output current linearly dependent on Iin • Serves as a current amplifier • Widely used circuit

  17. Current Sources/Mirrors npn current mirror amplifier

  18. Current Sources/Mirrors npn current mirror amplifier Amplifiers both positive and negative currents

  19. Current Sources/Mirrors n-channel Current Mirror npn Current Mirror

  20. Current Sources/Mirrors n-channel Current Mirror If process parameters are matched, it follows that Current mirror gain can be accurately controlled Layout is important to get accurate gain (for both MOS and BJT)

  21. Layout of Current Mirrors Example with M = 2 Standard layout Gate area after fabrication depicted

  22. Layout of Current Mirrors Example with M = 2 Standard layout Better Layout

  23. Layout of Current Mirrors Example with M = 2 Standard layout Better Layout This is termed a common-centroid layout Even Better Layout

  24. Current Sources/Mirrors n-channel current mirror current amplifier Amplifiers both positive and negative currents

  25. Current Sources/Mirrors multiple output n-channel current sink array multiple output p-channel current source array

  26. High-gain amplifier How can we build the current source? What is the small-signal model of an actual current source?

  27. Basic Current Sources and Sinks Basic Bipolar Current Sinks Basic Bipolar Current Sources Very practical methods for biasing the BJTs can be used Current Mirrors often used for generating sourcing and sinking currents

  28. Basic Current Sources and Sinks Small-signal Model of BJT Current Sinks and Sources Small-signal model of all other BJT Sinks and Sources are the same

  29. Basic Current Sources and Sinks Small-signal Model of BJT Current Sinks and Sources Small-signal model of all other BJT Sinks and Sources are the same

  30. Basic Current Sources and Sinks Small-signal Model of BJT Current Sinks and Sources Small-signal model of all other MOS Sinks and Sources are the same

  31. High-gain amplifier

  32. High-gain amplifier • Nonideal current source decreased the gain by a factor of 2 • But the voltage gain is still quite large Can the gain be made even larger?

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