lecture 12 n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Lecture 12 PowerPoint Presentation
Download Presentation
Lecture 12

Loading in 2 Seconds...

play fullscreen
1 / 19

Lecture 12 - PowerPoint PPT Presentation


  • 148 Views
  • Uploaded on

Lecture 12. ANNOUNCEMENTS Review session: 3-5PM Friday (10/5) in 306 Soda (HP Auditorium) Midterm #1 (Thursday 10/11, 3:30PM-5:00PM) location: 106 Stanley Hall: Students with last names starting with A-L 306 Soda Hall: Students with last names starting with M-Z. OUTLINE

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Lecture 12' - oshin


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
lecture 12
Lecture 12

ANNOUNCEMENTS

  • Review session: 3-5PM Friday (10/5) in 306 Soda (HP Auditorium)
  • Midterm #1 (Thursday 10/11, 3:30PM-5:00PM) location:
    • 106 Stanley Hall: Students with last names starting with A-L
    • 306 Soda Hall: Students with last names starting with M-Z

OUTLINE

  • Cascode Stage (cont’d)
    • supplementary remarks
  • Current Mirrors

Reading: Chapter 9.2

review cascode stage r out
Review: Cascode Stage Rout
  • The impedance seen looking into the collector can be boosted significantly by using a BJT for emitter degeneration, with a relatively small reduction in headroom.
another view of a cascode stage
Another View of a Cascode Stage
  • Instead of considering a cascode as Q2 degenerating Q1, we can also think of it as Q1 stacked on top of Q2 (current source) to boost Q2’s output impedance.
temperature and supply voltage dependence of bias current
Temperature and Supply-Voltage Dependence of Bias Current
  • Circuits should be designed to operate properly over a range of supply voltages and temperatures.
  • For the biasing scheme shown below, I1 depends on the temperature as well as the supply voltage, since VT and IS depend on temperature.
concept of a current mirror
Concept of a Current Mirror
  • Circuit designs to provide a supply- and temperature-independent current exist, but require many transistors to implement.

 “golden current source”

  • A current mirror is used to replicate the current from a “golden current source” to other locations.
current mirror circuitry
Current Mirror Circuitry
  • Diode-connected QREF produces an output voltage VX that forces Icopy1 to be equal to IREF, if Q1 is identical to QREF.

Current Mirror Circuitry

Current mirror concept

Generation of required VBE

bad current mirror example 1
Bad Current Mirror Example 1
  • If the collector and base of QREF are not shorted together, there will not be a path for the base currents to flow, so that Icopy is zero.
bad current mirror example 2
Bad Current Mirror Example 2
  • Although it provides a path for base currents to flow, this biasing approach is no better than a resistive voltage divider.
multiple copies of i ref
Multiple Copies of IREF
  • Multiple copies of IREF can be generated at different locations by applying the current mirror concept to multiple transistors.
current scaling
Current Scaling
  • By scaling the emitter area of Qj by a factor of n with respect to the emitter area of QREF, Icopy,j is scaled by a factor of n with respect to IREF.
    • This is equivalent to placing n unit-sized transistors in parallel.
fractional scaling
Fractional Scaling
  • A fraction of IREF can be created in Q1 by scaling up the emitter area of QREF.
example different mirroring ratios
Example: Different Mirroring Ratios
  • Using the concept of current scaling and fractional scaling, Icopy1 = 0.05mA and Icopy2 = 0.5mA, derived from a single 0.2mA reference current source (IREF).
improved mirroring accuracy
Improved Mirroring Accuracy
  • Use QF (rather than IREF) to supply the base currents of QREF and Q1, reduce the mirroring error by a factor of .
pnp current mirror
PNP Current Mirror
  • A PNP BJT current mirror can be used as a current-source load for an NPN BJT amplifier stage.
generation of i ref for a pnp bjt current mirror
Generation of IREF for a PNP-BJT Current Mirror
  • Neglecting base currents, the currents flowing through QM and QREF2 are the same.
current mirror with discrete bjts
Current Mirror with Discrete BJTs
  • If QREF and Q1 are discrete NPN BJTs, IREF and Icopy1 can differ dramatically, due to IS mismatch.