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Lecture 5

Lecture 5. ANNOUNCEMENTS HW1 will be considered as extra credit. HW3 is posted, due Tuesday 9/18. OUTLINE BJT (cont’d) Transconductance Small-signal model The Early effect BJT operation in saturation mode Reading: Chapter 4.4.3-4.5. Notes on PN Junctions.

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Lecture 5

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  1. Lecture 5 ANNOUNCEMENTS • HW1 will be considered as extra credit. • HW3 is posted, due Tuesday 9/18 OUTLINE • BJT (cont’d) • Transconductance • Small-signal model • The Early effect • BJT operation in saturation mode Reading: Chapter 4.4.3-4.5

  2. Notes on PN Junctions • Typically, pn junctions in IC devices are formed by counter-doping. The equations provided in class (and in the textbook) can be readily applied to such diodes if • NA net acceptor doping on p-side (NA-ND)p-side • ND net donor doping on n-side (ND-NA)n-side ID (A) VD (V)

  3. Transconductance, gm • The transconductance (gm) of a transistor is a measure of how well it converts a voltage signal into a current signal. • It will be shown later that gm is one of the most important parameters in integrated circuit design.

  4. Visualization of Transconductance • gm can be visualized as the slope of the IC vs.VBE curve. • The slope (hence gm) increases with IC.

  5. Transconductance and IC • For a given VBE swing (DV), the resulting current swing about IC2 is larger than it is about IC1. • This is because gm is larger when VBE = VB2.

  6. Transconductance and Emitter Area • When the BJT emitter area is increased by a factor n, IS increases by the factor n.  For a fixed value of VBE, IC and hence gm increase by a factor of n.

  7. Derivation of Small-Signal Model • The BJT small-signal model is derived by perturbing the voltage difference between two terminals while fixing the voltage on the third terminal, and analyzing the resultant changes in terminal currents. • This is done for each of the three terminals as the one with fixed voltage. • We model the current change by a controlled source or resistor.

  8. Small-Signal Model: VBE Change

  9. Small-Signal Model: VCE Change • Ideally, VCE has no effect on the collector current. Thus, it will not contribute to the small-signal model. • It can be shown that VCB ideally has no effect on the small-signal model, either.

  10. Small-Signal Model: Example 1 • The small-signal model parameters are calculated for the DC operating point, and are used to determine the change in IC due to a change in VBE.

  11. Small-Signal Model: Example 2 • In this example, a resistor is placed between the power supply and collector, to obtain an output voltage signal. • Since the power supply voltage does not vary with time, it is regarded as ground (reference potential) in small-signal analysis.

  12. The Early Effect • In reality, the collector current depends on VCE: • For a fixed value of VBE, as VCE increases, the reverse bias on the collector-base junction increases, hence the width of the depletion region increases. Therefore, the quasi-neutral base width decreases, so that collector current increases.

  13. Early Effect: Impact on BJT I-V • Due to the Early effect, collector current increases with increasing VCE, for a fixed value of VBE.

  14. Early Effect Representation

  15. Early Effect and Large-Signal Model • The Early effect can be accounted for, by simply multiplying the collector current by a correction factor. • The base current does not change significantly.

  16. Early Effect and Small-Signal Model

  17. Summary of BJT Concepts

  18. BJT in Saturation Mode • When the collector voltage drops below the base voltage, the collector-base junction is forward biased. Base current increases, so that the current gain (IC/IB) decreases.

  19. Large-Signal Model for Saturation Mode

  20. BJT Output Characteristics • The operating speed of the BJT also drops in saturation.

  21. Example: Acceptable VCC Range • In order to prevent the BJT from entering very deeply into saturation, the collector voltage must not fall below the base voltage by more than 400 mV.

  22. Deep Saturation • In deep saturation, the BJT does not behave as a voltage-controlled current source. • VCE is ~constant.

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