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Cascode Stage

Cascode Stage. OUTLINE. Review of BJT Amplifiers Cascode Stage Reading: Chapter 9.1. Review: BJT Amplifier Design. A BJT amplifier circuit should be designed to ensure that the BJT operates in the active mode, allow the desired level of DC current to flow, and

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Cascode Stage

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  1. Cascode Stage

  2. OUTLINE Review of BJT Amplifiers Cascode Stage Reading: Chapter 9.1

  3. Review: BJT Amplifier Design A BJT amplifier circuit should be designed to ensure that the BJT operates in the active mode, allow the desired level of DC current to flow, and couple to a small-signal input source and to an output “load”. Proper “DC biasing” is required! (DC analysis using large-signal BJT model) Key amplifier parameters: (AC analysis using small-signal BJT model) Voltage gain Av  vout/vin Input resistance Rin resistance seen between the input node and ground (with output terminal floating) Output resistance Rout resistance seen between the output node and ground (with input terminal grounded)

  4. Large-Signal vs. Small-Signal Models The large-signal model is used to determine the DC operating point (VBE, VCE, IB, IC) of the BJT. The small-signal model is used to determine how the output responds to an input signal.

  5. The voltage across an independent voltage source does not vary with time. (Its small-signal voltage is always zero.) It is regarded as a short circuit for small-signal analysis. The current through an independent current source does not vary with time. (Its small-signal current is always zero.) It is regarded as an open circuit for small-signal analysis. Small-Signal Models for Independent Sources Large-Signal Model Small-Signal Model

  6. Comparison of Amplifier Topologies Common Emitter Large Av < 0 - Degraded by RE - Degraded by RB/(b+1) Moderate Rin - Increased by RB - Increased by RE(b+1) Rout RC ro degrades Av, Rout but impedance seen looking into the collector can be “boosted” by emitter degeneration Common Base • Large Av > 0 - Degraded by RE and RS - Degraded by RB/(b+1) • Small Rin • - Increased by RB/(b+1) • - Decreased by RE • Rout RC • ro degrades Av, Rout • but impedance seen • looking into the collector • can be “boosted” by • emitter degeneration Emitter Follower • 0 < Av ≤ 1 - Degraded by RB/(b+1) • Large Rin • (due to RE(b+1)) • Small Rout • - Effect of source • impedance is • reduced by b+1 • - Decreased by RE • ro decreases Av, Rin, and Rout

  7. Common Emitter Stage

  8. Common Base Stage

  9. Emitter Follower

  10. Ideal Current Source An ideal current source has infinite output impedance. How can we increase the output impedance of a BJT that is used as a current source? Circuit Symbol I-V Characteristic Equivalent Circuit

  11. Recall that emitter degeneration boosts the impedance seen looking into the collector. This improves the gain of the CE or CB amplifier. However, headroom is reduced. Boosting the Output Impedance

  12. Cascode Stage In order to relax the trade-off between output impedance and voltage headroom, we can use a transistor instead of a degeneration resistor: VCE for Q2 can be as low as ~0.4V (“soft saturation”)

  13. Maximum Cascode Output Impedance The maximum output impedance of a cascode is limited by r1.

  14. PNP Cascode Stage

  15. False Cascodes When the emitter of Q1 is connected to the emitter of Q2, it’s not a cascode since Q2 is a diode-connected device instead of a current source.

  16. Short-Circuit Transconductance The short-circuit transconductance of a circuit is a measure of its strength in converting an input voltage signal into an output current signal.

  17. Voltage Gain of a Linear Circuit By representing a linear circuit with its Norton equivalent, the relationship between Vout and Vin can be expressed by the product of Gm and Rout. Norton Equivalent Circuit Computation of short-circuit output current:

  18. Example: Determination of Voltage Gain Determination of Gm Determination of Rout

  19. Comparison of CE and Cascode Stages Since the output impedance of the cascode is higher than that of a CE stage, its voltage gain is also higher.

  20. Voltage Gain of Cascode Amplifier Since rO is much larger than 1/gm, most of IC,Q1 flows into diode-connected Q2. Using Rout as before, AV is easily calculated.

  21. Practical Cascode Stage No current source is ideal; theoutput impedance is finite.

  22. Improved Cascode Stage In order to preserve the high output impedance, a cascode PNP current source is used.

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