Lecture 23
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Lecture 23. ANNOUNCEMENTS Midterm #2 results (undergrad. scores only): N=73; mean=63 ; median=63.5 ; std.dev.=8.42 You may pick up your exam during any TA office hour. Regrade requests must be made before you leave with your exam.

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Lecture 23

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Lecture 23

Lecture 23

ANNOUNCEMENTS

  • Midterm #2 results (undergrad. scores only):

    • N=73; mean=63; median=63.5; std.dev.=8.42

  • You may pick up your exam during any TA office hour.

    • Regrade requests must be made before you leave with your exam.

  • The list of misunderstood/forgotten points has been updated.

OUTLINE

  • BJT Differential Amplifiers (cont’d)

    • Cascode differential amplifiers

    • Common-mode rejection

    • Differential pair with active load

      Reading: Chapter 10.4-10.6.1

6

44

21


Cascode differential pair

Cascode Differential Pair

Half circuit for ac analysis


Telescopic cascode differential pair

Telescopic Cascode Differential Pair

Half circuit for ac analysis


Example

Example

Half circuit for ac analysis


Effect of finite tail impedance

Effect of Finite Tail Impedance

  • If the tail current source is not ideal, then when an input common-mode voltage is applied, the currents in Q1 and Q2 and hence the output common-mode voltage will change.


Effect of input cm noise ideal tail current

Effect of Input CM Noise Ideal Tail Current

  • There is no effect of the input CM noise at the output.


Effect of input cm noise non ideal tail current

Effect of Input CM NoiseNon-Ideal Tail Current

  • The single-ended outputs are corrupted by the input CM noise.


Comparison

Comparison

  • The differential output voltage signal is the same for both cases.

     For small input CM noise, the differential pair is not affected.

Ideal Tail Current

Non-Ideal Tail Current


Cm to dm conversion gain a cm dm

CM to DM Conversion; gain ACM-DM

  • If finite tail impedance and asymmetry (e.g. in load resistance) are both present, then the differential output signal will contain a portion of the input common-mode signal.


Example1

Example


Common mode rejection ratio

Common-Mode Rejection Ratio

  • CMRR is the ratio of the wanted amplified differential input signal to the unwanted converted input common-mode noise that appears at the output.


Differential to single ended conversion

Differential to Single-Ended Conversion

  • Many circuits require a differential to single-ended conversion.

  • This topology is not very good; its most critical drawback is supply noise corruption, since no common-mode cancellation mechanism exists. Also, we lose half of the voltage signal.


A better alternative

… A Better Alternative

  • This circuit topology performs differential to single-ended conversion with no loss of gain.


Active load

Active Load

  • With a current mirror as the load, the signal current produced by Q1 can be replicated onto Q4.

  • This type of load is different from the conventional “static load” and is called an “active load.”


Differential pair with active load

Differential Pair with Active Load

  • The input differential pair decreases the current drawn from RL by I, and the active load pushes an extra I into RL by current mirror action; these effects enhance each other.


Active load vs static load

Active Load vs. Static Load

  • The load in the circuit on the left responds to the input signal and enhances the single-ended output, whereas the load in the circuit on the right does not.


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