2 ShortCircuit Transconductance The shortcircuit transconductance of a circuit measures its strength in converting input voltage to output current CH 9 Cascode Stages and Current Mirrors 3 ID: 1031051
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2. CH 9 Cascode Stages and Current Mirrors2Short-Circuit TransconductanceThe short-circuit transconductance of a circuit measures its strength in converting input voltage to output current.
3. CH 9 Cascode Stages and Current Mirrors3Transconductance Example
4. CH 9 Cascode Stages and Current Mirrors4Derivation of Voltage GainBy representing a linear circuit with its Norton equivalent, the relationship between Vout and Vin can be expressed by the product of Gm and Rout.
5. CH 9 Cascode Stages and Current Mirrors5Example: Voltage Gain
6. CH 9 Cascode Stages and Current Mirrors6Comparison between Bipolar Cascode and CE StageSince the output impedance of bipolar cascode is higher than that of the CE stage, we would expect its voltage gain to be higher as well.
7. CH 9 Cascode Stages and Current Mirrors7Voltage Gain of Bipolar Cascode AmplifierSince rO is much larger than 1/gm, most of IC,Q1 flows into the diode-connected Q2. Using Rout as before, AV is easily calculated.
8. CH 9 Cascode Stages and Current Mirrors8Alternate View of Cascode AmplifierA bipolar cascode amplifier is also a CE stage in series with a CB stage.
9. CH 9 Cascode Stages and Current Mirrors9Practical Cascode StageSince no current source can be ideal, the output impedance drops.
10. CH 9 Cascode Stages and Current Mirrors10Improved Cascode StageIn order to preserve the high output impedance, a cascode PNP current source is used.
11. CH 9 Cascode Stages and Current Mirrors11MOS Cascode Amplifier
12. CH 9 Cascode Stages and Current Mirrors12Improved MOS Cascode AmplifierSimilar to its bipolar counterpart, the output impedance of a MOS cascode amplifier can be improved by using a PMOS cascode current source.
13. CH 9 Cascode Stages and Current Mirrors13Temperature and Supply Dependence of Bias CurrentSince VT, IS, n, and VTH all depend on temperature, I1 for both bipolar and MOS depends on temperature and supply.
14. CH 9 Cascode Stages and Current Mirrors14Concept of Current MirrorThe motivation behind a current mirror is to sense the current from a “golden current source” and duplicate this “golden current” to other locations.
15. CH 9 Cascode Stages and Current Mirrors15Bipolar Current Mirror CircuitryThe diode-connected QREF produces an output voltage V1 that forces Icopy1 = IREF, if Q1 = QREF.
16. CH 9 Cascode Stages and Current Mirrors16Bad Current Mirror Example IWithout shorting the collector and base of QREF together, there will not be a path for the base currents to flow, therefore, Icopy is zero.
17. CH 9 Cascode Stages and Current Mirrors17Bad Current Mirror Example IIAlthough a path for base currents exists, this technique of biasing is no better than resistive divider.
18. CH 9 Cascode Stages and Current Mirrors18Multiple Copies of IREFMultiple copies of IREF can be generated at different locations by simply applying the idea of current mirror to more transistors.