Cascode Stages and Current Mirrors
Following our study of basic bipolar and MOS amplifiers in previous chapters, we deal with two other important building blocks in this chapter. The “cascode”1 stage is a modified version of common-emitter or common-source topologies and proves useful in high-performance circuit design, and the “current mirror” is an interesting and versatile technique employed extensively in integrated circuits. Our study includes both bipolar and MOS implementations of each building block. Shown below is the outline of the chapter.
9.1 CASCODE STAGE
9.1.1 Cascode as a Current Source
Recall from Chapters 5 and 7 that the use of current-source loads can markedly increase the voltage gain of amplifiers. We also know that a single transistor can operate as a current source but its output impedance is limited due to the Early effect (in bipolar devices) or channel-length modulation (in MOSFETs).
How can we increase the output impedance of a transistor that acts as a current source? An important observation made in Chapters 5 and 7 forms the foundation for our study here: emitter or source degeneration “boosts” the impedance seen looking into the collector or drain, respectively. For the circuits shown in Fig. 9.1, we have