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
Coined in the vacuum-tube era, the term “cascode” is believed to be an abbreviation of “cascaded
triodes.”