Bias Circuit

Unit 2. Transistors and Voltage
Amplification
Radio transmitters and receivers have existed since before the end of the nineteenth century.
A practical form of wireless telegraph, attributed to G. Marconi, appeared in 1895, and
successful transmission across the Atlantic Ocean was achieved in 1901. However, in the early
part of the twentieth century, systems were limited by the lack of a means of voltage
amplification. The appearance of a voltage amplification device, the vacuum tube, dramatically
improved the concept, as microvolt signals could be boosted for receiving and transmitting.
In the middle of the twentieth century, the transistor appeared. The idea of transistors based
on a sandwich of pn junctions (BJT) and a field-effect transistor based on pn junctions (JFET)
and on a metal – oxide – semiconductor (MOS) structure (basically, a capacitor) were all
understood at the time. However, pn-junction devices became a practical realization much
sooner than the MOS structure, due to fabrication complications in producing the MOS device
as well as perhaps a perceived lack of need. The JFET served as an interim field-effect
transistor until the MOS technology evolved. It provided for a transistor with very high input
resistance and was used extensively as the input transistors for BJT opamps.
A textbook on radio, Elements of Radio, published in 1948 (Marcus and Marcus, 1948), makes
no mention of transistors. A 1958 text, (Millman, 1958), Vacuum-Tube and Semiconductor
Electronics, gives equal weight to vacuum tubes and BJTs in electronic circuits but makes no
mention of the field-effect transistor. Slightly later (Nanavati, 1963), in An Introduction to
Semiconductor Electronics, as the title suggests, vacuum tubes are dropped completely and
the only reference to a field-effect transistor is in one section of the last chapter and this refers
to a junction field-effect transistor. In 1965, in his textbook Analysis and Design of Electronic
Circuits, Chirlian devotes a small portion of the book to vacuum tubes, but most of the
emphasis is on circuits based on the BJT (Chirlian, 1965). No mention is made of the fieldeffect-transistor.
An example of a book in which BJTs and field-effect transistors of both types
were finally given balanced treatment was published in 1979 (Millman, 1979). Textbooks tend
to lag the industry a bit, and during the 1970s, MOSFET circuits were emerging rapidly, driven
by the simultaneous development of integrated circuits. The four editions of a text on analog
circuits by Gray and Meyer, (1977, 1984, 1993) and Gray, et al. (2001) serve well as a series
through which we observe a transition from mostly BJT to, in the last two editions, more-orless
equal treatment of BJT and MOSFET devices. A recent textbook on the subject of analog
integrated circuits (Johns and Martin, 1997) takes the approach that such circuits are now
totally dominated by MOSFETS but includes some BJT applications. BiCMOS, a combination of
MOSFET and BJT devices on the same integrated circuit, is growing in popularity as more ways
of taking advantage of the superior properties of the two transistor types are developed.
Since the earliest transistors, there has been persistent competition between BJT and MOS
transistors. It has been, to a large extent (along with many other considerations), a matter of
power consumption versus speed; the BJT has been faster but is associated with high power
consumption. The MOSFET has gradually taken over as the most important transistor, with
increased emphasis on integrated circuits and improved speeds.