Bias Circuit

2.1 BJT and MOSFET Schematic Symbols, Terminal
Voltages, and Branch Currents
The BJT can be either a pnp or an npn. The MOSFET similarly can be a pmos or an nmos. The
equivalents are npn and nmos and pnp and pmos. The following discussion is based on the npn
and nmos, as shown in Fig. 2.1. (All polarities and current directions are reversed for the pnp
and pmos. This provides for important versatility in applications.)
Figure 2.1. BJT npn and MOSFET nmos transistors. The terminal
configurations are designated common emitter and common source.
The BJT terminals are designated collector, base, and emitter while those of the MOSFET are
drain, gate, and source. The terminal configurations in Fig. 2.1 are, for the BJT, the common
emitter, and for the MOSFET, the common source, in amplifier-stage parlance. This suggests
that both the input (left side) and output (right side) are referred to the common terminal. For
example, for the BJT, the input terminal voltage is VBE and the output terminal voltage is VCE.
Similarly, for the MOSFET, we have VGS and VDS. Note that in the convention of subscripts in
electronics, the first subscript is assigned positive. This matches the assignments in the
diagram, and the plus and minus signs are superfluous.
Note also the convention for symbols for all currents and voltage.
Total voltage and current: vXY, iX
Dc, bias, quiescent, or operating point: VXY, IX
Signal or ac (RMS, peak): Vxy, Ix
General instantaneous signal: vx, ix
The voltage and current symbols in Fig. 2.4 are therefore for dc. For a voltage, a single
subscript means that this terminal (or node) voltage is referred to the common terminal. For
example, in the npn case above, VCE = VC.
Figure 2.4. Basic NMOS amplifier with resistor gate biasing and input
signal Vs. (a) Complete circuit. (b) Signal (or ac or incremental) circuit.
The signal circuit is obtained by setting the power supply (dc) node to
zero volts. (c) Linear signal circuit replaces the linear schematic
representation.