Unit – 1
Basic MOS physics and equivalent circuits and models:
MOSFET as a switch:
• Symbol for an n-type MOSFET, revealing three terminals: gate (G), source (S),
and drain (D).
• The latter two are interchangeable because the device is symmetric.
• When operating as a switch, the transistor “connects” the source and the drain
together if the gate voltage, VG, is “high” and isolates the source and the drain
if VG is “low.”
MOSFET structure:
MOS I/V characteristics:
Threshold Voltage:
• The dielectric, and the substrate form a capacitor, as VG becomes more
positive, the holes in the p-substrate are repelled from the gate area, leaving
negative ions behind so as to mirror the charge on the gate.
• In other words, a depletion region is created Under this condition, no current
flows because no charge carriers are available.
• As VG increases, so do the width of the depletion region and the potential at
the oxide-silicon interface
• In a sense, the structure resembles a voltage divider consisting of two capacitors
in series: the gateoxide capacitor and the depletion-region capacitor.
Derivation of I/V characteristics:
• Thus, the charge density at a point x along the channel can be written as.
• subject to boundary conditions V(0) = 0 and V(L) = VDS. While V(x) can be
easily found from this equation, the quantity of interest is in fact ID.
Multiplying both sides by dx and performing integration, we obtain
Since ID is constant along the channel,
Noise gain:
• Noise gain is the ratio of the output noise to the input-referred noise in an
operational amplifier (op-amp) or other analog circuits
• It determines how much the noise present in the system is amplified.
• This gain is influenced by the feedback network and is critical in predicting the
noise performance of the circuit.
ICMR:
• ICMR stands for Input Common Mode Range.
• ICMR is the range of input voltages over which both inputs of a differential
amplifier can swing while the amplifier maintains its normal operation.
• Outside this range, the performance degrades, and the amplifier may not
function correctly.
Voltage swing:
• Voltage swing is the difference between the maximum and minimum output
voltages that an amplifier can achieve. It is typically limited by the supply
voltages and the internal circuitry of the amplifier.
