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BETWEEN THE EXTREMES

Analog lives on the slope

Digital Uses the flat ends of the transfer curve: fully on or fully off
Analog Uses the steep middle, where a small gate change makes a large current change
Bias A fixed DC gate voltage that holds the transistor in saturation
Small signal The tiny wiggle riding on top of the bias, which is what gets amplified
Everything from a radio front end to a sensor readout starts with a biased transistor.

THE COMMON-SOURCE STAGE

One transistor, one resistor, gain

Av = −gm·RD

The input goes to the gate and the output is taken from the drain. A rise in gate voltage raises the current, which drops more voltage across the load resistor, so the output falls: the gain is negative. Bias too high and the transistor leaves saturation and the gain collapses; too low and it cuts off.

Gain is the slope of the transfer curve at the operating point.

WHAT SETS THE GAIN

gm, ro and the trade-offs

gm = 2ID ÷ Vov
gm (transconductance) How strongly the gate steers the current, in mA per volt
ro (output resistance) Set by channel-length modulation, so it falls as L shrinks (Unit 26)
Intrinsic gain gmro The most gain one transistor can give: it falls in short, fast devices
Sensor link A FET sensor readout is this same stage: a small gate-voltage shift from bound charge becomes a larger output change
More current gives more gm, but costs power: analog design is a budget of watts, speed and gain.

COPYING A CURRENT

The current mirror

Iout = (W2 ÷ W1)·Iref

Two matched transistors share one gate voltage. The reference transistor, wired as a diode, sets that voltage so that it carries the reference current; the second transistor then carries a current in proportion to its width. Current mirrors bias nearly every amplifier on a chip.

The copy stays good only while the output transistor remains in saturation.

CHECK THE WORDS

Bias, gain or clipping?

Bias sets the stage, gain does the work, clipping is the limit.

UNIT 13 STUDY COMPLETE

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Ready for the Fab Challenge?

You've used a transistor as an amplifier and as a current copier, the two blocks inside almost every analog chip.