WHY A REFERENCE

Anchoring the solution

E_ref must stay fixed

The reference electrode sets the solution's potential, and the gate voltage is applied between it and the source. If the reference potential wanders, the threshold appears to move exactly as if the pH had changed. The sensor can only be as stable as its reference.

A drifting reference looks identical to a changing pH.

REFERENCE OPTIONS

Stable, small, or both

🥈

Ag/AgCl

A silver wire coated with silver chloride in a fixed chloride solution. Stable and well defined

📍

Pseudo-reference

A bare Ag/AgCl or Pt wire in the sample. Tiny, but its potential shifts with the sample's chloride

🔬

Miniaturized

On-chip versions trade some stability for size, for portable devices

The choice is a trade between stability and size.

TWO WAYS TO READ

What the circuit measures

Constant current A feedback loop holds ID fixed and reports the gate voltage it needs. The output is ΔVth in millivolts

Constant voltage Hold VGS and VDS fixed and read the drain current. Simple, but the output is nonlinear

Constant-current output is in millivolts and tracks pH almost linearly.

SUBTHRESHOLD READOUT

Reading the current directly

10^21/70 ≈ 2×

Below threshold, the current changes about 10× for every 60 to 100 mV of gate shift. With a swing of 70 mV per decade, a 21 mV threshold shift changes the current by 10^21/70, or about 2×. So a 0.4 pH change doubles the current.

In subthreshold, a small pH change becomes a large, easy-to-read current change.

A REAL MEASUREMENT

Putting it together

1

Dip

The sensor and the reference electrode sit in the sample

2

Bias

Set VDS and hold the gate bias through the reference electrode

3

Read

Record ΔVth or ΔID, then convert to pH with a calibration line

Calibrating in known buffers turns millivolts into pH.

UNIT 12 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered the stable reference that sets the gate potential, and the two ways to read the signal.