Analog Basics for Digital Designers

Ground-up guide to CMOS circuit design — transistors, gates, delay, power, memory, datapath and test

Why a Digital Designer Meets Analog

A chip looks digital from the outside: ones and zeros. But the edges of that digital world are analog. Clocks come from analog loops. Signals enter and leave through analog drivers. Sensors, references, and high-speed links are analog. So even a pure digital designer keeps bumping into analog blocks. Understanding them, at least in outline, makes you far more useful. This chapter gives a plain-language tour of the analog ideas a digital designer should know: current mirrors, the differential pair, voltage references, converters between analog and digital, comparators, and how supply noise hurts analog. No heavy math — just the shapes and the reasons.

The Current Mirror

Analog circuits love to copy a current. A current mirror does exactly that: set up one current, and the mirror makes a matching copy somewhere else. It uses two matched transistors sharing the same gate voltage. The first sets the current; the second copies it. Why copy currents? Because a steady, known current is the lifeblood of analog. It biases amplifiers, sets speeds, and feeds references. The mirror's accuracy depends on the two transistors being well matched, which is why analog layout places them carefully, side by side, same orientation.

Worked example — a scaled mirror

A mirror can also scale. If the copy transistor is made three times the size of the reference, it carries three times the current.

reference current = 10 µA
copy transistor = 3× size
copied current = 3 × 10 = 30 µA

So one reference current can spawn several scaled copies for different blocks. That is why mirrors are everywhere in analog design.

The Differential Pair

The differential pair is the heart of most amplifiers. It is two matched transistors sharing a common current source at the bottom. Two inputs come in, one to each transistor. The pair responds to the difference between the inputs, not their absolute level. This is powerful. Noise that hits both inputs equally cancels out, because the pair only cares about the difference. So a differential pair rejects shared (common-mode) noise. That makes it the building block for clean amplifiers, comparators, and sense amplifiers.

Worked example — common-mode rejection

Both inputs sit at 0.6 V, and a 0.05 V noise bump hits both equally.

input difference before noise = 0 V
after noise: (0.6+0.05) - (0.6+0.05) = 0 V
the pair sees no change → noise rejected

Only a true difference between the inputs moves the output. Shared noise disappears. That is the whole appeal.

Voltage References

Many circuits need a steady voltage that does not move with temperature or supply. A plain divider will not do — it drifts. A reference circuit makes a fixed voltage that holds steady across conditions. A common idea (the bandgap reference) cleverly adds two voltages that drift in opposite directions with temperature. One rises with heat, the other falls. Add them in the right ratio and the drifts cancel, leaving a nearly flat voltage. Digital designers rarely build these, but they rely on them: data converters and sensors all need a stable reference.

Converting Between Analog and Digital

The real world is analog: sound, light, voltage from a sensor. The chip is digital. Converters bridge the two.

An ADC (analog-to-digital converter) measures an analog voltage and outputs a digital number. A DAC (digital-to-analog converter) takes a digital number and outputs an analog voltage. Their key specs are resolution (how many bits, i.e. how fine the steps) and speed (how many conversions per second).

Worked example — ADC step size

A 10-bit ADC measures a 1.0 V range.

levels = 2^10 = 1024 steps
step size = 1.0 V / 1024 ≈ 0.98 mV per step

Each digital code represents about 1 mV. More bits give finer steps but cost area, power, and design effort. This resolution-versus-cost trade is central to converter design.

The Comparator

A comparator is a simple, vital analog block. It compares two voltages and outputs a digital high or low depending on which is larger. It is the bridge from a smooth analog level to a hard digital decision. Comparators sit inside ADCs, sense amplifiers, and many control circuits. A good comparator is fast and decides cleanly even when the two inputs are very close. When inputs are nearly equal it can momentarily hesitate (metastability), so designs give it time or add stages, much like a synchronizer in digital logic.

Why Supply Noise Hurts Analog

Digital logic is forgiving of supply noise — it only needs to tell high from low. Analog is not. A reference, an amplifier, or a converter measures small voltages precisely. Noise on the supply leaks into these small signals and corrupts the result. This is why analog blocks get special care: separate, quiet supplies; guard rings and isolation from noisy digital neighbours; and differential designs that reject shared noise. A digital designer who places a noisy block next to a sensitive analog one can ruin the chip's accuracy.

Worked example — noise versus signal

A sensor signal is 5 mV. Supply noise coupling in is 1 mV.

noise / signal = 1 mV / 5 mV = 20%

A 20% error from noise alone — unacceptable for a precise measurement. The same 1 mV is invisible to a 1 V digital signal. That gap is why analog needs quiet supplies and isolation.

Interview Q&A

voltage so that the second copies (and can scale) the current set in the first. Analog circuits need steady, known currents to bias amplifiers and feed references, and the mirror provides accurate copies — its accuracy depends on good transistor matching.

source that respond to the difference between two inputs, not their absolute level. Because noise hitting both inputs equally cancels, the pair rejects common-mode noise, making it the core of amplifiers, comparators, and sense amplifiers.

that stays steady across temperature and supply. A plain divider drifts; a reference (e.g. a bandgap) adds two oppositely-drifting voltages so the drifts cancel, giving a flat output that converters and sensors depend on.

measures an analog voltage and outputs a digital number; a DAC takes a digital number and outputs an analog voltage. Their key specs are resolution (number of bits, i.e. step fineness) and speed (conversions per second) — a 10-bit ADC over 1 V resolves about 1 mV per step.

or low based on which is larger, bridging analog levels to digital decisions. It is used in ADCs, sense amplifiers, and threshold detectors; when inputs are nearly equal it can briefly hesitate (metastability), so designs allow settling time or add stages.

low, so small noise is harmless, but analog measures small voltages precisely, and supply noise corrupts those tiny signals — 1 mV of noise is nothing to a 1 V digital line but a 20% error on a 5 mV sensor signal. Analog therefore needs quiet separate supplies, isolation, and differential design.

Key Takeaways

• A current mirror copies (and scales) a reference current to bias analog blocks; it relies on transistor matching.
• The differential pair responds to the difference of two inputs and rejects shared noise — the heart of amplifiers.
• Voltage references (e.g. bandgap) give a steady voltage by cancelling temperature drift; converters depend on them.
• ADCs and DACs bridge analog and digital; resolution (bits) and speed are the key trade-offs.
• Analog hates supply noise — small signals get corrupted — so it needs separate quiet supplies, isolation, and differential design.