Analog Circuit Design

Analog design is where the precision work lives. Digital circuits deal in ones and zeros; analog circuits must get a signal to behave exactly right across temperature, supply voltage, component aging, and manufacturing tolerance. The goal is usually to amplify, filter, or measure something to a specified accuracy — and then verify it actually achieves that in the worst case. Small systematic errors compound through gain stages and become large errors at the output.

Transistor Amplifier Configurations

Beyond op-amps, discrete transistor stages are still essential for high-frequency work, driving large currents or voltages, or situations needing specific impedance characteristics:

Configuration	Voltage gain	Input impedance	Output impedance	Phase shift	Primary use
Common Emitter (BJT)	High (−RC/RE)	Moderate (β×RE)	Moderate (RC)	180° (inverting)	Voltage amplification, general purpose
Common Collector (Emitter Follower)	~1	High (β×RE)	Low (RE/β)	0° (non-inverting)	Impedance buffer, current driving
Common Base (BJT)	High (RC/RE)	Very low (~26mV/Ic)	High	0° (non-inverting)	RF amplifiers, current buffer
Common Source (FET)	High (−gm×RD)	Very high (GΩ)	Moderate (RD)	180° (inverting)	FET voltage amplification
Common Drain (Source Follower)	~1	Very high	Low (1/gm)	0°	Impedance buffer with FET input

Common Emitter with Emitter Degeneration

Voltage gain Av ≈ −RC / RE   (with degeneration, approximate)
Input impedance ≈ β × RE     (at low frequencies)
Quiescent collector current: Ic ≈ (VCC × R2/(R1+R2) − 0.7V) / RE

The emitter resistor RE provides local negative feedback that stabilises the operating point against β variation and temperature drift. Without it (bypassed with a capacitor), Av = −gm×RC = −RC×Ic/26mV — much higher gain but very sensitive to temperature and transistor β.

Instrumentation Amplifiers

When amplifying small differential signals in the presence of large common-mode noise — thermocouple outputs, Wheatstone bridge measurements, biosignals — an instrumentation amplifier is the correct tool. A simple difference amplifier has limited CMRR because resistor mismatch degrades rejection; the INA topology uses three op-amps with laser-trimmed resistors to achieve very high CMRR:

Gain = 1 + 2R/RG   (R is internal, RG is external)

INA128 with RG=10kΩ: Gain = 1 + 100k/10k = 11
INA128 with RG=1kΩ:  Gain = 1 + 100k/1k = 101

CMRR: 80–120dB (rejects common-mode, amplifies only difference)

Part	Gain range	Vin (cm) range	Vos (max)	CMRR (min)	Notes
INA128 / INA129	1–10,000	±10V	25 µV	90 dB	Classic precision INA. ±15V supply.
INA333	1–1,000	−0.1 to VCC+0.1V	25 µV	100 dB	Single supply, low power — good for battery applications
AD8221	1–1,000	−Vs+2.1V to +Vs−1.1V	25 µV	80 dB	Low noise, rail-to-rail output. High-performance medical/industrial.
INA826	1–1,000	−0.1 to VCC+0.1V	100 µV	80 dB	Tiny, cheap. Good for data acquisition.

Noise in Analog Circuits

Every resistor and active component adds noise. Understanding noise sources lets you design for a required signal-to-noise ratio:

Noise type	Source	Formula	Spectrum
Thermal (Johnson) noise	Any resistance	V_n = √(4kTRB)	White (flat)
Shot noise	DC current through junction	I_n = √(2qI_DC×B)	White
Flicker (1/f) noise	Semiconductor surfaces, contacts	Increases as 1/f	Pink (rises at low freq)
Popcorn (burst) noise	Defects in ICs	Random step changes	Irregular

Thermal noise example: R=10kΩ, T=300K, B=20kHz (audio bandwidth)
V_n = √(4 × 1.38×10⁻²³ × 300 × 10,000 × 20,000)
V_n = √(3.31×10⁻¹²) ≈ 1.82 µV RMS

Op-amp noise: input-referred voltage noise density en (nV/√Hz)
Total input noise in bandwidth B:
  V_noise = en × √B

TL071: en = 18 nV/√Hz → in 20kHz audio BW: V_noise = 18 × √20000 = 2.55 µV
NE5532: en = 5 nV/√Hz → V_noise = 5 × √20000 = 0.71 µV

Noise Budget Strategy

Total system SNR requirement → input-referred noise budget → op-amp selection

Example: 12-bit ADC, 3.3V reference, required SNR = 72dB (12-bit ideal)
LSB = 3.3V / 4096 ≈ 806 µV
Required noise < LSB/2 ≈ 400 µV RMS at the ADC input

With gain of 10 before the ADC:
Required input-referred noise < 40 µV RMS

Available bandwidth = 1kHz:
Required en < 40µV / √1000 Hz ≈ 1.26 µV/√Hz = 1260 nV/√Hz
Any op-amp with en << 1260 nV/√Hz is adequate → even LM358 (en ≈ 40nV/√Hz) is fine here

Transimpedance Amplifier (TIA)

Converts current to voltage. Essential for photodiode circuits, current-mode sensors, and output of charge-based sensors like accelerometers:

Bandwidth: f_-3dB ≈ 1/(2π × Rf × Cf)   (Cf in parallel with Rf to prevent oscillation)
Stability: Cf ≥ √(Cd / (2π × Rf × GBW))  (Cd = photodiode capacitance)

Example: Rf=1MΩ, Cd=20pF, GBW=10MHz (op-amp):
Cf ≥ √(20×10⁻¹² / (2π×10⁶×10⁷)) ≈ 0.56pF → use 1pF
f_-3dB ≈ 1/(2π×10⁶×10⁻¹²) ≈ 160kHz

ADC Input Conditioning

A complete ADC input chain must address multiple issues:

Problem	Solution
Signal range doesn't match ADC input range	Scaling amplifier — gain or attenuation to fill 0 to Vref
Signal has DC offset	Differential input or subtraction of offset before scaling
Frequencies above Nyquist present	Anti-aliasing low-pass filter before ADC
Source impedance too high	Voltage follower or low-impedance buffer before filter
Impedance mismatch causes RC error	Source impedance must be ≪ ADC sample-and-hold input impedance
Ground loops / common-mode interference	Differential sensing + instrumentation amplifier

PCB Layout for Analog

Analog circuits are much more sensitive to layout than digital. These rules matter:

Rule	Reasoning
Solid ground plane on one layer	Low-impedance return path; reduces ground bounce and electromagnetic pickup
Decoupling caps within 2mm of each IC VCC pin	Inductance in series with a cap renders it ineffective at high frequencies
Keep high-gain signal traces short and direct	Long traces act as antennas — pick up interference from adjacent signals
Guard ring around high-impedance nodes	Driven ground ring around op-amp + input, TIA input prevents leakage from adjacent net biasing the node
Route digital clock signals away from analog paths	Switching edges radiate EMI that couples into sensitive nodes
Separate analog and digital sections of the PCB	Common return current flows under traces — keep digital return currents from passing through analog section
Place filter caps at output, not input of amplifier	Filtering at the output removes noise; filtering at the input sources are already at the op-amp input — mismatch biases can worsen noise

Precision Voltage References

A good ADC is only as accurate as its reference. Zener-based shunt references and bandgap references are the two families:

Part	Voltage	Initial accuracy	Tempco	Notes
LM385-2.5	2.5V	±1.5%	20 ppm/°C	Micropower shunt reference. Good enough for 8-bit work.
TL431	2.5–36V adj.	±0.5%	25 ppm/°C	Programmable with two resistors. Ubiquitous in power supply feedback.
LM4040-5.0	5V	±0.1%	50 ppm/°C	Precision shunt. Good for 10-bit ADC reference.
REF3325	2.5V	±0.2%	10 ppm/°C	TI series reference. Good 12-bit performance.
MAX6350	5V	±0.02%	1 ppm/°C	Precision for 16-bit ADC. Low noise, laser-trimmed.

References

• Horowitz & Hill — The Art of Electronics, 3rd ed. Chapters 3–8
• Motchenbacher & Connelly — Low-Noise Electronic System Design
• Franco — Design with Operational Amplifiers, 4th ed.
• Analog Devices — Linear Circuit Design Handbook ↗
• TI — Op Amp Applications (PDF) ↗