Temperature Effects and Drift in Analog Circuits – The Silent Performance Killer

Analog circuits don’t live in ideal worlds — they live in temperature. Every transistor, resistor, and capacitor changes its behavior when the environment warms up or cools down. These changes may seem small, but in precision designs, they can silently degrade performance, shift bias points, or even cause functional failure. This phenomenon is called temperature drift, and managing it is one of the hardest parts of analog design.

1. What is Temperature Drift?

Temperature drift refers to the gradual change in circuit parameters (like gain, offset, or bias current) as temperature varies. It is usually expressed as a temperature coefficient (TC), such as µV/°C, ppm/°C, or %/°C, depending on the parameter.

Example: An amplifier with 10 µV/°C offset drift will see a 1 mV shift across a 100°C range — enough to ruin precision measurements.

2. Why Temperature Causes Variation

Semiconductor parameters like carrier mobility, threshold voltage (V_th), and saturation current (I_DSAT) are temperature-dependent. As temperature rises:

• Mobility (µ) decreases → transconductance drops.
• V_th decreases → devices conduct more current.
• Leakage current increases exponentially.

The result is a complex tug-of-war between opposing effects — one reason analog simulation across temperature corners is mandatory.

3. Temperature Dependence in Common Circuit Blocks

a) Resistors

All resistors have a Temperature Coefficient of Resistance (TCR). Metal-film resistors have low TCR (±25 ppm/°C), while polysilicon resistors can be as high as ±500 ppm/°C. High TCR causes bias current and gain drift in amplifiers.

b) Transistors

MOSFETs experience changes in both threshold voltage and mobility. Bipolar devices show an exponential increase in I_C with temperature due to saturation current variation — the reason bandgap references exist.

c) Capacitors

Capacitor value changes with dielectric constant variation. For example, ceramic (X7R) capacitors can vary ±15% over temperature, while NP0 types remain nearly constant.

d) References and Bias Circuits

Reference voltages (like bandgaps) and current mirrors are especially sensitive to temperature. Proper compensation is required to ensure stable operation across process, voltage, and temperature (PVT) corners.

4. Temperature Drift in Amplifiers

In op-amps, the main temperature-dependent errors are:

• Offset Drift: Changes in input offset voltage with temperature (µV/°C).
• Gain Drift: Variation in open-loop gain or resistor ratio with temperature.
• Bias Current Drift: Input bias currents increase with temperature, introducing DC errors.

5. Temperature Compensation Techniques

• 1. Use PTAT (Proportional To Absolute Temperature) and CTAT (Complementary To Absolute Temperature) elements: Combine both to achieve temperature-independent references, as used in bandgap circuits.
• 2. Matched Layout: Place symmetrical devices in close proximity to ensure equal temperature exposure.
• 3. Temperature-Stable Resistors: Use low-TCR resistors in gain-setting networks.
• 4. Auto-Zero and Chopper Stabilization: Eliminate offset drift in precision amplifiers through dynamic correction.
• 5. Calibration: Use post-silicon digital calibration to measure and correct drift behavior during production testing.

6. Real-World Validation Example

In analog validation labs, engineers perform temperature sweep testing using thermal chambers. Devices are measured from –40°C to +125°C, while key parameters like gain, offset, and current are logged. The slope of the trendline gives the temperature coefficient.

Example: An amplifier output drifts by 1.2 mV from –40°C to 125°C. Drift = (1.2 mV / 165°C) = 7.27 µV/°C.

This data is compared with simulation results to confirm model accuracy and layout symmetry performance.

7. Interview Questions on Temperature Effects

• How does temperature affect MOSFET threshold voltage?
• What is offset drift and how can it be minimized?
• What are PTAT and CTAT circuits?
• Why do bipolar transistors show exponential current increase with temperature?
• How is temperature drift validated in silicon?

Conclusion

Temperature drift is the silent performance killer in analog circuits. It doesn’t announce itself with oscillations or visible distortion — it slowly degrades precision until your design fails a corner case. Great analog engineers learn to predict, measure, and neutralize it through compensation, calibration, and clever design symmetry. The perfect analog circuit isn’t one that ignores temperature — it’s one that respects it.