Introduction / Context:
Direct-coupled (DC) amplifiers are widely used whenever the signal includes very low frequencies or a constant (zero-hertz) component, such as in sensor conditioning, instrumentation, and operational amplifier stages. A common practical observation is that DC amplifiers have a tendency to be unstable unless carefully designed. This question asks you to judge that statement from an electronics design perspective and explain why it is generally considered correct.
Given Data / Assumptions:
- Amplifiers under discussion pass down to 0 Hz (no coupling capacitor), so offsets and low-frequency noise matter.
- Device parameters (input offset voltage, input bias current) are non-ideal and temperature-dependent.
- Long-term drift and 1/f (flicker) noise become significant near DC.
Concept / Approach:
The apparent 'instability' of DC amplifiers is not just about oscillation. It includes output wandering due to input offset, temperature drift, bias current, resistor tolerance, and power-supply variation. Small DC errors, when multiplied by high closed-loop gains, can drive outputs toward saturation, where the stage can no longer respond linearly. Compensation networks must ensure adequate phase margin over a bandwidth that includes very low frequencies, while trimming and auto-zeroing minimize offset and drift.
Step-by-Step Solution:
Identify DC error sources: input offset voltage, input bias/offset current, resistor mismatch.Recognize temperature and time dependence: offsets and bias currents vary with temperature and age.Understand loop behavior: at low frequencies the loop gain may be very large, magnifying small DC errors.Result: output drift or saturation unless proper offset trimming, chopping/auto-zero, and compensation are used.
Verification / Alternative check:
Compare a capacitor-coupled AC amplifier (which blocks DC) with a DC-coupled stage. The AC stage avoids DC drift at its output; the DC stage must actively manage it via design techniques.
Why Other Options Are Wrong:
'False': contradicts common practice and datasheet guidance; designers routinely mitigate DC drift.'Only with feedback': instability issues exist with or without feedback; feedback can help but also requires compensation.'Only at high frequencies': DC issues are fundamentally low-frequency problems.'Depends entirely on load': load matters, but core drift/offset and loop-gain causes are internal.
Common Pitfalls:
Ignoring input bias currents through large resistances, neglecting temperature coefficients, and assuming that high open-loop gain always improves stability at DC.
Final Answer:
True
Discussion & Comments