ELEC4602 Chap.3 MOS Devices, Noise and Current Mirrors
MOS Devices, Noise and Current Mirrors
Week 3 joins three ideas that should be revised together. MOS bias establishes an operating region and therefore the device behaviour available to a circuit. Small-signal parameters describe how that biased device responds to a perturbation. Noise sources then place a floor under the useful signal, while current mirrors distribute the bias that keeps later amplifier stages in their intended regions.
The raw whiteboard text is partly corrupted, so this chapter emphasises reliable qualitative relationships rather than reconstructing unreadable equations.
What this chapter covers
- 01
MOS bias and operating regions
- 02
Large-signal and small-signal viewpoints
- 03
Parasitic capacitance and switching behaviour
- 04
White and low-frequency noise
- 05
Device noise referred through a circuit
- 06
Current-mirror bias and output branches
Worked example · free
Separate a bias fault from a noise fault
- +1Check the DC operating point and device regions first, because a device outside its intended region invalidates the expected small-signal response.
- +1Verify the current-mirror reference and output branches so the bias error is not being distributed through the whole front end.
- +1Once the circuit is biased correctly, measure the remaining variation across frequency to separate broadband and low-frequency behaviour.
- +1Report the evidence in that order, avoiding a claim that device noise explains clipping that is actually caused by lost headroom.
Key terms
- Operating point
- The operating point is the DC bias state around which circuit behaviour and small variations are analysed.
- Operating region
- An operating region is the bias-defined MOS mode that determines the applicable large-signal behaviour.
- Small-signal model
- A small-signal model linearises device response around a chosen operating point for incremental analysis.
- Thermal noise
- Thermal noise is broadband random variation associated with dissipative device behaviour.
- Low-frequency noise
- Low-frequency noise grows in importance toward the lower end of the spectrum and can dominate slow precision signals.
- Current mirror
- A current mirror uses a reference branch to establish related bias currents in one or more output branches.
MOS Devices, Noise and Current Mirrors FAQ
Why must I check the operating point before small-signal analysis?
The small-signal model is built around a particular bias state. If the device is actually in another region, the assumed incremental parameters and gain path no longer describe the circuit. Reporting a small-signal result without first checking the operating point can make a mathematically neat answer physically irrelevant.
How are current mirrors connected to amplifier performance?
A mirror establishes or distributes bias currents, which set the operating points of later stages. If a mirror lacks headroom or a device leaves its intended region, the amplifier can lose gain, output range or symmetry. Diagnose the bias network before blaming the signal path alone.
What should a noise explanation include?
Name the source family, the frequency range in which it matters, the node where its effect is observed and the circuit operation that transfers it there. A useful answer distinguishes deterministic offsets or clipping from random variation and states how a measurement would separate competing explanations.
Exam move
Organise this week as a layered diagnostic tree. The first layer is DC: label the device terminals, determine the intended operating region and check whether the current mirror can sustain that state. The second layer is incremental: state what changes around the operating point and which small-signal quantity connects input to output.
The third layer is dynamic: identify the parasitic capacitances that matter during a transition or at high frequency. The fourth layer is noise: separate broadband sources from low-frequency behaviour and trace each source to the observation node. Practise explaining the same circuit four times, once from each layer, so you do not mix a bias statement with a noise statement.
For current mirrors, follow reference establishment, copying action, output compliance and loading in that order. End every practice answer with a verification move: a DC check, an incremental response, a transient observation or a spectrum. That sequence turns scattered device facts into a coherent analysis method. Build a branch comparison for every mirror exercise.
Keep the reference path fixed, change one output condition and record bias, region and load before interpreting current error. For a noisy or clipped waveform, test deterministic operating limits first and describe the remaining variation only after restoring a valid signal path. Add bandwidth and observation node to any noise statement, because a source matters through its transfer to the measured output.
Finish with a short verdict that says which layer the evidence supports and which adjacent explanation the controlled test ruled out.
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