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| 1 | +Small Signal Analysis Guide |:zap:| |
| 2 | +#################################### |
| 3 | + |
| 4 | +This guide explains how to perform small signal AC analysis using Tidy3D's TCAD simulation capabilities. |
| 5 | + |
| 6 | +Overview |
| 7 | +======== |
| 8 | + |
| 9 | +Small signal analysis (SSAC) is a technique used to analyze the frequency response of electronic devices around a DC operating point. It's particularly useful for: |
| 10 | + |
| 11 | +- Analyzing frequency-dependent behavior of semiconductor devices. |
| 12 | +- Computing small-signal parameters like transconductance and output conductance. |
| 13 | +- Studying AC characteristics of transistors, diodes, and other electronic components. |
| 14 | + |
| 15 | +Key Components |
| 16 | +============== |
| 17 | + |
| 18 | +GroundVoltageSource |
| 19 | +------------------- |
| 20 | + |
| 21 | +The :class:`GroundVoltageSource` class provides an explicit way to specify ground references in simulations: |
| 22 | + |
| 23 | +.. code-block:: python |
| 24 | +
|
| 25 | + import tidy3d as td |
| 26 | + |
| 27 | + # Create a ground boundary condition |
| 28 | + ground_bc = td.VoltageBC(source=td.GroundVoltageSource()) |
| 29 | +
|
| 30 | +ACVoltageSource |
| 31 | +--------------- |
| 32 | + |
| 33 | +The :class:`ACVoltageSource` class defines small-signal AC excitations: |
| 34 | + |
| 35 | +.. code-block:: python |
| 36 | +
|
| 37 | + # Define DC bias voltages and AC source amplitude |
| 38 | + bias_voltages = [0.0, 0.5, 1.0, 1.5, 2.0] # V |
| 39 | + ac_amplitude = 1e-3 # 1mV (small signal amplitude) |
| 40 | + |
| 41 | + ac_source = td.ACVoltageSource( |
| 42 | + voltage=bias_voltages, |
| 43 | + amplitude=ac_amplitude |
| 44 | + ) |
| 45 | +
|
| 46 | +SteadyChargeDCAnalysis with SSAC |
| 47 | +------------------------------ |
| 48 | + |
| 49 | +The :class:`SteadyChargeDCAnalysis` class now supports small signal analysis through the ``ssac_freqs`` parameter: |
| 50 | + |
| 51 | +.. code-block:: python |
| 52 | +
|
| 53 | + ssac_frequencies = [1e3, 1e4, 1e5, 1e6] # AC analysis frequencies (Hz) |
| 54 | +
|
| 55 | + isothermal_spec = td.SteadyChargeDCAnalysis( |
| 56 | + ssac_freqs=ssac_frequencies, |
| 57 | + # ... other parameters |
| 58 | + ) |
| 59 | + simulation = td.HeatChargeSimulation( |
| 60 | + analysis_spec=analysis, |
| 61 | + # ... other parameters |
| 62 | + ) |
| 63 | +
|
| 64 | +Best Practices |
| 65 | +============== |
| 66 | + |
| 67 | +1. **Ground Reference**: Specify exactly one ground reference using :class:`GroundVoltageSource`. If no ground is specified, the smallest voltage source will be considered as the ground voltage. |
| 68 | + |
| 69 | +2. **Small Signal Amplitude**: Keep the AC amplitude small (typically 1-10 mV) to ensure linear operation. |
| 70 | + |
| 71 | +3. **Frequency Range**: Choose frequencies that cover the relevant bandwidth of your device. |
| 72 | + |
| 73 | +4. **Convergence**: Use appropriate tolerance settings for the charge analysis to ensure convergence. |
| 74 | + |
| 75 | +5. **Validation**: The simulation will automatically validate that one :class:`ACVoltageSource` is present when ``ssac_freqs`` is specified. |
| 76 | + |
| 77 | +Common Use Cases |
| 78 | +================ |
| 79 | + |
| 80 | +- **MOSFET Analysis**: Study transconductance and output conductance vs. frequency. |
| 81 | +- **Diode Characterization**: Analyze junction capacitance and conductance. |
| 82 | +- **BJT Small-Signal**: Compute h-parameters and frequency response. |
| 83 | + |
| 84 | +Troubleshooting |
| 85 | +============== |
| 86 | + |
| 87 | +- **No AC Source Error**: Ensure at least one :class:`ACVoltageSource` is present when using ``ssac_freqs``. |
| 88 | +- **Multiple Grounds Error**: Only one ground reference is allowed per simulation. |
| 89 | +- **Convergence Issues**: Adjust tolerance settings or decrease the discretization size. |
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