Designing Inductive Degenerated Cascode LNA in Cadence Virtuoso for 2.4 GHz

marcelodesouzaee wrote:

Using Cadence Virtuoso, explain how to design an inductive degenerated cascode LNA achieving noise matching. Frequency range is 2.4 GHz, gain is 15 dB and noise figure is 2 dB @ElektrodaBot

Initial Assessment of Available Information
Multiple sample answers, both online and offline, describe a similar procedure for designing an inductively degenerated cascode LNA using Cadence Virtuoso. The online responses bring up-to-date guidance on component sizing, noise figure (NF) analysis, and layout considerations, which are particularly important in modern nanoscale CMOS processes. All information aligns in describing a systematic approach that includes selecting the cascode topology, performing manual calculations for rough sizing, then iteratively refining the design through simulation and layout optimization.

Below is a comprehensive, step-by-step synthesis showing how to design an inductive degenerated cascode LNA in Cadence Virtuoso for a 2.4 GHz application, aiming at 15 dB gain and 2 dB noise figure.

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Direct Answer to the Question
In Cadence Virtuoso, you can achieve a 2.4 GHz inductively degenerated cascode LNA with approximately 15 dB gain and a 2 dB noise figure by following these key steps:

1. Select an appropriate CMOS technology node and model files.
2. Create the inductive degenerated cascode topology in the schematic.
3. Size the transistors (common-source and cascode devices) based on initial hand calculations for transconductance (gm) and noise considerations.
4. Insert source (Ls), gate (Lg), and drain (Ld) inductors and refine their values to meet noise and impedance matching requirements at 2.4 GHz.
5. Bias transistors such that M1 operates with sufficient gm for low noise, while M2 ensures high-frequency isolation and overall gain.
6. Run S-parameter and noise simulations in Cadence Spectre to verify and iteratively refine matching, gain, and noise figure.
7. Optimize the layout to minimize parasitics and preserve fidelity at 2.4 GHz, and verify final performance with post-layout extraction.

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Detailed Problem Analysis

1. Topology and Design Requirements
You are designing for:
• Center frequency: 2.4 GHz (common in Wi-Fi and Bluetooth).
• Target gain: ~15 dB.
• Maximum noise figure: ~2 dB.
• Typical source/load impedance: 50 Ω.

An inductively degenerated cascode topology provides:
• High gain at microwave frequencies (due to lower Miller effect in a cascode).
• Potential for simultaneous noise and power matching via source degeneration inductance.
• Improved reverse isolation and stability.

2. Hand Calculations and Initial Sizing

2.1 Transistor Sizing
• Choose MOSFET M1 for the common-source input with W/L that yields a transconductance gm suitable for low noise figure and matching. For 2.4 GHz, typical gm values might range from 10 to 20 mS depending on the bias current.
• Cascode transistor M2 can be sized slightly smaller (e.g., 0.8× width of M1) since it primarily provides isolation at high frequency.

2.2 Inductive Degeneration (Ls) and Gate Inductor (Lg)
• The input impedance for an inductively degenerated common-source stage can be approximated as:
Z_in ≈ jωLg + (1/gm) + jωLs
• You want Z_in ≈ 50 Ω (real) at resonance to minimize reflection (S11) and approach noise matching. Hence, choose Ls so that the real part of Z_in is near 50 Ω at 2.4 GHz, and use Lg to resonate out any residual imaginary component.

2.3 Cascode Configuration
• Cascode transistor M2 (common-gate) prevents the large voltage swing at the drain of M1 from feeding back to the input, stabilizing the gain and reducing the Miller effect.

3. Building the Schematic in Cadence Virtuoso
1. Create a new cell view, selecting the correct PDK (e.g., 65 nm or 130 nm CMOS).
2. Place M1 (common source), M2 (common gate, forming the cascode).
3. Insert the inductors: Ls (source of M1 to ground), Lg (gate to input matching network), Ld at the common drain node (drain of M2) or as part of the output matching network.
4. Include bias circuits:
• A current source or resistor network to set the drain current of M1.
• Gate bias for M2, typically at a fixed DC voltage to ensure saturation.
5. Connect 50 Ω ports at the input and output (through AC coupling capacitors if desired) to replicate typical measurement conditions.

4. Simulation Strategies

4.1 S-Parameter Analysis
• In the ADE (Analog Design Environment), set up an S-parameter analysis (sp).
• Sweep from ~1 GHz up to ~3 GHz to capture the 2.4 GHz band thoroughly.
• Plot S11 (input match) and S21 (gain). Aim for:
– S11 < -10 dB at 2.4 GHz.
– S21 ~ 15 dB around 2.4 GHz.

4.2 Noise Figure Analysis
• Run “pnoise” or “sp” noise simulations to measure the noise figure.
• At 2.4 GHz, you want NF < 2 dB. If NF is higher, adjust:
– Bias current (increasing gm can reduce noise but increases power).
– Transistor size (larger transistor can reduce noise but raises parasitic capacitances).
– Values of Ls and Lg for better noise/power matching.

4.3 Stability Analysis
• Use the STB analysis (stability) or calculate the K-factor to ensure unconditional stability. Ensure K > 1 from ~1 GHz to at least several GHz above the operating frequency.

5. Layout Guidelines
• In high-frequency designs, the layout can drastically shift inductances and capacitances, so:
– Keep interconnects and ground paths short.
– Use wide, short metal routing to reduce parasitic inductances and resistance.
– Place inductors far enough apart to reduce mutual coupling effects.
– Consider adding guard rings and ground shielding for noise isolation.

6. Post-Layout Extraction and Optimization
1. Extract the layout (using the foundry’s extraction tool) to account for parasitic capacitances and resistances.
2. Re-run S-parameter and noise simulations on the extracted netlist.
3. If the post-layout results deviate significantly from the schematic-level design, tune the inductors or transistor sizes to restore performance.

7. Current Information and Trends
• Modern CMOS nodes (e.g., 28 nm, 16 nm) offer transistors with higher fT (transition frequency), enabling LNAs with lower noise figures at microwave frequencies. However, advanced nodes also amplify layout parasitic sensitivity.
• Integrated passives (on-chip inductors) typically have modest Q-factors. For the best performance, advanced packaging or external inductors can help, but that increases cost and complexity.

8. Supporting Explanations and Details
• Always perform temperature and process corner analyses (PVT corners) to guarantee robust operation.
• Consider power consumption vs. performance. LNAs for battery-powered devices might need lower current, but that can increase the noise figure.
• The cascode’s main advantage is improved isolation. In addition, it helps the first transistor maintain a more constant VDS, reducing its contribution to noise at higher frequencies.

9. Practical Guidelines
• Start with small test structures or a ring oscillator design kit (RODK) to characterize transistor fT, gm, and parasitics if the foundry data is limited.
• For each iteration, systematically document changes to transistor widths, bias currents, or inductor values, so that improvements can be tracked effectively.

10. Suggestions for Further Research
• Investigate concurrent matching techniques for wideband LNAs.
• Explore differential LNAs for improved common-mode noise rejection and stability.
• Look into advanced noise canceling or feedback topologies if <2 dB NF is required in the same frequency band.

11. Brief Summary
Designing an inductively degenerated cascode LNA for 2.4 GHz involves carefully sizing and biasing the input transistor for both noise and impedance matching, adding a cascode device to stabilize gain and mitigate Miller effects, and tuning source/gate inductors to achieve ~15 dB gain and ~2 dB noise figure. Iterative simulations in Cadence Virtuoso, followed by a careful layout and post-layout extraction, are critical steps for ensuring real-world performance meets these specifications.

Questions for the User
1. Which CMOS technology node and PDK are you currently using for this design?
2. Have you performed any layout versus schematic (LVS) or parasitic extraction steps yet?
3. Are there additional constraints such as low power consumption, supply voltage limitations, or integrated passive device availability that we should account for?

By clarifying these questions, we can refine the LNA design process further and target specific practicalities or challenges in your environment.

marcelodesouzaee wrote:

Consider 65 nm tech. Use sp simulation to find the optimal source admittance. @ElektrodaBot

Initial Assessment of Available Information
The user wants to determine the optimal source admittance (Y_s,opt) for an inductively degenerated cascode LNA in a 65 nm CMOS technology, operating around 2.4 GHz. The goal is to use S-parameter (sp) simulations in Cadence Virtuoso to extract the noise parameters—particularly Y_s,opt, which provides the best noise match. Offline and online sample answers converge on a systematic approach involving sp simulation plus noise parameter extraction. The difference is that one of the online samples was not able to provide specific guidance, but another online sample did. Offline samples provide detailed procedures.

Direct Answer to the Question
Use sp simulation in Cadence Virtuoso with SpectreRF to extract the noise parameters of your 65 nm LNA. From those parameters, retrieve the optimal source admittance (Y_s,opt). Then tailor the input network—inductive source degeneration (L_s) plus any input matching components (L_g, possibly a capacitor)—so the effective source admittance at the transistor input equals Y_s,opt. This ensures near-minimum noise figure at your target frequency.

Below is a thorough, step-by-step guide to help you accomplish this.

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Detailed Problem Analysis

1. LNA Topology and Goals
• Technology: 65 nm CMOS.
• Frequency: 2.4 GHz (common for ISM band applications).
• Desired performance: simultaneous power match (low S11) and low noise figure (~2 dB).

An inductively degenerated cascode LNA typically has:
1. A common-source transistor M1 with source inductor L_s for setting input impedance and noise performance.
2. A cascode transistor M2 (common-gate) for isolation and improved gain-bandwidth product.
3. Matching networks at the input (L_g, possibly additional capacitors) and output (L_d, possibly additional matching).

2. Setting Up the Schematic
1. Include M1 and M2 transistors from the 65 nm PDK, ensuring correct model files.
2. Insert source inductor (L_s) between the source of M1 and ground.
3. Insert gate inductor (L_g) at the input node of M1 for impedance transformation.
4. Add a drain inductor (L_d) as part of the output matching network on the drain of M2.
5. Connect a 50 Ω port (Port 1) at the LNA input and a 50 Ω port (Port 2) at the LNA output for standard test conditions.
6. Set up biasing for M1 and M2 to ensure the device is biased in saturation (with the correct drain current ID for the targeted transconductance, gm, which strongly impacts noise).

3. Configuring the sp Analysis in Cadence Virtuoso

1. In the Analog Design Environment (ADE), select SpectreRF as the simulator.
2. Add an sp analysis (“Choose Analysis” → “sp”):
• Frequency Sweep: typically 1 GHz to 3 GHz to capture 2.4 GHz and surrounding frequencies.
• Port Impedances: define the input/output ports as 50 Ω (typical system impedance).
• Activate Noise Parameters: in many PDKs, you can enable noise parameters output under the sp analysis (or use a separate Noise analysis if your flow requires it). This is critical for obtaining γ_opt, Fmin, and Rn.

3. Save S-parameters and noise parameters across the frequency sweep.

4. Extracting the Optimal Source Admittance (Y_s,opt)
After running the sp simulation, look for the noise-parameter outputs:
• F_min(ω) or NF_min(ω): the minimum noise figure vs. frequency.
• γ_opt(ω): the optimum reflection coefficient for noise.
• R_n(ω): the equivalent noise resistance of the device.

From γ_opt(ω), you can compute Y_s,opt(ω) via:

1) Convert γ_opt to impedance or admittance.
2) Recall that
Γ = (Z_s - Z_0) / (Z_s + Z_0),
and Y_s = 1 / Z_s.
3) With Z_0 = 50 Ω as the reference, you can convert γ_opt to Z_opt or Y_opt.

At 2.4 GHz, you’ll get either Y_opt(2.4 GHz) directly (some PDKs or simulation setups provide it in the result browser) or compute it in the Cadence Calculator by applying the above formula to γ_opt.

5. Matching the LNA Input to Y_s,opt

1. Real Part (G_s,opt):
• The source inductor L_s and transistor gm form the real part of the input impedance. Adjust L_s and transistor bias (which sets gm) so that the real part of the input admittance meets G_s,opt.

2. Imaginary Part (B_s,opt):
• L_g helps null out the unwanted imaginary part at the gate. The net parallel/series combination of L_s, L_g, and transistor parasitics yields the imaginary part of Y_in. Adjust L_g until you align the input admittance’s imaginary part with B_s,opt.

3. Iterations & Param Sweeps:
• You can param-sweep L_s, L_g, or transistor W/L in the schematic while observing how the input reflection coefficient (S11) and noise figure change.
• Keep an eye on S21 (gain) to ensure the design still meets your ~15 dB target.

6. Verifying the Design
• Re-run the sp simulation and compare your new input admittance with Y_s,opt.
• Plot S11 on a Smith chart to confirm near 50 Ω if you also want power matching, or check how close the actual source admittance is to Y_s,opt for noise matching.
• Check noise figure at 2.4 GHz to see if you meet or approach the F_min value.
• Confirm the gain and look at stability (K-factor, Δ, or run an stb analysis in Cadence).

7. Current Information and Trends
• In 65 nm technology, on-chip inductors typically have moderate Q (6–10), so keep layout parasitics minimal.
• Some advanced flows allow you to co-optimize Q-factor by choosing thick top metals for inductors.
• Tools like Cadence ADE GXL or Virtuoso’s Optimization Cockpit can automate this impedance/noise matching process.

8. Supporting Explanations and Details
• The difference between power match and noise match can be significant, especially at microwave frequencies. Often, the minimal noise figure point does not coincide exactly with the best S11. Designers typically find an acceptable compromise or use external tuning.
• The optimum source admittance often requires a specific gm in the transistor to minimize noise, so adjusting bias current can significantly alter Y_s,opt.

9. Practical Guidelines
• Start with approximate hand calculations or standardized design formulas for inductive degeneration.
• Use parametric analysis in ADE to sweep L_s over a reasonable range (e.g., ±10% to ±20% around an estimated starting value).
• Match the measured Y_in(2.4 GHz) to Y_s,opt(2.4 GHz) from the simulation results.

10. Possible Disclaimers or Additional Notes
• Post-layout extraction (RC or RLC) is vital in 65 nm for accurate results. The presence of routing parasitics can significantly shift your optimum admittance and degrade the final noise performance if not accounted for.
• The numeric values of Y_s,opt may vary significantly with transistor size and bias conditions.

11. Suggestions for Further Research
• Explore multi-technology solutions (e.g., using external high-Q passives if the on-chip inductors are too lossy).
• Investigate advanced noise-canceling topologies if sub-2 dB noise figure is essential in 65 nm with minimal power.
• Look into differential LNAs for better supply noise rejection and improved linearity.

Brief Summary
1. Configure an sp analysis in Cadence Virtuoso (SpectreRF) and enable noise parameters.
2. Extract γ_opt (optimum reflection coefficient) or Y_s,opt (optimum source admittance) at 2.4 GHz.
3. Adjust L_s and L_g to transform your 50 Ω source to Y_s,opt, thus minimizing noise figure.
4. Validate final S11, S21, NF, and stability, ensuring the design meets the targeted 15 dB gain and 2 dB NF.

Questions for the User
1. Have you confirmed the bias current and transistor sizing for the desired gm range before attempting noise matching?
2. Are there constraints on chip area or external passives that might limit your inductor choices?
3. Do you have a post-layout extraction flow in place to verify the final LNA performance and optimize parasitic effects?

By following this procedure in Cadence Virtuoso, you will systematically determine and implement the optimal source admittance for your 65 nm inductively degenerated cascode LNA to minimize noise while maintaining appropriate gain and input match.