Hello,

In ADS (Advanced Design System), when a full nonlinear model for a transistor is unavailable, several alternative methods and modeling approaches can still be employed to estimate and analyze its linearity characteristics.

Here are some approaches you can consider:

• Using S-parameters with Harmonic Balance (HB) simulations:While S-parameters alone represent the linear behavior of a device, they can be utilized within a Harmonic Balance simulation if combined with other nonlinear models or elements present in the circuit. You can create a custom non-linear model, even a simplified one, and connect it with the S-parameter file for the linear aspects of the transistor to perform HB simulations. This allows you to investigate effects like harmonics, intermodulation distortion, and compression by observing the output spectrum and power levels for different input power levels.
• X-parameters:X-parameters are a generalization of S-parameters that characterize nonlinear behavior under large signal conditions, including magnitude and phase information for all harmonics and intermodulation products. If you have measured X-parameters for the transistor, you can directly use them in ADS to simulate the nonlinear performance, such as OIP3, even without a detailed internal nonlinear model. X-parameters are particularly useful for designing amplifiers and other nonlinear circuits, as they capture complex nonlinear interactions and allow for faster simulations than traditional Harmonic Balance, according to UPCommons.
• Behavioral Modeling:You can create a behavioral model in ADS using equations or lookup tables that approximate the transistor's nonlinear characteristics based on available data, even if it's not a full physical model. This might involve fitting analytical functions, like polynomials or transcendental functions, to the measured input-output relationships of the device under various conditions. While less precise than a full nonlinear model, behavioral models can offer valuable insights into the device's linearity and compression behavior.
• Load-Pull Data Analysis:If you have access to load-pull measurements for the transistor, you can extract linearity parameters, such as optimal load impedances for maximum output power or linearity, from the measured data. Although load-pull data may not constitute a full nonlinear model, it provides crucial information for designing matching networks and assessing linearity at different operating points.
• Simplified Nonlinear Models:Even if a comprehensive nonlinear model is unavailable, you can start with a basic transistor model and introduce some empirical or simplified nonlinearities to it. This could involve adding voltage-dependent elements, such as capacitors or resistors, whose values change with the operating conditions, to capture some aspects of the device's nonlinear behavior.

By leveraging these alternative approaches within ADS, you can still gain valuable insights into the linearity of the transistor and make informed design decisions, even in the absence of a complete nonlinear model.

Thanks,

Hello,

To assess the linearity of an LNA designed with the EC2612-99F pHEMT in ADS, focusing on the nonlinear model, you'll primarily check for Output Power (P1dB) and Input Third-Order Intercept Point (IP3). These are key parameters for characterizing linearity in nonlinear circuit analysis. Additionally, you might examine gain compression and intermodulation distortion (IMD).

Here's a breakdown of the steps involved:

1. Small-Signal Analysis (S-parameters):

• Initial Check:Begin with a small-signal simulation using S-parameters to evaluate the LNA's basic performance (gain, return loss, stability). This provides a foundation for understanding the LNA's behavior before delving into nonlinear effects.
• Frequency Response:Examine the S-parameters over the desired frequency range (e.g., 3GHz to 10GHz) to identify potential issues like gain roll-off or instability.

2. Nonlinear Analysis (Harmonic Balance):

• Two-Tone Test:A crucial step is using a two-tone input signal in a harmonic balance simulation. This helps determine the LNA's linearity by analyzing the resulting intermodulation products (IMD).
• Output Power (P1dB):Simulate the LNA's output power as a function of input power. P1dB is the input power at which the output power is 1 dB less than the small-signal gain.
• Input Third-Order Intercept Point (IP3):Calculate IP3, which is a measure of the LNA's ability to handle strong input signals without generating excessive distortion. It's typically determined by extrapolating the fundamental and third-order intermodulation products in the two-tone test.
• Gain Compression:Analyze how the LNA's gain changes as the input power increases. Significant gain compression indicates a loss of linearity.
• Harmonic Distortion:Observe the power levels of harmonics generated by the LNA. Excessive harmonic power indicates nonlinearity.

3. Load-Pull Analysis (Optional):

• Load Impedance: If you have access to load-pull data for the transistor, you can assess how the load impedance affects linearity. This can help optimize the matching network for both gain and linearity.

4. Stability Analysis:

• Low-Frequency and Microwave Instabilities: Use linear and nonlinear stability analysis methods to identify potential oscillations at low and high frequencies.

5. Simulation Setup:

• Accurate Model: Use the EC2612-99F pHEMT's nonlinear model in ADS for accurate simulation.
• Appropriate Bias: Ensure the LNA is biased correctly for the desired operating conditions.
• Proper Simulation Settings: Choose appropriate simulation settings (frequency range, input power levels, etc.) to capture the relevant nonlinear behavior.

6. Design Optimization:

• Iterative Process:Based on the simulation results, adjust the LNA's design (e.g., component values, transistor size, feedback networks) to improve linearity.
• Trade-offs:Be aware of potential trade-offs between linearity, gain, noise figure, and power consumption.

By following these steps and carefully analyzing the simulation results, you can effectively evaluate and optimize the linearity of your LNA design using the EC2612-99F pHEMT in ADS, according to research papers on ADS and LNA design.

Thanks,