Example of Complex Load in Power Amplifiers

In the design and optimization of power amplifiers, understanding and managing the load impedance is critical for achieving high performance and efficiency. Here, we present a detailed example that illustrates the steps involved in determining the optimal complex load for a power amplifier, taking into account parasitic elements and the desired operating frequency.

Problem Statement

Given the following parameters for a power amplifier:

• Output Power (P_out): 1W
• Supply Voltage (V_DD): 4V
• Parasitic Capacitance (C_p​): 5pF
• Operating Frequency (F): 1GHz

Solution

Step 1: Maximum Current Calculation

First, we calculate the peak voltage (V_max​) across the transistor, which can be approximated as twice the supply voltage for a class B amplifier:

Step 2: Optimum Resistive Load Calculation

Next, we determine the optimum resistive load (R_opt​) using the peak voltage and current calculated:

Step 3: Calculation of Inductive Component

The total impedance Z_L​ includes a reactive component to counteract the parasitic capacitance. The inductance L needed to resonate with C_p at the operating frequency can be calculated as:

Step 4: Quality Factor (Q) and Load Impedance

The quality factor (Q_p​) of the inductor is given by:

Using the quality factor, we can derive the equivalent series resistance (R_S​) and inductance (L_S​):

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Conclusion

In this example, we have demonstrated the calculation of the maximum current, optimal load impedance, and its resistive and reactive components for a power amplifier operating at 1GHz with a parasitic capacitance of 5pF. These calculations are crucial for designing a power amplifier that maximizes output power and efficiency by effectively managing the complex load.

Learn more about this topic by taking the complete course ‘Linear RF Power Amplifier (PA) Design Theory and Principles online course – RAHRF562’. Watch the course videos for more detailed understanding. Also checkout other courses on RF system and IC design on https://rahsoft.com/courses/. Rahsoft also provides a certificate on Radio Frequency. All the courses offer step by step approach.