The power amplifier (PA) is the core component of a transmitter. Regardless of the industry we are in, the goal is to amplify the signal and transmit it as far as possible, ensuring good signal quality, transmitter efficiency, and so forth. The PA plays a decisive role in these aspects. Let’s discuss the matching design of PAs.

The primary objectives of PA matching:

• Maximum power transfer
• Improvement of system signal-to-noise ratio (SNR)
• Reduction of amplitude and phase errors

Regarding PA matching design, you can find simulation examples using ADS (Advanced Design System) online, so this article won’t cover them in detail. Instead, we need to understand the purpose of the simulation steps.

 

The goal of simulation matching is to achieve maximum power transfer for the PA. The method for achieving maximum transfer is conjugate matching of the input and output. The image above shows a case where the convergence circle was not found.

The key to conjugate matching is finding the optimal impedance point. During source-pull and load-pull, the critical task is to adjust the center and radius to find the convergence point by adjusting the position of the center and impedance to locate the convergence circle.

 

Once the optimal impedance is found, you can proceed with the conjugate matching design according to the principles of conjugate matching.

For PA matching, especially broadband matching, the following methods are generally used:

 

1. Bode-Fano Criterion

 

 

– To achieve a wider passband for a given load, it is necessary to sacrifice the reflection coefficient within the passband. 

– The reflection coefficient within the passband cannot be zero unless the bandwidth is zero. 

– When R or C increases, the matching quality (bandwidth or reflection coefficient) decreases, so high-Q circuits match better than low-Q circuits.

 

 

Matching according to the equal-Q circle follows the Bode-Fano criterion.

 

2. Balun Impedance Transformation

 

 

Design an impedance transformer to achieve impedance transformation from R1 to R2;

Simulation results for different impedances:

50 ohms, length 12mm:

 

30 ohms, 12mm:

 

 

When doing Balun Matching, besides the impedance of the line affecting the match, the length of the line also matters:

 

30 ohms, 5mm:

 

 

When performing Balun Matching, to shorten the line length, a magnetic core is typically used to reduce the length. It’s important to pay attention to the selection of the magnetic core inductance and power capacity. Due to circuit losses, the magnetic core may accumulate heat, which can cause a rapid increase in the core temperature. In severe cases, this can lead to a decrease in the core’s permeability, affecting the low-frequency response of the coaxial transformer.

 

3. Balanced Amplifier

 

A balanced amplifier can achieve better gain flatness and significantly improve the input and output VSWR (Voltage Standing Wave Ratio). It is a practical broadband PA circuit structure. A balanced amplifier consists of two 3dB broadband quadrature couplers and two amplifiers.

 

Advantages of a balanced amplifier:

1. When designing an individual amplifier, you can prioritize gain flatness and output power. Even if the input and output VSWR of a single amplifier are not ideal, as long as both amplifiers are consistent, the VSWR of the balanced amplifier will still be good.
2. The output power of a balanced amplifier is twice that of a single amplifier. For broadband amplifiers with relatively low output power, using a balanced amplifier for power combining can increase the power output.
3. The reliability of a balanced amplifier is high. Even if one amplifier fails, the circuit can still operate normally, though the gain will decrease by 6dB.
4. The stability of a balanced amplifier is good, as there are no reflected waves between the ports, making the circuit theoretically absolutely stable.
5. Negative Feedback

The gain of RF power transistors decreases as frequency increases. Typically, with each octave increase, the gain drops by about 6dB. In narrowband circuits, the gain drop with increasing frequency can be negligible, but in multi-octave circuits, it is necessary to consider suppressing the low-frequency gain.

By applying negative feedback from the output to the input, the low-frequency gain can be reduced, compensating for the gain drop with increasing frequency. This allows the amplifier to achieve flat gain across the entire frequency band, effectively broadening the operating bandwidth of the PA.