Feature: RF design
The key role of isolators in millimetre-wave power amplifiers
By Greg Rankin, US-based freelance technology writer C
ommunications standards continue to upgrade and, with it, we are increasingly seeing them move to the higher frequencies of the spectrum – or milimitre
wave (mmWave) bands. At frequencies of 24-100GHz, 5G and 6G systems will benefit from very high speeds and bandwidth and low latency, but have limited range and poor penetration through obstacles. When working at mmWave frequencies,
power amplifiers are frequently equipped with isolators at their output ports. However, the purpose of this is oſten misunderstood. Te primary reason isolators are used has less to do with the impedance matching of the amplifier, but rather the impedance matching of the load. While an amplifier may function
optimally with a well-matched load, such as a power meter, its performance can degrade significantly when the load is
30 October 2025
www.electronicsworld.co.uk
not well matched. Since power amplifier manufacturers can’t dictate the specific load impedance in their customers’ systems, they incorporate isolators to maintain consistent performance.
Identifying the challenge Te fundamental issue is shown in Figure 1. In a controlled factory setting, when the amplifier is connected to a well-matched power meter, it delivers consistent RF output across the designated frequency band. Under these conditions, it operates within its expected temperature range and draws predictable DC power. However, problems emerge when
the amplifier is deployed in the field. Variability in the load impedance can result in signal reflections that travel back toward the amplifier, disrupting its operating parameters. Tis leads to reduced output power, increased operating temperatures,and higher power consumption. To address these challenges, manufacturers incorporate isolators at
the amplifier’s output port. Te isolator creates a stable load environment for the amplifier, regardless of any impedance mismatches from the connected system; see Figure 2. Isolators do introduce insertion
loss, which can be counterbalanced by increasing the amplifier’s output power. However, doing so comes with a trade- off of elevated operating temperatures. Nevertheless, isolators ensure performance remains consistent across broad frequency ranges.
Insertion loss A notable drawback of conventional isolators, particularly in higher millimetre-wave bands, is their significant insertion loss. Traditional isolators operating in the D-band (WR-6.5GHz, 110-170GHz) can have insertion losses exceeding 3dB. Part of the issue is that the design of traditional isolators dates back more than five decades, with very few improvements over that time.
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