Why MMICs Behave as High Impedance Below Their Intended Frequency Range

Monolithic Microwave Integrated Circuits (MMICs) are typically designed for use in the VHF to microwave range — often from 50 MHz up to several GHz. However, during our experiments with active receive antennas and broadband amplifier stages for HF and below, we tested several MMICs at frequencies well under their design range — even down to 100 kHz.

What we observed was unexpected: MMICs often behave as high-impedance devices at these lower frequencies. This article dives into why that happens, and why it’s actually advantageous in receive-only applications such as E-field probes and short vertical whips.

Design Assumptions and Real-World Behavior

MMICs are internally matched for 50-ohm systems — but only within their specified operating frequency. This broadband matching is accomplished through internal reactive elements, like capacitors and inductors, optimized to form low-reflection impedance networks between 50 MHz and 3 GHz.

Below this range, the matching networks lose effectiveness. Instead of maintaining a 50-ohm input, the device appears more like a floating gate or base. The result? A surprisingly high input impedance, often in the hundreds to thousands of ohms. In many cases, the behavior is dominated by parasitic inductances and the lack of any resonant matching at low frequencies.

Why It’s Not Capacitive

One might assume that the input of an MMIC should appear capacitive at lower frequencies, due to the gate or base capacitance of the internal transistor. However, in practice, the parasitic inductance and series components (such as bias chokes or ESD protection structures) tend to dominate, especially at lower frequencies. The result is not a classic capacitive load, but rather a more complex, often high-Z resistive or weakly inductive profile.

Ideal for High-Z Antennas

This input behavior makes MMICs surprisingly well-suited for coupling with high-impedance antennas — such as E-probes (electrically short verticals), end-loaded whips, and active short dipoles. These antennas often produce signal voltages into high-Z nodes, where conventional 50-ohm terminations would load the signal too much.

The high input impedance of the MMIC ensures that the voltage generated by the antenna is preserved, resulting in better signal transfer and minimal distortion. Even though the amplifier is mismatched from a power-transfer perspective, the voltage transfer is optimal — which is exactly what matters in a receive-only chain.

Takeaways

  • MMICs behave as high-Z devices below their rated frequency range due to ineffective internal matching and parasitic components.
  • They often do not present a capacitive load at low frequencies, but a complex impedance often dominated by inductive parasitics.
  • This is beneficial for active RX antennas that produce voltage-mode signals into high impedance.

If you're designing an active antenna like the EchoTracer or VerticalVortex, this is exactly what you want — and it saves you the effort of inserting additional buffer stages or impedance transformers.

Understanding this behavior lets you unlock the full potential of MMICs even far below their intended GHz playground.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.