Why MMICs Behave as High Impedance Below Their Intended Frequency Range

Monolithic Microwave Integrated Circuits (MMICs) are commonly used as “drop-in” RF gain blocks and front-end amplifiers, and they’re usually characterized for VHF through microwave bands (often tens of MHz up to several GHz). Some MMIC amplifiers are even specified down to DC, but the key point is that the manufacturer’s guaranteed impedance match and S-parameters only apply over the stated datasheet frequency range.

During our experiments with active receive antennas and broadband amplifier stages for HF and below, we intentionally tested several MMICs far under their characterized range — down to around 100 kHz.

What we observed was counterintuitive at first: many MMIC gain blocks can present a high input impedance at these very low frequencies. Once you understand why, it becomes a useful trait for receive-only applications such as E-field probes and short vertical whips.

Design Assumptions and Real-World Behavior

Many “gain-block” style MMIC amplifiers are designed to interface with 50-ohm systems across their specified band. That “close to 50 Ω” behavior is usually achieved through a mix of on-chip techniques such as resistive feedback, emitter/source degeneration, and small matching or stabilizing networks.

Below the datasheet band, that controlled impedance behavior is no longer guaranteed. In particular:

• Internal (or required external) DC-blocking capacitors behave like open circuits as frequency drops, effectively disconnecting the RF input at LF.
• Any matching or stabilization components optimized for RF can become ineffective (or simply irrelevant) at LF.
• The remaining signal path may be dominated by whatever “DC-ish” paths exist inside the MMIC: leakage, bias networks, ESD structures, or feedback resistors.

The practical result is that the input can look more like a lightly-loaded transistor gate/base node than a tidy 50 Ω termination — i.e., a high-impedance port that may measure anywhere from hundreds of ohms to many kilohms (or higher), depending on the part and its internal architecture.

Why It Often Doesn’t Look Like a Simple Capacitor

It’s true that the intrinsic input of a transistor is largely capacitive (gate/base capacitances). However, those capacitances are typically very small (often on the order of a few pF or less). At LF, a few pF corresponds to an extremely large reactance. For example, 1 pF is about 1.6 MΩ at 100 kHz.

So while the input is still “capacitive” in a physical sense, the capacitance can be so small that it behaves almost like an open circuit at LF. In that situation, what you measure as “input impedance” is often dominated by other parallel or series effects, such as:

• Internal feedback resistors (common in broadband gain blocks), which can make the input appear more resistive than expected.
• Bias and protection structures (e.g., ESD diodes or bias networks) that provide a weak resistive path.
• AC coupling (internal or external), which can create a clear low-frequency cutoff and a very high apparent input impedance below it.

In other words, it’s not that the MMIC input “isn’t capacitive” — it’s that at LF the capacitance is often so small that it no longer behaves like the kind of capacitive load people intuitively expect (tens to hundreds of pF). Any resistive/leakage paths can easily dominate what you see on the bench.

Ideal for High-Z Antennas (Voltage-Mode Reception)

This behavior can be advantageous for high-impedance, voltage-mode antennas — such as E-field probes (electrically short verticals), end-loaded whips, and active short dipoles. These antennas often generate a voltage on a high-impedance node, and a conventional 50 Ω termination would heavily load that node and collapse the available voltage.

A MMIC that presents a high input impedance at LF/HF will load the antenna much less, preserving more of the antenna’s open-circuit voltage. In receive-only chains, that often matters more than power matching: you want good voltage transfer into the amplifier input, not maximum power transfer into 50 Ω.

One practical caveat: for broadband active antennas, the effective input capacitance still matters as you move up in frequency. Even if the LF impedance looks “very high,” a few pF can become a meaningful load at the upper HF range. So it’s still worth paying attention to input capacitance, protection, biasing, and strong-signal handling.

Takeaways

• Many MMIC gain blocks can appear high impedance well below their specified band, especially if AC coupling or RF-optimized matching is involved.
• At LF, the input is still fundamentally capacitive, but the capacitance is often so small that it behaves nearly open-circuit; measured impedance can look mostly resistive due to feedback, leakage, bias, or ESD structures.
• This can be beneficial for active RX antennas that are naturally voltage sources into high impedance, because it preserves antenna voltage and reduces loading.

If you're designing an active antenna like the EchoTracer or VerticalVortex, leveraging this “high-Z at LF” characteristic can simplify the front end — often reducing the need for additional buffer stages or impedance transformers — as long as you still account for input capacitance, protection, filtering, and strong-signal behavior.

Understanding when and why this happens lets you get useful performance from 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.