Measuring Amplifier Gain

The NanoVNA-H can measure amplifier gain (S21), input match (S11), and output match (S22 with reversed connection). This tutorial covers small-signal amplifiers — preamplifiers, low-noise amplifiers (LNAs), and buffer amps — where the NanoVNA-H’s low output power (~-10 dBm) falls within the amplifier’s linear range.

What You Will Learn

• Measuring forward gain (S21) and gain flatness
• Checking input match (S11) and reverse isolation
• Safely connecting high-gain amplifiers
• Interpreting Smith chart impedance data for amplifier inputs
• Diagnosing common amplifier problems from S-parameter traces

Danger

The NanoVNA-H input can be damaged by excessive RF power. NEVER connect the NanoVNA-H to a power amplifier’s output while the PA is transmitting. The NanoVNA-H’s maximum safe input is approximately +10 dBm. For high-gain amplifiers, you may need an attenuator between the amplifier output and the NanoVNA-H CH1 port.

Equipment Needed

• NanoVNA-H (calibrated)
• DC power supply for the amplifier under test
• SMA cables and adapters (matched to your amplifier’s connectors)
• External attenuator, 10-20 dB (if amplifier gain exceeds 30 dB — to protect the NanoVNA-H input)
• Bias tee (if the amplifier requires DC fed through the RF path)

Connection Setup

For a standard two-port amplifier measurement:

NanoVNA CH0 (stimulus) --> Amplifier Input --> Amplifier Output --> NanoVNA CH1 (receiver)
                                                       |
                                                 DC Power Supply

If the amplifier gain exceeds 30 dB, insert an attenuator to keep the signal at CH1 below +10 dBm:

Amplifier Output --> 10-20 dB Attenuator --> NanoVNA CH1

Remember to subtract the attenuator value from the measured S21 if it was not included during calibration.

Calibration

Accurate gain measurement depends on calibrating through the same cable path you will use for the amplifier.

1. Set the sweep range

   Choose a range that covers the amplifier’s operating band with some margin on each side.

   For a 2m LNA (144-148 MHz):

   • START: 100M (100 MHz)
   • STOP: 200M (200 MHz)

   For a broadband MMIC:

   • START: 1M (1 MHz)
   • STOP: 1G (1 GHz)

2. Connect cables without the amplifier

   Run the cable from CH0 through any adapters and connectors, directly to CH1 — the same path the signal will take, minus the amplifier.

3. Perform THRU calibration

   Navigate to CAL > CALIBRATE > THRU to set the 0 dB reference.

   For the most accurate results, perform full SOLT calibration (see Full Calibration).

4. Decide how to handle the attenuator

   If you need an external attenuator:

   • Option 1 (recommended): Include the attenuator in the calibration path. S21 then reads amplifier gain directly.
   • Option 2: Exclude the attenuator from calibration. You must subtract the attenuator’s loss from every S21 reading.

   Option 1 is more accurate because attenuator loss varies with frequency.

Measuring Gain (S21)

1. Configure the S21 trace

   Set a trace to FORMAT S21 (THRU) > LOGMAG.

   Adjust the scale so the expected gain range fits on screen. For a 20 dB amplifier, a scale of 5 dB/div with the reference at 20 dB works well.

2. Connect the amplifier

   • Amplifier input to CH0 cable
   • Amplifier output to CH1 cable (or to attenuator, then CH1)

3. Power on the amplifier

   Apply DC power. The S21 trace should show positive dB values (above the 0 dB line) across the amplifier’s operating band.

4. Place a marker at the peak

   Use a marker to read the maximum gain and its frequency.

5. Record gain at key frequencies

   Place additional markers at the band edges and center frequency of interest.

Typical Gain Values

Amplifier Type	Expected Gain
2m LNA	15-25 dB
HF preamp	10-20 dB
MMIC (MAR-series)	10-15 dB
Buffer amp	3-10 dB

Measuring Gain Flatness

Gain variation across the operating band matters for wideband applications.

1. Place marker 1 at the band center

   Note the gain value.

2. Place markers at the band edges

   The difference between the highest and lowest gain readings across the band is the gain flatness (or gain ripple).

3. Use delta markers for direct readout

   Set marker 2 as a delta reference to marker 1. The delta values at band edges give the gain variation directly.

Good amplifiers show +/- 0.5 dB flatness across their intended band. A downward slope at higher frequencies is typical for BJT-based amplifiers due to gain-bandwidth product rolloff.

Measuring Input Match (S11)

1. Enable S11 trace

   Set a trace to FORMAT S11 (REFL) > LOGMAG.

2. Terminate the amplifier output

   The amplifier’s output should be connected to a 50-ohm load (or left connected to CH1, which presents roughly 50 ohms).

3. Read return loss in the operating band

   More negative values mean a better match. Place a marker at the worst (least negative) point in the passband.

Input Return Loss Interpretation

Return Loss	VSWR	Match Quality
-6 dB	3.0:1	Poor — significant mismatch
-10 dB	2.0:1	Acceptable for broadband designs
-15 dB	1.4:1	Good
-20 dB	1.2:1	Very good

Measuring Reverse Isolation

Reverse isolation indicates how much signal leaks backward through the amplifier (from output to input).

1. Swap the amplifier connections

   Connect the amplifier output to CH0 and the amplifier input to CH1.

2. Read S21 (which now represents S12)

   The trace should show negative dB values. Good amplifiers show -20 to -40 dB of reverse isolation.

3. Note any frequencies with poor isolation

   Isolation dips can indicate potential instability.

Interpreting the Smith Chart

Switch the S11 trace to Smith chart format to see the amplifier’s input impedance directly.

• Center of chart: perfect 50-ohm match
• Right side: high impedance (common for FET inputs at low frequencies)
• Left side: low impedance
• Circular arcs: frequency-dependent matching — the tighter the arc, the more the impedance varies across the band

If the impedance is far from 50 ohms, a matching network at the input will improve system performance. See LC Matching for design guidance.

Gain vs. Supply Voltage

If the amplifier’s performance is sensitive to supply voltage:

1. Set the sweep running continuously

2. Vary the DC supply voltage slowly

   Watch S21 change in real time. Note the voltage where gain stabilizes.

3. Record gain at several voltage points

   This reveals the amplifier’s minimum operating voltage and the point of diminishing returns.

Troubleshooting

S21 Shows Negative Values (Loss Instead of Gain)

• Amplifier not powered — check DC supply connections and current draw
• DC supply polarity reversed
• Amplifier oscillating (unstable, draws excessive current)
• Sweep frequency range is outside the amplifier’s operating band
• Cables connected to the wrong ports

S21 Shows Very High Gain with Noisy Trace

• The amplifier is likely oscillating. Check for oscillation peaks by widening the sweep range.
• Add decoupling capacitors (0.1 uF ceramic + 10 uF tantalum) close to the amplifier’s power pins.
• Check for poor grounding or excessively long leads.

Gain Drops Sharply at High Frequencies

• Normal behavior for most amplifiers (gain-bandwidth product limitation)
• Verify that cable loss is not introducing additional rolloff — recalibrate if cables have changed
• Check that the calibration covers the full frequency range of interest

Erratic or Jumping Trace

• Loose cable connections — hand-tighten all SMA connectors
• Amplifier intermittently oscillating
• Insufficient power supply decoupling
• DC supply current limit tripping

Tip

Enable the S21 Phase trace alongside S21 LogMag to detect potential instability. Rapid phase rotation near the band edges, especially if accompanied by a gain peak, can indicate that the amplifier is close to oscillation.

Tip

For LNAs used in satellite or weak-signal work, noise figure matters more than gain above about 15 dB. The NanoVNA-H cannot measure noise figure directly, but gain and input match measurements help verify the amplifier is functioning correctly before installing it in the signal chain.

Documenting Amplifier Performance

Record your results for future reference:

Parameter	Value
Amplifier model	_________
Supply voltage	_________ V
Supply current	_________ mA
Gain at center frequency	_________ dB at _________ MHz
Gain flatness (across band)	+/- _________ dB
Input return loss (worst)	_________ dB
Reverse isolation	_________ dB
External attenuator used	_________ dB (if any)

Next Steps

• Your First S21 Measurement — Basic transmission measurement setup
• Characterizing a Filter — Similar two-port technique for passive devices
• Full Calibration — Maximize measurement accuracy with SOLT
• LC Matching — Design matching networks for poorly matched amplifiers