What is the difference between input VSWR and output VSWR on an amplifier datasheet?
Input and Output VSWR Explained
Every two-port RF component has both input and output impedance, characterized by S11 and S22 respectively. For amplifiers, these two parameters serve different roles and are optimized for different criteria.
The input VSWR determines how much of the incoming signal is reflected back to the source. Poor input VSWR causes mismatch loss (reduced signal reaching the amplifier), gain ripple when interacting with the source impedance, and for LNAs, can degrade noise figure if the source impedance is pushed away from the device's optimal noise impedance. Input VSWR is typically 10 to 20 dB return loss for broadband amplifiers.
The output VSWR determines how well the amplifier delivers power to its load. Poor output VSWR reduces the power delivered to the load and creates reflected signals that travel backward through the amplifier, potentially causing instability or oscillation if the reverse isolation (S12) is insufficient. Output VSWR also affects gain flatness through interaction with the load impedance, creating ripple as frequency changes the electrical length between mismatches.
Frequently Asked Questions
Can the amplifier oscillate with high VSWR?
Yes. If the product of forward gain (S21) and reflected power exceeds unity due to poor output match and insufficient reverse isolation (S12), the amplifier can become conditionally unstable and oscillate. Check the stability factor (K > 1 and |Δ| < 1) under actual load conditions.
Which VSWR matters more for LNAs?
Input VSWR matters more because it affects the source impedance seen by the transistor, which impacts noise figure. However, the LNA input is often deliberately mismatched (noise match rather than conjugate match) to minimize NF, resulting in relatively poor input VSWR (8-12 dB RL).
How does a pad improve VSWR?
An attenuator pad between stages improves the effective VSWR at both interfaces because the pad absorbs reflected signals. A 3 dB pad improves the effective return loss by 6 dB (round trip through the pad). This is the simplest way to improve inter-stage VSWR at the cost of gain and noise figure.