# Can a Unilateral Amplifier be Unstable?

When designing microwave amplifiers, stability is an important factor to consider. Stability is usually measured with the K factor. In a unilateral network, S_12 = 0, and this makes the K factor infinite.

However, we also know that being unilateral means Gamma_in = S_11, and Gamma_out = S_22. What if the network is unilateral, but either S_11 or S_22 are larger than 1? Will the amplifier be unstable despite being unilateral? Is this situation possible at all?

• I've never used the term with bilateral which has a similar meaning as bipolar nor seen any stable s11>1 as this implies the feedback amplifies any input to the input. ( Return Gain not Loss) Hence unstable But I never thought of using s parms for an oscillator – Tony Stewart EE75 Dec 22 '19 at 18:38
• @TonyStewartSunnyskyguyEE75, usually you use S-parameters to describe amplifiers, and when you design an amplifier you want to make sure you didn't accidentally design an oscillator instead. – The Photon Dec 22 '19 at 18:58
• @ThePhoton Γ_in = S_11 is true only when the port 2 is matched, not in the general case. – Ali Lavasani Dec 22 '19 at 18:59
• @Ali, OK, fair enough. But you could equally well talk about $S_{11}$ of the whole 1-port sub-system composed of the amplifier and its load. If you're going to parse the definitions of $\Gamma_{in}$ and $S_{11}$ this carefully, you should probably also give us the exact definition of "unstable" you're using. – The Photon Dec 22 '19 at 19:04
• it will certainly be distorted if unilateral and s11 or s22>1 and not useful – Tony Stewart EE75 Dec 22 '19 at 19:17