Loop Gain Envelope Evaluation For Rapid MMIC Amplifier Stability Analysis

A common method for performing linear Monolithic Microwave Integrated Circuit (MMIC) amplifier stability analysis is to evaluate the loop gain for each active device. In order to ensure stability is achieved for a wide range of system applications, loop gain analysis is typically performed using purely reflective terminations at the amplifier input and output from low frequency up to frequencies where device oscillation is no longer possible. All input and output phase combinations must be evaluated at each frequency, resulting in significant computational burden to the designer. This paper presents a method for evaluation of the loop gain envelope at a particular frequency. Rather than evaluating the loop gain for all combinations of input and output reflection coefficient phases, one reflection coefficient phase is defined and the phase of the other which results in a point on the envelope is analytically evaluated. Therefore, rather than evaluating the loop gain for N 2 reflection coefficient combinations at each frequency, only N evaluations are necessary. The reduction in computations allows rapid evaluation of phase margin, allowing optimization of individual device stability within a MMIC during the design phase. The necessary equations for evaluation of the loop gain envelope at a particular frequency are provided. The method is applied to a three-stage MMIC amplifier as an example.