The first issue that needs to be addressed when designing a high-frequency amplifier is whether the circuit performance specifications can be met with the selected technology. In case the termination on one of the ports of the active device is fixed, the problem is usually solved using the gain and mismatch limits available in the literature. Otherwise, when the loads at the ports of the active device are not constrained, an analytical criterion is available in the literature to address the problem. However, it is not well known, so it is little exploited. Moreover, a proof of the criterion is not available in the literature. This paper presents a simple geometric proof of the criterion referenced. Closed-form expressions of the terminations determining the fulfilment of the amplifier design goals are determined and the stability of the amplifier in relation to the fulfilment of the performance specifications is addressed. Finally, it is shown that the criterion and the analysis developed in the paper are useful tools for the development of design methodologies of microwave amplifiers operating in more general conditions.

The stability criteria based on the Rollett parameter ($K$) plus one auxiliary condition is yet the most popular method used by designers to evaluate the stability of a two-port network although other criteria are available. In this paper, novel analytical proofs of the criteria utilizing$K$and a new geometrical proof of the relationship between the stability of the ports of a circuit are proposed. They are obtained by making use of relationships between the reflection coefficients at the ports of the network and novel expressions that bind stability circles and unit mapping circles. The developed relations make the proposed proofs easier and more immediate than the traditional ones. Moreover, a new auxiliary condition having the same complexity and a greater geometrical insight with respect to the traditional ones is used.

This work is focused on the design of broadband amplifiers by exploiting dissipative reciprocal matching networks. Unlike the classical approach, which makes use of lossless reciprocal matching networks, there is no need to trade-off the gain flatness with the input/output matching levels. In this contribution the flat gain condition is obtained partially by exploiting the mismatch loss at a certain section and partially by leveraging on ohmic losses. In particular, two different matching schemes have been analytically studied. Taking advantage of the more promising scheme of the two presented, the design of a linear power amplifier operating over the 4-40 GHz decade, is illustrated. The amplifier showed measured input/output return loss better than 9/10 dB respectively, a gain of 8 ± 0.6 dB and an output power level of 25.5 ± 0.5 dBm at 1 dB compression point. Incidentally, unconditional stability is obtained without considering additional stabilization networks or components devoted to the purpose.