Accurate Substrate-Related Packaged Transistor Modeling for 24GHz Circuit Design

S-parameter measurements of packaged transistors are performed in dedicated microwave test fixtures and they provide the S-parameters with respect to reference planes usually extending to the transistor capsule. Most packaged transistors have typically an extra piece/length of the metal leads extending further from those reference planes, underneath the capsule, to enable the connection of each of the leads with the encapsulated transistor die. During measurements in a common test fixture these extra metal pieces remain basically suspended in the air. When a transistor is placed on a microwave substrate the leads are usually resting on transmission lines printed on the top layer. As a result, the overall electrical conditions regarding the transistor´s leads and package will differ to those present in the test fixture. This paper presents results showing how severe the impact of these lead extensions can be in the case of a 24GHz 2-stage amplifier design. It is shown how the extra package lead lengths can be accurately modeled with S-parameter blocks generated with an EM simulator. The demonstrated modeling method can lead to significant reductions in the number of design cycles at 24 GHz