Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 68

System amplifiers are simply gain blocks used to boost signal levels in a system, while providing reverse isolation. They can have higher noise figures than LNAs as they are used in signal paths where the signal is well above the noise floor. They often follow an LNA stage, frequently after some pre‐filtering. They are also often used in the frequency converters as a local oscillator (LO) amplifier to isolate the RF signal from leaking out the LO port, or as an isolation amplifier to prevent LO leakage out the RF port. These tend to be broad band amplifiers, with good input and output match, emulating an idealized gain block. The important figures of merit for such amplifiers are gain (S21), input and output match (S11, S22), and isolation (S12). Occasionally, directivity of an amplifier is defined as isolation (a positive number in dB) minus the gain (in dB), or S12/S21. It is a measure of the effects of a load apparent at the input of the amplifier, or how the output impedance is affected by the source impedance (Mini‐circuits n.d.) and is important in cases where other system components have a poor or unstable match. Since these amplifiers have wide bandwidths, it is important that they have good stability as they can have a variety of load impedances applied. Other figures of merit for system amplifiers can include gain flatness (deviation of the gain from nominal value), 1 dB compression point (the power at which the gain drops by 1 dB), harmonic distortion, and two‐tone third‐order IM, sometimes expressed as third‐order intercept point (see Section 1.3).