BJT-BJT, FET-BJT, and FET-FET

Cascode amplifiers have three configurations: bipolar junction transistor-bipolar junction transistor (BJT-BJT), field-effect transistor (FET)-BJT, and FET-FET. Although the high-frequency behaviors of these configurations are not the same in practice, in most textbooks only the BJT-BJT configuration is analyzed. High-frequency response of the BJT-BJT cascode amplifier is limited by three factors: 1) the source impedance or the output impedance of the previous stage; 2) the output impedance or the load of the amplifier; and 3) the dc bias current of the amplifier. In order to cope with these limitations, this article presents a modified cascode amplifier. In this new configuration, only a single transistor is added to the elements of each of the aforementioned cascode amplifiers. Corresponding to each configuration, a modified configuration results in greater or approximately equal gain and higher bandwidth.