High frequency model of transistor

 We discussed small signal analysis of BJT  under the key word "Small signal analysis". When input signal to an amplifier is in the range of ten to hundred  Kilo Hertz, a small signal-low frequency model of the transistor can be used for analysis. But as the frequency increases, internal capacitance of the transistor will strongly effect it's performance. A low frequency model cannot work well in this situation. To accommodate these performance changes of the transistor, a separate model  is developed for high frequency operations. This high frequency model is given in Fig 1. A high frequency hybrid-pi model is  also known as Giacoletto model.