We ll next work out an important relationship between charge control parameters and hybridpi parameters. In Figure 10-5a we see a circuit that will help us work out this relationship. The small-signal hybrid-pi model for this circuit is shown in Figure 10-5b.

Figure 10-5: Circuit for finding relationship between charge control and hybrid-pi parameters. (a) Transistor circuit. (b) High-frequency hybrid-pi model.

If we run the transistor at very high frequencies, the hybrid-pi model finds the base current:

For very high frequencies, the charge control model predicts (ignoring space-charge capacitances):

We find the following relationship:

Next, by looking at the charge control equations at very low frequencies, we find:

So, we now have a relationship between the charge control parameters ? _F and ? _BF and the hybrid-pi parameters ? _F and ? _T.