# Applying hybrid-pi model of an npn-BJT to a pnp BJT in small signal analysis

When doing the small signal analysis of a circuit containing a pnp BJT connected to either the emitter or collector (or base?) of a npn BJT, is it possible to apply the hybrid-pi model for the npn-BJT and apply a minus sign to the current as in -gmVbe since current flows opposite?

simulate this circuit – Schematic created using CircuitLab

• I1 is 19 amps? Dang, that's going to produce some seriously high voltages in that circuit. Nov 9, 2013 at 2:16

You can take the NPN model and apply Vbe = -Veb (that's by definition). Then, on the equations for the NPN model you'll have ib = -Veb/rb, ic = -gm Veb and ie = ib + ie;
As you see, It's a matter of avoiding minus signs. Just like in large signal analysis we say VEB = 0.6V instead VBE = -0.6V for a PNP transistor.