> Well, I have been using a 2N222s model with Cje=22 pF. The BFR93 gives
> 2pF, i.e. one order of magnitude less and the BFP 405 gives 3.7 fF.
> Now it is about time to make some measurements...
Ok, so I tested this
measuring Vb and Ve. Transistors: 2N2222a and BFR93a.
Generator is adjusted to Vdc=2.3V and 5Vpp.
At 1 MHz, the emitter signal waveform is a nice trapezoidal signal with
a steeper slope when leaving the upper clipping level. The simulator
predicts rounded tops and a somewhat upwards sloping bottom.
At 27 MHz the Spice model predicts negative signal excursions. I could
not observe them. But if I add some 5-10 pF at the emitter resistance to
account for the probe, simulations and reality almost match.
Generator is adjusted to Vdc=0 and 5Vpp.
At 27 MHz, Spice model predicts sinusoidal signal centered around 0
approx +-2.3. On the scope I see max +1.8 and min -1.5, i.e. there is
some loss but the qualitative shape is ok. Of course, this is not what
the circuit is expected to do, but indicates the model is ok.
At 1 MHz, Spice model predicts bottom clipping but with positive slope
of 1.9V/us. Measurements more or less confirm this.
At 27 MHz, a quite sinusoidal output signal of 4Vpp can be measured
adjusting Vdc=1.3. The simulator predicts the same, although with a
4.5Vpp amplitude. So, the model seems ok.
The Spice models are ok and they do model large signal behavior. It was
my error to assume that there could be no negative signal excursions:
this is only true at low frequencies. Somehow I thought that 27 MHz was
dc for a 5 GHz transistor. But his is not the case for a high input
impedance circuit, as JT pointed out.
Thanks to all.