# Trying to plot input and output resistance for a single BJT in LTspice

I'm trying to plot a single NPN transistors's own input and output resistances in common emitter configuration looking into the base from input and emitter. I showed below with arrows that the plot I'm trying to achieve is how the input impedance and output impedance varies with Vbe:

So the collector voltage V3 is fixed at 5V. The input voltage V1 = Vb is fixed at 1V. And what actually varied in DC sweep is the emitter voltage Ve. So this causes the Vbe to vary between 300mV to 900mV as follows:

To see the dynamic input resistance the green plot is D(V(Vb))/D(I(V1)); and for the output resistance(seen as shown in the right arrow) it is D(V(Ve))/D(I(V2)).

Is this way of plotting correct? The green plot is almost zero regardless of Vbe. Is that expected?

And here below is if I don't use derivatives:

I'm confused which plots really represents the input output resistances.

I use this circuit:

And $$\\large r_{e} = \frac{d(V_{BE})}{d(I_E)} \approx \frac{V_T}{I_E} \approx\frac{25.86492 \textrm{mV}}{I_E}\$$ plot at $$\27^{\circ}C\$$

And $$\\large r_{\pi}= \frac{d(V_{BE})}{d(I_B)} \approx (\beta +1)r_e\$$

• This looks very good! Is this correct about your jargon?. rpi is the dynamic/AC input resistance looking from the base; and re is the output dynamic/AC resistance looking from the emitter or load. ? – cm64 Jan 16 at 14:44
• Yes, you are right electronics.stackexchange.com/questions/367321/… – G36 Jan 16 at 14:46
• I plot your answer with respect to Vbe and found out that at Vbe around 700mV, the rpi is around 5 Ohm and re is 1.5k Ohm. This model looks very nice to see the effect of load to input and output impedance. Thanks – cm64 Jan 16 at 14:50

I do the same except I prefer to use log-scale and recognize Re like Rce has a bulk resistance limit at max current and the power rating of the device is often inversely related to this bulk resistance. Rce=k/Pd for k ~ 0.25 to 1 Also the base uses a series equivalent bias resistance so Re increases by Rb/hFE unless for example, shunted by a cap for a common base.

$$\\large r_{e} = \frac{d(V_{BE})}{d(I_E)} +\frac{R_B}{h_{FE}} \approx \frac{V_T}{I_E} +\frac{R_B}{h_{FE}} \approx\frac{25.86492 \textrm{mV}}{I_E}+\frac{R_B}{h_{FE}}\$$ plot at $$\25^{\circ}C\$$