Electronic – Sinusoidal voltage source not being sinusoidal in LTSpice

analogltspice

I'm trying to obtain the output resistance (resistance from collector node) of the following circuit

enter image description here

By inspection, it should be the output resistance of a common emitter amplifier with degeneration

$$R_o = r_o(1+g_mR_1)$$

This transistor has

$$I_c = 20 mA $$
$$V_A = 36$$
$$r_o = \frac{36 V}{22mA} \approx 2k\Omega$$
$$ g_m = \frac{20 mA}{25 mV} \approx 720 m\Omega^{-1}$$

So its output resistance should be

$$R_o = 2k\Omega(1+720 m\Omega^{-1}250\Omega) \approx 360 k\Omega$$

However, the simulations are giving strange results. I tried simulating a small sinusoidal voltage at the collector and measuring the current as to get the resistance of the node, but trying to get the voltage amplitude below 1 V yields very strange results. Here are some captures of the voltage at the collector, which should be sinusoidal, but it isn't. The frequency of operation is 1 kHz

1000 mV ampltiude:

enter image description here

100 mV amplitude:

enter image description here

10 mV amplitude:

enter image description here

Best Answer

When needing high dynamic range outputs in .TRAN, the .opt plotwinsize=0 card should almost always be added to the schematic. That disables waveform compression which is, by default 300 points (you can check that by R-Click on the waveform window and select View > Mark data points). What you have there is a mV ranged signal on top of V worth of DC.

The downside is (may be) that the .raw file will get larger, in which case the .save card can be used.

If that doesn't help, try what @jonk, and then @SpehroPefhany said, in that order.

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