# Why does the colllector current decrese with decreasing collector-emitter voltage in this circuit?

I'm trying to understand how the circuit below works so I ran a simulation with TINA and I got the curves shown on the right (C3 is already charged).

Considering the characteristic curve of that BJT (shown below), I can't understand how it is possible for Vce to decrease with decreasing Ic.

My reasoning is as follows (the lines are not the ones corresponding to the circuit above, but they are close enough to expect a similar behavior):

The red line is the DC load (Vtank = 0) and the green line is the one I would expect after Vtank (EDIT: I made a mistake here, this Vtank is 180° out of phase with respect to the one shown above, but the discussion still stands), increases from 0. The yellow dot is the initial Q point. Since the Q point must be on the green line after a certain time, and since the simulation says that both Vce and Ic decrease, I'm having trouble understanding what's going on.

Of course Ic can't simply increase either because otherwise Vbe would go down (through Re) and Ic would be immediately interrupted.

EDIT:

After trying with a lower frequency and with a higher Re I got this:

Here is a bigger one.

So what seems to be happening is that locally (higher frequency) the transistor is behaving like I expected but non locally (lower frequency) the tank oscillations define the behavior. I would like someone to confirm this.

• Ant-why do you expect a shift of the Q point due to reactive elements (L and C)? The Q point remains constant. – LvW Jan 23 '15 at 8:34
• And where is the input stimulus? I think that what you are looking at is the switch on decay of the LC circuit oscillating. Also, how did you draw the red line - what fixed base current did you assume? – Andy aka Jan 23 '15 at 9:21
• @LvW he might be referring to the dynamic Q point, and Andy that's what I was thinking, at first I thought it could be an oscillator but it's missing an inductor in series with C3 to oscillate. – Vladimir Cravero Jan 23 '15 at 9:22
• The dynamic Q point would still go thru the yellow dot but the green line would have a different slope. – Andy aka Jan 23 '15 at 9:24
• @Ant, another question: Why do you expect a decreasing Vce in case of a lower Ic? – LvW Jan 23 '15 at 9:42

In the top simulation, your $I_c$ is only changing by 10uA for a 20mV. This is just the effect of the Early voltage and the finite output resistance of the transistor. From your data I calculate an Early voltage of about 80 V (very reasonable). I don't know what's going on with the LC tank, but it isn't the cause of the $I_c$ variation.

• Couldn't it be that the tank voltage is making the Q point "run" along a constant Ib curve? but then, why is Ib oscillating too? – Ant Jan 28 '15 at 2:42
• The voltage on the collector is making the Q point run along a constant Ib curve, but those curves aren't perfectly horizontal, so the collector current changes too. This is exactly the Early effect. From adding together the Vre, Vce, and Vtank curves it looks like the voltage on C2 is changing too. That would then change the base voltage and current a little. – Austin Jan 28 '15 at 22:51
• @Austin_Steiner I tried removing C2 and I get the same curves, I think there might be some Early effect going, but not completely, otherwise Ib shouldn't change. – Ant Feb 1 '15 at 1:45
• It looks like something strange is going on. If you add the voltage variation from Vre, Vce, and Vtank, you get about 0.12V, even though the source should be holding it at 9 V. Can you check this with a probe? I've never used (or heard of) TINA before so I can't think of any specific reason that would happen. Also, notice that the variation in Ib is out of phase with the variation in Ic, so I don't think it's the cause of the change in Ic (there shouldn't be much phase shift at 500kHz). – Austin Feb 1 '15 at 5:57
• I still think the Ic/Vce relationship is just output resistance. The bigger question is why is Vce changing in the first place. Try changing the inductor and capacitor values in the tank to see if they change the oscillation frequency. – Austin Feb 1 '15 at 5:59