# A question on AC amplifier

• genxium
In summary, the speaker is confused about the negative value of ic/ib in the second picture of the oscilloscope graph and asks if it is caused by the capacitance between the base electrode and collector electrode. They try changing the sense resistors and discuss the concept of phase. They have also tried similar experiments on other software and devices but still get the same result. They are looking for an explanation for this issue.

#### genxium

Although this question may seem too simple, I still want to ask it on the forum because I’m really confused……

Here are the easiest amplifiers circuits for DC and AC currents, and what’s shown in the oscilloscope I’ve got in Multisim (corresponding), I want to know why ic/ib is negative in the second picture of oscilloscope graph, is it caused by the capacitance between base electrode and collector electrode? If so , what exactly the division is for the current (around the transistor)?

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genxium said:
Although this question may seem too simple, I still want to ask it on the forum because I’m really confused……

Here are the easiest amplifiers circuits for DC and AC currents, and what’s shown in the oscilloscope I’ve got in Multisim (corresponding), I want to know why ic/ib is negative in the second picture of oscilloscope graph, is it caused by the capacitance between base electrode and collector electrode? If so , what exactly the division is for the current (around the transistor)?

Hmm. That is indeed weird. The way you've set up your voltage probes, they should show Ic and Ib in phase, as they do for the DC case.

My only idea at this point is that you are using way too small voltages for the source drive in the AC case (in the DC case too, but whatever). That may be introducing calculation errors into the simulator. I've seen that happen before.

Increase the 1 Ohm sense resistors to more like 1k Ohm, and try to get at least 1Vpp out of the collector circuit. Do the waveforms end up in phase now?

I changed those "1 Ohm"s to 1k and 5k to test,and I got the same result,I drawed a picture of currents through the transistor and guessed it might be just easy as the situation in DC circuit,I would post it as long as I get my laptop

I guess that the currents might go through the transistor in this way,when Vb>Ve at some moments : (picture shown below)

Firstly I got some problems understanding what the "phase" actually is and was quite confused about the directions of currents, but after a discussion with my friends, we all think that there is no "phase" for a certain moment ("phase" should be a concept along with the whole time line), so the picture above maybe right.

Always expecting answers for this problem~ I'm still not sure about that, and I've also done some other similar experiments about AC amplifiers, on Qucs, even on real devices (also with transistor 2N2222A), but I just got the same result: ic/ib is negative...

Any explanation will be appreciated~