Understanding AC Amplifiers: DC vs. AC Circuits and Oscilloscope Analysis

[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)?

 

[berkeman]

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?

 

[genxium]

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

 

[genxium]

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~

 

[DeeNos]

Thank you for your question. It is great to see that you are exploring the differences between DC and AC circuits and using an oscilloscope to analyze the signals.

In the first picture of the oscilloscope graph, you can see a DC circuit with a constant input voltage and a constant output voltage. This is because in a DC circuit, the input voltage is constant and does not change over time. Therefore, the output voltage also remains constant. In this case, the current flowing through the circuit is also constant, as shown by the flat line on the oscilloscope.

In the second picture, you can see an AC circuit with a varying input voltage and a varying output voltage. In an AC circuit, the input voltage changes over time, causing the output voltage and current to also change. The negative ic/ib ratio in the second picture is due to the phase difference between the input and output signals. This is caused by the capacitance between the base and collector electrodes, which affects the current flowing through the transistor.

To understand this better, it is important to know that in an AC circuit, the input voltage and current are out of phase with each other. This means that the current does not flow through the circuit at the same time as the voltage is applied. This is due to the capacitance between the base and collector electrodes, which causes the current to lag behind the voltage.

In the second picture, the negative ic/ib ratio indicates that the current is lagging behind the voltage. This is because the input voltage is increasing while the current is still decreasing, resulting in a negative ratio.

In summary, the negative ic/ib ratio in the second picture is caused by the phase difference between the input and output signals in an AC circuit, which is influenced by the capacitance between the base and collector electrodes. I hope this helps to clarify your confusion. Keep exploring and experimenting with different circuits and signals to deepen your understanding of amplifiers and oscilloscope analysis.