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

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genxium
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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)?

Circuit schematic of a common-emitter transistor amplifier with a 2N2222A NPN transistor Q1, resistors R1 1Ω, R2 1Ω, R3 and R4 10kΩ, R5 1Ω, supply VCC 12V, and an oscilloscope XSC1 connected to input and output nodes.

Screenshot of the XSC1 oscilloscope window in Multisim showing a black screen with a blue trace and a red trace as flat horizontal lines near the top half of the grid, with readings of 929.911 uV for Channel A and 163.261 mV for Channel B at 0.000 s.

Multisim schematic of a common-emitter transistor amplifier with a 2N2222A NPN transistor, 12V VCC, resistors R1 1Ω, R2 1Ω, R3 and R4 10kΩ, R5 1Ω, a 12mVrms 60 Hz source V1, and an oscilloscope XSC1 showing a sine wave.

Multisim oscilloscope window XSC1 showing a red sine wave and a blue sine wave of similar frequency, with the blue wave of smaller amplitude and inverted relative to the red, on a black screen with a 20 ms/Div timebase.
 
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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)?

Simulator schematic of a common-emitter transistor circuit with a 2N2222A NPN transistor Q1, a 12V VCC supply, bias resistors R3 and R4 at 10kΩ, emitter resistor R1 and base resistor R2 at 1Ω, collector load R5 at 1Ω, and an XSC1 oscilloscope probed at the base and emitter.

Oscilloscope-XSC1 window showing a black screen with two flat horizontal traces: a blue line near the upper area and a red line lower, both at constant levels with no waveform. Readout shows Time 0.000 s, Channel_A 929.911 uV, Channel_B 163.261 mV.

Simulator schematic of a common-emitter transistor amplifier with a 2N2222A NPN transistor, 12V VCC rails, 10kΩ bias resistors R3 and R4, 1Ω resistors R1, R2 and R5, a 12mVrms 60 Hz source V1, and an oscilloscope XSC1 probing the base and emitter nodes.

Oscilloscope window (XSC1) showing a red sine wave of higher amplitude and a blue sine wave of lower amplitude over a black grid, with Channel A at 50 pV/div and Channel B at 10 pV/div and a 20 ms/div timebase.

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)

Schematic of a transistor-like network with nodes b, c and e; ib_sum flows into node b, ix flows through a capacitor-like element to c, ic_sum leaves c, ib flows through a blue element to e, and ic flows through an orange diamond element to e, with e grounded.


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~
 

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