Having problem with finding BJT Vbase

[negativefeedback]

Hello,

I am having trouble with calculating the base voltage of the attached circuit. I tried to do node voltage method but I wasn't succesful. Could anyone help with the calculation?

Thanks.

[Zero999]

Post your working so far.

What does the simulator say? Use it to check your working.

[negativefeedback]

Hi, I am having problem to setup equations. I cant match simulation results with hand calculations. I am asking how to construct equations for upper and lower loops.
Best.

[Ian.M]

Its not solvable without knowing the characteristics of the transistor.  Here's a LTspice SIM that gives a different Vb for each of generic NPN (SPICE default), 2N3904 and 2N3055.

[negativefeedback]

we can say general 2n2222 with 0.7VBE

[free_electron]

An approximation would be to assume the transistor is in saturation and not consider the base current. So you have (24 volts / (3300+1000+330)) * 330 across R3.  So Vr1 is 0.7 volt higher than that. You can now find the rest.
Of course this will deviate in real life as we did not take base current and setpoint of the transistor into account. But since that is unknown...

Chuck it in a simulator.

Or is this one of these 'homework puzzles' teacher like to give to their students ? In that case i would do the calculation for a transistor with a gain of 1000000 ( darlington ). it will be pretty darn close -evil grin- if they can't be bothered to specify what transistor then i pick the one that makes my work the simplest .

[Ian.M]

we can say general 2n2222 with 0.7VBE

Real transistors don't have a fixed Vbe as it depends on device characteristics, currents and temperatures.  If you are going to use a 'kindergarten' level model of a NPN transistor that assumes Vbe=0.7V whenever current is flowing into the base, you'll also need to assume a fixed h_FE to make it solvable.  2N2222 h_FE usually falls in the range 35 to 300 depending on collector current Ic, device to device gain variation and temperature.

free_electron has just posted a nice approach to getting the limiting values as h_FE tends to infinity.  You *COULD* use that to approximate Ic then plug in numbers from the graphs in the OnSemi 2N2222A datasheet to refine your answer.