# Transistor circuit analysis problem

#### taarkikguy

Joined Dec 1, 2019
2
Circuit diagram- here
Okay so we need to calculate V which is Vb1e1 I guess. If we can get Ic1 then Vb1e1=R5*Ic1 if I'm correct.
We have B=100 so Ic1=B*Ib1 ie 100Ib1. If we are to apply KVL, how do we go about it?
What is the overall voltage in all the loops?
Also, is it correct to use voltage division rule and say that
Va=12*(10k/(10k+20k+10k))= 3V
Vb=12*(20k/40k)= 6V
and, Vc=12*(10k/40k)= 3V
How do I proceed with the voltage division rule method or is it even the correct method. If not then how to solve using the KVL method?
If someone can help it would be great. Thanks

#### Jony130

Joined Feb 17, 2009
5,091
What you got is an approximation solution (not exact). Your calculations do not include the base current effect.
Try to apply the Thevenin's theorem to R1 and R2 and write some KVL after you simplified the circuit.

#### WBahn

Joined Mar 31, 2012
24,974
If Va and Vb are the voltages at the points marked A and B in the diagram, how can they be anything other than 12 V.

If they aren't those voltages, which voltages are they?

#### RBR1317

Joined Nov 13, 2010
509
If we are to apply KVL, how do we go about it?
KVL would imply using mesh analysis. But I don't like mesh; never touch the stuff. A good way to start is redraw the circuit with the appropriate transistor model. Here is the equivalent circuit with each transistor replaced by a super-node & a current-controlled current source. Also a screenshot of the Maple math application performing the nodal analysis. (Units: V-volt, I-mA, R-KΩ) So, is the correct answer Vout=6.4835?

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#### taarkikguy

Joined Dec 1, 2019
2
Yeah, it's 6.5V. thanks for the detailed solution.

• Zeeus

#### Zeeus

Joined Apr 17, 2019
481
KVL would imply using mesh analysis. But I don't like mesh; never touch the stuff. A good way to start is redraw the circuit with the appropriate transistor model. Here is the equivalent circuit with each transistor replaced by a super-node & a current-controlled current source. Also a screenshot of the Maple math application performing the nodal analysis. (Units: V-volt, I-mA, R-KΩ) So, is the correct answer Vout=6.4835?
Hi Please look at last post here : https://forum.allaboutcircuits.com/threads/transistor-circuit.165390/page-2#post-1456792
Calculating Dc

Thanks

#### WBahn

Joined Mar 31, 2012
24,974
KVL would imply using mesh analysis. But I don't like mesh; never touch the stuff. A good way to start is redraw the circuit with the appropriate transistor model. Here is the equivalent circuit with each transistor replaced by a super-node & a current-controlled current source. Also a screenshot of the Maple math application performing the nodal analysis. (Units: V-volt, I-mA, R-KΩ) So, is the correct answer Vout=6.4835?
Please don't just give out solutions -- at least not unless you are going to go take their exams for them and do all of their work for them after they graduate, too.

#### WBahn

Joined Mar 31, 2012
24,974
Yeah, it's 6.5V. thanks for the detailed solution.
By inspection and assuming β = ∞ you get Vout = 6.53 V. Be sure you can at least do that much on your own. If you can't, then ask and I'll walk you through it step by step.

#### RBR1317

Joined Nov 13, 2010
509
Please don't just give out solutions -- at least not unless you are going to go take their exams for them and do all of their work for them after they graduate, too.
I don't do homework; however, after some period of time it ceases to be homework. Is 7 days not enough time?

#### WBahn

Joined Mar 31, 2012
24,974
I don't do homework; however, after some period of time it ceases to be homework. Is 7 days not enough time?
Fair point. I'd recommend at least two weeks, since that is a very common amount of time between when assignments are released and the due date.