# Transistor analysis with 2 voltage sources

I want to calculate the DC currents of this circuit, and based on the superpostion theorem, I thought of ignoring Vqdc9 first, in order to calulate I2, I21 and I22, as shown in figure 2. After that, I ignored Vqdc2 and obtained I9, I91 and I92. Then, I assumed the current throught R6 is the difference between I91 and I21. Is it correct? I Assumed Vbe = 700mV

First 5 equations --> Vqdc9 is replaced by an open circuit, thus, the voltage on node 5, V5 is Vqdc2 - the voltage drog over R5. The current throught R5 is I2 which is decomposed into I21 and I22, I21 goes through R6 and then into the base of Q10, while I22 goes straight into the base of Q3.

On the second set of equations I assumed Vqdc2 as an open circuit and the voltage on node 4, V4, is Vqdc9 - the voltage drop on R7. The current through R7 is I9 which has 2 parts, I91 and I91, as I91 goes through R7 and then the base of Q3 while I92 goes into the base of Q10.

Just added image 3, I want calculate the bias current and voltages based in the model drawn on image 3.

• You can only use superposition on linear circuits. Transistors are not linear except possibly in small-signal conditions. Are you sure that your transistors are operating linearly and you have the correct model for their linear behavior? What does your simulation say? Mar 30, 2022 at 16:01
• Simulation says it's within the foward active region, Vc > Vb > Ve, for both of them. Mar 30, 2022 at 16:05
• OK, so read all of the currents from your simulation and verify your equations. Mar 30, 2022 at 16:08
• @jonk Yes but only after the BJTs were replaced with an appropriate linear model. Maybe the OP did that, maybe they didn't. Maybe they didn't do it correctly. Anyway, I would like to see their work. Mar 30, 2022 at 17:00
• Your V1 and V3 voltage source are connected backward than they should be.
– G36
Apr 1, 2022 at 17:15