# how to find equations for current of circuit with transistors?

using Ohm's law, i need to find the current for $$I_C , I_B$$ in terms of: $$V_B, V_C, V_{DC1},V_{DC2}$$ for the following ciruit.

my solution is: $$V_{DC2}+{I_B}{R_B}={I_C}{R_C}+{V_{DC1}}+{V_E}$$ But this seems to be wrong. why? Thank you

• You're overthinking this. Notice that $I_C$ is the same as the current through one of the resistors. And you can write Ohm's law for this resistor in terms of two of the voltages you are given. Same thing for $I_B$. Commented Apr 28, 2016 at 2:19
• You need the DC current gain of the transistor. Commented Apr 28, 2016 at 2:20
• @soosaisteven, no he doesn't, if he's allowed to take the node voltages as given. Commented Apr 28, 2016 at 2:21
• @The Photon Do you mean that i can simply make $$I_B=V_B/R_B,,I_C=V_C/R_C$$? Thank you for your response Commented Apr 28, 2016 at 2:26
• No, $V_C$ is the voltage between the collector node and ground, and the resistor is not connected between those two nodes. Commented Apr 28, 2016 at 2:27

## 1 Answer

This is a horrible question, because it's not related to how we'd normally calculate the operating point of this circuit. It's so annoying that rather than try to teach you anything, I'm just going to give you (part of) the answer. I'm doing this because the question isn't well-posed to teach you anything anyway.

You can get $I_C$ very simply as

$$I_C = \frac{V_{DC1}-V_C}{R_C}$$

That's just a simple statement of Ohm's law for RC, in terms of the voltages you were asked to find the current in terms of. So it answers the question as stated.

You can get the current for $I_B$ by a very similar statement of Ohm's law for $R_B$. I'll let you do that part yourself.

In the real world, of course, this has nothing to do with how you'd find $I_C$ in a real circuit, so just give your instructor the answer he wants but don't imagine you're learning much from it.