Thevenin for BJT base resistors

I want to use Thevenin's theorem with the two resistors connected to the base.

How can I combine those two resistors with Thevenin? Do you have any other better idea to combine them? (circled with red in the picture)

(PS: I don't ask the solution of the question. I'm only asking individually the calculation of those two resistors.)

• Why don't you start by redrawing just the circuit that you want to convert to its Thevenin equivalent. Include $V_1$ and the -1 V source. Clearly mark the two nodes where you want to find the equivalence. Jun 5 at 13:41
• I actually drawed like that initially but I could not understand to combine both of them. I already know if both of the resistor nodes are connected to the ground. However in this specific example, one of the node is connected with -1V which confused my mind. Jun 5 at 13:42
• @ElliotAlderson , what could it be your suggestion please for that -1V node? Jun 5 at 13:43
• The -1V is measured with respect to ground, so you just need an ideal voltage source between that node and ground. Orient the source and set its value such that you get -1V at the bottom of R2. Jun 5 at 13:45
• Rth is R1||R2 thus if R1 = 30k and R2 is 60k --->Rth = 60k/3 = 20k
– G36
Jun 5 at 14:13

What you need to do at the beginning is to disconnect the input voltage divider from the circuit and find the open voltage ($$\V_{TH}\$$):

simulate this circuit – Schematic created using CircuitLab

Now try to find $$\V_{TH}\$$.

Can you do it?

• @36 (Vi-(-1v))*(60k / 30k + 60k) = ... Is that form correct please? Jun 5 at 14:18
• No this is not correct. OK, can you find the current value that is flowing in this circuit?
– G36
Jun 5 at 14:23
• Sure, i can apply voltage divider rule for that: (R1/R1+R2)*(Vi+1V) Jun 5 at 14:24
• It seems, I really memorized quite everything while i was learning thevenin in the last term. I'm really sorry for that Jun 5 at 14:26
• No, back to the basics. The current is the circuit is equal to $I = \frac{V_I - (-1V)}{R_1 +R_2}$ do you see it? Also, do you notice that Vth is a voltage measured between the middle point and GND? electronics.stackexchange.com/questions/392010/…
– G36
Jun 5 at 14:30