# Why is Rc parallel to RL?

While determing the AC load line, the author takes Rc to be parallel to Rl. How is it possible since the two resistors have different nodes?

• Please inline images, or reproduce the circuit schematic using the on-site schematic tool. Commented Aug 24, 2019 at 11:42
• Where is the tool? Commented Aug 24, 2019 at 11:43
• When editing your question, there should be an icon at the top with a little circuit diagram and a pencil. Click that. Commented Aug 24, 2019 at 11:44
• I am using a phone so can't see Commented Aug 24, 2019 at 11:45
• Try read here electronics.stackexchange.com/questions/298560/…
– G36
Commented Aug 24, 2019 at 15:10

## 2 Answers

During the small signal analysis, you need to replace the DC power source by ground because a DC component has no AC component.

The next thing you do is replace the capacitor by short circuit. Doing so, your circuit looks like :

simulate this circuit – Schematic created using CircuitLab

Now you can see both of the resistor's nodes are connected to BJT's collector and the ground. So, they are in parallel.

• How can we determine the current through El in this case? Is it Ie or Ic? Commented Aug 25, 2019 at 6:34
• When two resistors share a common node, they are in parallel if seen from this common node and they are in series if seen from one of the "outer" nodes.
– LvW
Commented Aug 25, 2019 at 8:51

When performing small-signal analysis, DC sources have no AC component so their variation in time is zero i.e. they represent a short circuit, towards ground in this case.

Therefore, $$\R_C\$$ and $$\R_L\$$ share the same potentials: the BJT collector and ground.

• I don't understand. Which DC source are you talking about? Commented Aug 24, 2019 at 12:12
• @user29463: $V_{cc}$ in your case.
– edmz
Commented Aug 24, 2019 at 12:13