# Simple amplifier circuit, norton, thevenin questions

I have the above circuits and I have some questions related to them:

1. I drew above the Norton and Thevenin equivalents where $R_{no}$ is the resistance of the whole amplifier circuitry, excluding the load. Are they correct?

2. Is $V_{th} = V_1$? If not what is it equal to and why?

3. I simulated the circuit and have two sets of values for $V_{out}$, $I_{load}$ and $R_{load}$. How can I go about finding $R_{no}$? I tried various approaches but none of them seem to work.

Here is one approach I tried:

$$V_{load} = I_{load} R_{load}$$ $$V_{th} = \frac{V_{th}R_{no}}{R_{no}+R_{load}} + \frac{V_{th}R_{load}}{R_{no}+R_{load}}$$ $$V_{th} = \frac{V_{th}R_{no}}{R_{no}+R_{load}} + V_{load}$$ $$V_{th}R_{load} = V_{load}(R_{no}+R_{load})$$ $$\frac{V_{load_1}(R_{no}+R_{load_1})}{R_{load_1}}=\frac{V_{load_2}(R_{no}+R_{load_2})}{R_{load_2}}$$

However for some reason I do not get the expected value for $R_{no}$ from this answer. I cannot see what is wrong with the derivation though?!

• Seems like something might be going wrong with how you are treating the opamp. Check this answer out electronics.stackexchange.com/questions/73292/… Feb 25, 2016 at 20:25
• @bss36504 So you mean $V_{out} =V_{th}\neq V_1$? I can see how that might make sense but it does not help me find $R_{no]}$ Feb 25, 2016 at 21:29

## 1 Answer

For your first question, yes of course that's how the thevenin and Norton equivalent circuits will look like , the main concern is how to find the value of Vth or Ino and Rno.

I don't think Vth=V1 and we must consider the gain of the opamp to obtain the correct Vth equations. The link provided in the comments by bss36504 makes a lot of sense as it takes the opamp out and replaces it by an equivalent voltage source. Once this is done it makes things lot more easier. For that you should first derive the gain Vout/Vin with just Rs in the rigt hand side. With Nodal analysis I think this should be possible.

In your formula that you have tried to derive I think you have missed Iload in the 3rd step.