I Checking for negative feedback in a circuit

AI Thread Summary
The discussion centers on understanding feedback in an inverting summing amplifier circuit. The instructor explains that an increase in output voltage (v_out) leads to an increase in the inverting input voltage (v-), which then causes the next output to decrease, indicating negative feedback. There is confusion regarding the absence of a voltage divider in the inverting summing amplifier, as the two voltage sources are in parallel, complicating the analysis. Participants explore the relationship between the input voltages and the output, questioning how v- can increase with v_out without a clear voltage divider. The conversation emphasizes the need for clarity in circuit diagrams and the derivation of circuit equations for better understanding.
Milotic
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Screen Shot 2021-04-11 at 6.25.11 PM.png

This is the circuit in question.
During lecture, when checking whether this was negative feedback, my instructor said that if the v_out increases, then v- increases as well, which would lead to the next v_out decreasing because v_out = A(v+ - v-). I get how if v- increases the next v_out would decrease and that'd give a negative feedback, but the problem is, I don't understand how v_out increasing (initially) also leads to the v- increasing.

Actually, there was a similar question we went over just before:
Screen Shot 2021-04-11 at 6.32.35 PM.png

Here, my instructor said the same thing; if the v_out increased, then v- also increases, leading to smaller v_out the next time. Here, I thought I understood why: there's a voltage divider from V_s to v_out, where (v- - v_out) = R2 * V_s /(R1+R2). Since none of the resistances nor the voltage source V_s change, if v_out increases, v- would have to increase as well, or that was my understanding. (Is my understanding right?)

However, going back to the circuit for Inverting Summing Amplifier, we don't exactly have a voltage divider, do we? Because the two voltage sources V1 and V2 are in parallel, I don't see how I could combine them (I think you can only combine voltage sources in series). So assuming I'm right in not seeing any voltage dividers in Inverting Summing Amplifier, what makes v- go up as v_out goes up?
 
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Milotic said:
However, going back to the circuit for Inverting Summing Amplifier, we don't exactly have a voltage divider, do we? Because the two voltage sources V1 and V2 are in parallel, I don't see how I could combine them (I think you can only combine voltage sources in series). So assuming I'm right in not seeing any voltage dividers in Inverting Summing Amplifier, what makes v- go up as v_out goes up?

Since it is an inverting summing amplifier, its function is of course to add all the voltages connected to its inverting input terminal through the corresponding resistor, and then perform inverting amplification. Is it not obvious from the wiring diagram?

Next, I think you can try to derive the circuit equation.
 
@alan123hk well, I must say sometimes I have trouble understanding wiring diagrams right, but after reading your comment I drew things out again this is what I interpreted the inverting summing amplifier's (partial) circuit to be—is this correct? Since V1 and V2 seem to be kind of in series maybe I could add those two voltage sources(?) to have some form of voltage divider, I guess, but I also have the v_out in parallel with V2, which makes me doubt that.
invert.jpeg


what I thought initially was this below:
inverter2.jpeg


also, what do you mean by circuit equation?
 
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Milotic said:
I must say sometimes I have trouble understanding wiring diagrams right, but after reading your comment I drew things out again this is what I interpreted the inverting summing amplifier's (partial) circuit to be—is this correct? Since V1 and V2 seem to be kind of in series maybe I could add those two voltage sources(?) to have some form of voltage divider, I guess, but I also have the v_out in parallel with V2, which makes me doubt that.

Ignore the op amp first. Please see the picture below.

Add1.jpg


If V1 is now increased, and V2 and V3 remain unchanged, of course V4 will increase. I believe you should agree with this. Now we increase V2 and V3 sequentially, and the other two voltages remain unchanged, which will cause V4 to increase sequentially as V2 and V3 increase. So, isn't this a simple resistor circuit that adds these three voltages together?
 
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