# Operational amp (Inverting)

I need help with question a and b. For a i know that gain =-R2/R1 and i know you just assume a resistor values so 5kohms and 1kohms for R2 And R1 respectively. What i dont understand is why you can just ignore the R3 and R4 resistors. Never really got the whole concept of op amps. And for question b im not sure how to find R3 and R4 by using the op amp. Thanks

• For (b) Assume the non-inverting '+' input takes no current so you can ignore the opamp and just treat it as a potential divider. An opamp with negative feedback is trying to keep both inputs at the same potential so if Va is 0.5V and the non-inverting input is 0.5 what voltage do you need at vb? What happens when you increase or reduce Va? – Warren Hill Oct 27 '16 at 6:54
• why can we just assume theres no current in vp? – I have no clue Oct 27 '16 at 7:07
• Because an ideal op-amp has no current in either of its inputs A real op-amp will have some but it will be very small. – Warren Hill Oct 27 '16 at 7:11

In an ideal opamp with negative feedback, the two inputs are forced to be the same voltage.

As written for example in The Art of Electronics: "The output attempts to do whatever is necessary to make the voltage difference between the inputs zero."

Given this rule, you can calculate the values for your dc biasing circuit.

edit: Question a is doubtful as with switch S1 open, you can not have any gain (because you input signal is not connected).

• but for a why can i just ignore r3 and r4 sorry? – I have no clue Oct 27 '16 at 7:13
• As I said in my answer, the answer is behind "virtual ground" concept. – Rohat Kılıç Oct 27 '16 at 7:15
• R3 and R4 make a DC (fixed) voltage at the +input of the opamp. They are not dependant on the output nor the input voltage. Thus they do not have influence on the gain. – Douwe66 Oct 27 '16 at 7:15
• oh ok thanks, so since there not the actual inverting amplifier, they dont matter and they are just extra components essentially? (for this q) – I have no clue Oct 27 '16 at 7:21
• Well, there is no feedback from the output to the + input of the opamp, which means the voltage at that node is fixed by the voltage divider. – Douwe66 Oct 27 '16 at 7:24

R3 and R4 are biasing resistors to establish a suitable DC operating point - they serve the same purpose as the voltage divider for transistorized gain stages. Here, such biasing is necessary because your opamp is powered with one single voltage source only.

But don`t forget that the DC voltage produced by R3 and R4 will appear at the opamps output - amplified by the factor (1+5) if SW1 is closed. If SW1 is open we have unity gain feedback only.