The following circuit is a non-inverting amplifier with a normal op-amp. I know how the "classic" one works: Vout/Vin = (R1+R2)/R2. However, there is a third resistor R3. As far as I know, it has something to do with an offset voltage. May someone explain what it exactly does and how to calculate it (+ the amplification here)? The resistor values I have chosen are random.
Additionally, I have a second question. I recently saw an capacitor within a feedback. It wasn't an op-amp but a differential transistor amplifier. Basically, there was the same feedback like on the circuit above with R1 and R2 + a capacitor between R2 and GND (in series). What's the purpose for that?
Best Answer
As you can see, there is a gain component and an offset for the circuit you posted.
$$\frac{(10-V_{in})}{R_3} + \frac{(V_{out} - V_{in})}{R_1} = \frac{V_{in}}{R_2}$$
$$(10-V_{in}) R_1 R_2 + (V_{out} - V_{in}) R_3 R_2 = V_{in} R_1 R_3$$
$$10 R_1 R_2 - V_{in} R_1 R_2 + V_{out} R_3 R_2 - V_{in} R_3 R_2 = V_{in} R_1 R_3$$
$$10 R_1 R_2 + V_{out} R_3 R_2 = V_{in}(R_1 R_3 + R_1 R_2 + R_3 R_2)$$
$$V_{out} R_3 R_2 = V_{in}(R_1 R_3 + R_1 R_2 + R_3 R_2) - 10 R_1 R_2$$
$$V_{out} = V_{in}\frac{(R_1 R_3 + R_1 R_2 + R_3 R_2)}{R_3 R_2} - 10 \frac{R_1}{R_3}$$