Electronic – Why is the amplifier circuit amplifying more than I expect

amplifieroperational-amplifier

My Question

Why is my amplifier circuit amplifying more than I expect, and what can I do to fix it?

What I want to accomplish

I want to amplify an input that, at most, is 1.5[V] to, at most, be 2[V].

What I have tried

I have the below circuit set up. When I measure the voltage of OUT against GND, I get values that are 7 times higher than the value IN.

I used the following formula:

$$V_o = V_i * (1 + \frac{R_2}{R_1})$$

Plugging in \$2\$ for \$V_o\$ and \$1.5\$ for \$V_i\$ evaluates to:

$$R_1 = 3R_2$$

I tried using 300[Ω] and 100[Ω] for \$R_1\$ and \$R_2\$ respectively, which yielded a different, but also undesirable, gain. I recall that it was a bit lower.

What I got

Measuring the voltage at IN and OUT against GND using a multimeter gives me about 0.5[V] for IN and 3.5[V] for OUT.

schematic

simulate this circuit – Schematic created using CircuitLab

Best Answer

Change to the following to get a non-inverting amplifier with gain = \$1 + R_2/R_1\$

schematic

The difference is that \$R_2\$ is connected to the op amp's inverting input instead of ground.


Please see Scott Seidman's answer for an explanation of what the incorrect circuit was doing.