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
.
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\$
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.