# Summing amplifier with one op amp

I want to design amplifier that do the following summation

$$V_o = 2V_1 +3V_2 +5V_3$$

the difficult arose when design with one op-amp and all voltages are referenced to the ground .

I tried with making non inverting amplifier with gain = 7 (any gain more than 5), then putting three resistor in the non inverting input (R1 for $V_1$ and R2 for $V_2$ and R3 for $V_3$) making superposition three times i have three equation.

But i can't solve them; it's very difficult for me. Does anyone have an idea?

• Since when is 7 the LCM of 2, 3, and 5? – Ignacio Vazquez-Abrams Nov 25 '16 at 17:52
• my assumption is that i have maximum gain is 5 so V3 will be applied to voltage divider (R2//R1 , R3) so its value will decrease so when it multiplied with the non inverting gain A , A must be greater than 5 – Mohamd ahmed Nov 25 '16 at 17:57
• What are the values of V1, V2, and V3? – EM Fields Nov 25 '16 at 18:18