Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 10 marks

Year: Dec 2013

Figure shows the inverting configuration with three inputs $V_a, V_b,\ \ and \ \ V_c$. Depending on the relationship between $R_a, R_b, \ \ and \ \ R_c$ the circuit can be used as a summing amplifier/ scaling amplifier or as an averaging amplifier.

Let $V_a$ be the output voltage. Using KCL at node $V_2$ we have

$$I_a+I_b+I_c=I_B+I_F$$

Since the op-amp is ideal, we have

$$\dfrac{V_a-V_3}{R_a}+\dfrac{V_b-V_3}{R_b}+\dfrac{V_c-V_3}{R_c}=\dfrac{V_3-V_0}{R_F} \\ I_B=0 \ \ and \ \ V_1=V_2 \cong0V \\ \therefore V_0=-\bigg(\dfrac{R_F}{R_a}V_a+\dfrac{R_F}{R_b}V_b+\dfrac{R_F}{R_c}V_c \\ or \ \ \ \ \dfrac{V_a}{R_a}+\dfrac{V_b}{R_b}+\dfrac{V_c}{R_c}=-\dfrac{V_0}{R_F}$$

Now if $R_a=R_b=R_c=R_F=R,$ we have from equation

$$V_0=-(V_a+V_b+V_c)$$

Inverting configurations with three inputs

Hence, the amplifier shown acts as the summing amplifier/scaling amplifier or averaging amplifier.

The output characteristics of an inverting amplifier, as shown in figure above is that the output voltage is of opposite polarity to the input voltage. Hence, it is called the inverting amplifier.