If an averaging amplifier has 4 inputs, the ratio Rf/Ri must be:

To determine the ratio Rf/Ri for an averaging amplifier with 4 inputs, we need to consider the circuit configuration.

1. First, let's understand the concept of an averaging amplifier. It is a type of operational amplifier (op-amp) circuit that calculates the average of multiple input voltages.

2. In an averaging amplifier, the inputs are connected to the inverting terminal of the op-amp through resistors. The resistors are labeled as Ri1, Ri2, Ri3, and Ri4 for the 4 inputs.

3. The output of the op-amp is connected to a feedback resistor labeled as Rf.

4. The ratio Rf/Ri determines the gain of the amplifier. In this case, since we want to calculate the average of the inputs, the gain should be equal for all inputs.

5. To achieve equal gain for all inputs, the value of Rf should be equal to the sum of all the Ri resistors. In other words, Rf = Ri1 + Ri2 + Ri3 + Ri4.

6. Therefore, the ratio Rf/Ri for the averaging amplifier with 4 inputs is 1:4.