Purpose of attenuation before op-amp input

Question

What is the purpose of putting attenuation circuit before op-amp input? I cant find the actual reason for this topology. Is there any reason behind it? Why we need to attenuate the incoming signal but at the end we amplify it.

Signal input -> attenuation circuit (reduce incoming signal level) -> op amp input (amplify signal level)

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sorry for late response.

I attached my circuit diagram. Basically it just a simple voltage source (from MIC) + bandpass filter with positive rectifier (range within 5KHz bandwidth) and this signal will input to ADC for detection.

Based on circuit below, I confirmed the attenuation and gain

Attenuation level : -15.56dB Gain level : 29dB

If based on above level, can we just set gain of the amplifier to be 13.44dB (gain level - attenuation level)?

I'm thinking to reduce the component used by deleting the attenuation circuit before the amplifier input.

.

Answer 1

In addition to Bimpelrekkie's answer, here is another very common scenario:

^{simulate this circuit – Schematic created using CircuitLab}

Here we are monitoring a system power rail (typical in avionics). As most ADCs have reference voltages of 5V (or less) we need to change the effective range of the supply rail being monitored.

We don't want to waste power so I am using high value resistors, but the source resistance to an ADC typically needs to be quite low (a few k ohms at most usually), so the sense point in the voltage divider is buffered to properly drive such an ADC.

I haven't taken the trouble here to convert the signal to a 0V to Vref span and in fact it is rarely necessary in such a situation.

Answer 2

I cant find the actual reason for this topology

I can, here's an example:

This is an LM386 audio amplifier with an attenuator at its input. The attenuator is the volume control.

Since we have no idea what the input voltage from the source is this allows us to set the volume control such that the output level is where we want it to be. Within certain limits of course. Also the input voltage cannot be too small or too large.

If the volume control was not present at the input we would not be able to prevent the amplifier from saturating (clipping) when the input level is too high.

Also, since this is a power amplifier, attenuating at the output is impractical as the power level is much higher (compared to the input), attenuating the signal at the output would mean severe power loss. You first make a lot of power and then waste most of it for a low volume. So efficiency is also a good reason to do the signal attenuation before amplification.