# Can someone help me on this simple circuit. please ?

Joined Mar 10, 2022
1
Hi, I'm a first-year student, and I need to explain a really simple circuit. I understand the circuit but I don't know why they put this 100k ohm resistor connected to the ground just after the coupling capacitor. can someone explain me, please?

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#### MrChips

Joined Oct 2, 2009
29,853
Welcome to AAC!

You have to appreciate that we have dozens of new threads and thousands of viewers daily on AAC forums.
Hence a thread title such as "Question about microphone amplifier" is much more useful than "Can someone help me on this please?"

Also, is this question related to school work?

#### crutschow

Joined Mar 14, 2008
33,363
If the amplifier is an ideal device with infinite input impedance, then that should help you to understand the reason for the resistor.

#### MrChips

Joined Oct 2, 2009
29,853
There are some very important clues just reading that circuit diagram.

1) Buffer gain = 1
Hence there is no amplification. Voltage out = Voltage in.
The output drives an LED via a 470Ω series resistor.
What is the purpose of this buffer?

2) There are no values stated for C1 and C2 while other important values are given.

3) There is no other information given about the amplifier except for Gain = 100.

4) The circuit shows two 5V supplies.

#### drjohsmith

Joined Dec 13, 2021
816
Hi, I'm a first-year student, and I need to explain a really simple circuit. I understand the circuit but I don't know why they put this 100k ohm resistor connected to the ground just after the coupling capacitor. can someone explain me, please?

This could be "assumed" as an op amplifier,

Remember the basic rules of the perfect op amp

a) input impedance of the amp is infinate

Now look at the classic circuit for a gain +100 amplifier,

Im guessing this is home work
so this is going to be a question,

If the amp input is infinite impedance,
it has no effect on the circuit,

so take it out of the circuit.

Now what will the voltage be on the C1 / 100K resistor be
now take the resistor out,

Draw us a nice picture,

#### DickCappels

Joined Aug 21, 2008
10,104
The resistor both sets the low frequency response and establishes the amplifiers average DC voltage as varying around ground.

#### MisterBill2

Joined Jan 23, 2018
16,609
The resistor is there because the unknown amplifier circuit requires a DC path for the input. But given that it is an unknown circuit, it may not really be required. In fact, given what we know about the box, which is that the gain is 100, that resistor is not needed, unless that capacitor, C1, has some slight leakage.

#### crutschow

Joined Mar 14, 2008
33,363
The resistor is there because the unknown amplifier circuit requires a DC path for the input. But given that it is an unknown circuit, it may not really be required.
Even if the amp has infinite input impedance you would still want the resistor to establish a DC level.
A capacitor terminal should never be left floating.