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Microphone amplifier Circuit

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chenjk09

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Hi guys,

I have some question
from the link shown below.
https://people.ece.cornell.edu/land/courses/ece4760/FinalProjects/s2005/ik48_ua22/mysite/sw_hw.htm

Q1) first order RC high pass filter and it is built by a 1uF capacitor (C1) and a 1kΩ resister R2 in series.
How can the High pass filter connected in series?
Q2) why need 3 opamp can't i just use one opamp increase the value for the resister for the gain?

I want to built and microphone amplififier circuit connect to MCU. I doing a sound recorder project.

Thank
 
1 filters can be connected in series or parallel. not sure what is strange about that.
2 you could use one opamp. not sure why they chose three, perhaps they had limited range of values to choose from. more likely they just cascaded three simple circuits to get what they needed without bothering to redesign.
 
Hi,

thank for the reply.

1. how can i test using multisim?
2. if i want to redesign there not cal for gain in the data sheet. anyone can advice me how to cal the gain for 1 opamp?

3. anyone got a simple microphone amplifier Circuit for connect to MCU? (easy to undestand)

Thank
 
Last edited:
I don't use Multisim, but the idea is the same: capture schematic, assign values and run simulation. If you have more specific question you should be - specific.

As for amp, you need to describe what is your goal.
If you plan on sampling analog input like in your lik, there are two things you need:
- amplification of mic signal to match sampling circuit or ADC.
- filter to eliminate aliasing.

Mic output is usually low (mV) and ADC input range may be 0-5V for example (depends on your ADC and Vref). You would want to get the peak to peak of amplified signal to be large but within the limits of the ADC input range. Above circuit has three identical stages each with same gain. One can use single stage to get needed gain. If needed, gain can be adjustable (by replacing feedback resistor by trimmer pot for example).

OpAmp circuits are discussed everywhere, even Wiki has some info on it (and small example on deriving gain):
https://en.wikipedia.org/wiki/Opamp

note that opamps normally use dual supply. using them with single supply is done by creating virtual ground. circuit in your link does it too (10k/10k dividers).
gain of single stage is

Ainverting= - Rfbk/Rin = - R5/R2 = -R7/R6 = -R9/R8 = -10

three stages have combined gain of (-10)^3=-1000.

if it was non-inverting configuration, gain would be
Anoninv= 1 + (Rfbk/Rin)

As mentioned filtering is used to fix aliasing problem. Higher order filters have superior performance (sharper cutoff). One popular configuration is Sallen-Key.
Wiki shows an example (second order):
https://en.wikipedia.org/wiki/Sallen–Key_topology

Also you can get two opamps in DIP8 package.
 
1) The mic has a very low value 1k resistor (R1) to power it. It should be 4.7k if the supply is 5V. R1 should be fed from a series 470 ohm resistor to +5V with a 47uF filter capacitor to ground.

2) The first opamp is inverting and has a very low 1k input resistance due to R2. Then R2 shorts a lot of the signal from the mic. The first opamp should be non-inverting with a 22k to 47k input resistance.

3) The circuit needs only ONE "virtual ground" voltage divider that feeds all the opamps. It should have a 47uF filter capacitor to ground.
 
View attachment 65182
View attachment 65183
Hi,

I have the gain of -1000
base on gain = Vout/Vin
Vin = 20Vpp
Vout = Vin * gain
Vout = 20Vpp * 1000 = 20000
I done simulation of the circuit using multisim shown in attach.
Vin = 4*5 = 10Vpp
Vout = 0.5 * 5 = 2.5
Gain = Vout/Vin = 2.5/10 = 0.25
0.25 is not equal to 1000 (gain)


I don't use Multisim, but the idea is the same: capture schematic, assign values and run simulation. If you have more specific question you should be - specific.

As for amp, you need to describe what is your goal.
If you plan on sampling analog input like in your lik, there are two things you need:
- amplification of mic signal to match sampling circuit or ADC.
- filter to eliminate aliasing.

Mic output is usually low (mV) and ADC input range may be 0-5V for example (depends on your ADC and Vref). You would want to get the peak to peak of amplified signal to be large but within the limits of the ADC input range. Above circuit has three identical stages each with same gain. One can use single stage to get needed gain. If needed, gain can be adjustable (by replacing feedback resistor by trimmer pot for example).

OpAmp circuits are discussed everywhere, even Wiki has some info on it (and small example on deriving gain):
https://en.wikipedia.org/wiki/Opamp

note that opamps normally use dual supply. using them with single supply is done by creating virtual ground. circuit in your link does it too (10k/10k dividers).
gain of single stage is

Ainverting= - Rfbk/Rin = - R5/R2 = -R7/R6 = -R9/R8 = -10

three stages have combined gain of (-10)^3=-1000.

if it was non-inverting configuration, gain would be
Anoninv= 1 + (Rfbk/Rin)

As mentioned filtering is used to fix aliasing problem. Higher order filters have superior performance (sharper cutoff). One popular configuration is Sallen-Key.
Wiki shows an example (second order):
https://en.wikipedia.org/wiki/Sallen–Key_topology

Also you can get two opamps in DIP8 package.
 
The first opamp has a gain of -10 only when fed from an extremely low source impedance which a microphone is not.
An electret mic might have an impedance of 3k ohms so the 1k input resistance of the first opamp will attenuate the mic signal. A non-inverting opamp with a much higher input resistance should be used.

The opamps are not powered.

The LMC7111 opamp is extremely slow and barely amplifies 5kHz.

The output lowpass filter cuts the 5kHz signal almost in half.
 
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