# Electret microphone signal amplification with NPN transistor.. (two stage?)

#### abdulwahab.hajar

Joined Jun 14, 2016
93
Hello everyone
I hope you're all well...

I'm trying to amplify an electret microphone signal using an NPN transistor and have a circuit which managed to give me a gain of 100 approximately...
However I need a gain of approximately 400..... I amplified my original input signal by a common emitter configuration... the circuit is attached below...
The varying voltage source represents my microphone input...
I was obviously able to amplify my input signal of 10mV by small signal amplification... I can't further amplify by another amplifier stage right? because my output from the first stage is around 1.2V which is no longer a small signal... so how can I further amplify this to around 4V?

Furthermore, please if you have any notes regarding the circuit do tell!!
Thank you

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#### DickCappels

Joined Aug 21, 2008
10,240
To get a second stage that you can use to amplify the output of your first stage (the preamp) by a factor of about 3.3, copy the circuit you already have and leave out C2 in the second stage, change R4 in the second stage to 3.3k, Now the gain is the ratio of R4/R5. Change R3 in the second stage to 47k to set quiescent the DC level of the output of the second stage to 4.5 volts.

#### AnalogKid

Joined Aug 1, 2013
11,160
OR, remove C2 and change the first stage resistors to have a gain of 20, and have an identical stage after it for an overall gain of 400.

ak

#### abdulwahab.hajar

Joined Jun 14, 2016
93
To get a second stage that you can use to amplify the output of your first stage (the preamp) by a factor of about 3.3, copy the circuit you already have and leave out C2 in the second stage, change R4 in the second stage to 3.3k, Now the gain is the ratio of R4/R5. Change R3 in the second stage to 47k to set quiescent the DC level of the output of the second stage to 4.5 volts.
Yes sir, that actually worked.... thank you....
thing is shouldn't linear amplification only apply to small signals? or is that something I misunderstood?
And if you don't mind me asking, why did we remove the capacitor....?

#### DickCappels

Joined Aug 21, 2008
10,240
You removed the capacitor to lower the gain of the stage. The gain of the stage is roughly the impedance n series with the collector divided by the impedance in series with the emitter since essentially the same current goes through both impedances.The capacitor in parallel with the emitter resistor lowered the impedance seen by the emitter.

Speaker amplifiers are a good example of an application of large linear signals. You can drive a linear amplifier so hard that is no longer linear, maybe that is what you were thinking of.

#### AnalogKid

Joined Aug 1, 2013
11,160
shouldn't linear amplification only apply to small signals? or is that something I misunderstood?
Misunderstood. As long as the amplifier output signal does not exceed the power supply voltages (minus some inefficiencies), a linear amplifier can operate at any power level. Many audio power amplifiers now are switching circuits, but not that long ago the best way to get 300 W of good clean audio was to buy a 300 W linear amplifier from Crown.

The Crown Pulse 2x1100 amplifier has switching power supplies but linear amplifiers. Each channel can pump 1500 W into a 2 ohm load, or the two channels can be "bridged" to drive a single 4 ohm load with 3000 W

https://www.scribd.com/document/80890624/Crown-Pulse-Sevice-Manuals
http://diagramasde.com/im2/87074_schematic-atari2600-1000-png-schematic-atari2600-1000-png.jpg

ak

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#### ian field

Joined Oct 27, 2012
6,536
Hello everyone
I hope you're all well...

I'm trying to amplify an electret microphone signal using an NPN transistor and have a circuit which managed to give me a gain of 100 approximately...
However I need a gain of approximately 400..... I amplified my original input signal by a common emitter configuration... the circuit is attached below...
The varying voltage source represents my microphone input...
I was obviously able to amplify my input signal of 10mV by small signal amplification... I can't further amplify by another amplifier stage right? because my output from the first stage is around 1.2V which is no longer a small signal... so how can I further amplify this to around 4V?

Furthermore, please if you have any notes regarding the circuit do tell!!
Thank you
You probably need more than 1 transistor for that. If you study the parameters; you can work the electret into a grounded base amplifier. That gives you the most voltage gain, but the output current is slightly less than just the electret. You can hang an emitter follower on the collector for a bit of current gain.

#### Bordodynov

Joined May 20, 2015
3,182

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#### abdulwahab.hajar

Joined Jun 14, 2016
93
Thank you everyone, you have all been of great help..
I appreciate it