problem in amplying the LM386 Amplifier wtih 200 db gain

[abhishekdixit]

hi,
i want to make a small amplifier which can amplify 200db gain of my 0.1V input, so i made the circuit according to datasheet. but practically when i measure my amplified output so i found that my amplifier is able to amplify my input only 10 to 11 times. so i just want to know that what should i do to increase my gain from 10-11dB to 200dB. my circuit is exactly as the datasheet shown.



with regards,
Abhishek Dixit

 

[keith1200rs]

I think you have your calculations wrong. 0.1V with 200dB of gain will give you 1GV!

Keith

 

[abhishekdixit]

hello keith,
what do you meanby 1GV???? Please tell me..

 

[keith1200rs]

200dB is a gain of 10,000,000,000 times

Keith

 

[abhishekdixit]

hello keith,
ok but i check my output voltage on oscilloscope, and i found that if i applied 0.1V analogue input at the input, then i am getting 1V 0r 1.5 V analogue without clipping..now i have to amplify it more than this, due to this i designed this 200dB type amplifier, but still its not working.

abhishek

 

[pebe]

keith1200rs is quite right.
An LM386 cannot possibly give a gain of 200dB. The open loop gain of an op-amp is only about 100dB, so where did you get the figure of 200dB from?

 

[keith1200rs]

There are examples on page 6 of the datasheet of circuits with various gains. It seems like you have a gain of 10 (= 20dB) and need a gain of 50 or 100. There are examples there to help.

20dB is a voltage gain of 10
40dB is a voltage gain of 100
60dB is a voltage gain of 1000
....
100dB is a voltage gain of 100000
....
200dB is a voltage gain of 10000000000

Keith

 

[pebe]

abhishekdixit,
The circuit given in the LM386 data sheet gives a voltage gain of 200, - not 200dB.

A voltage gain of 200 is 46dB.

 

[abhishekdixit]

hii pebe,
i saw the datasheet & you are right, the gain is 46dB, but can you tell me please what is meaning of 200 here??? if the my gain is 46 dB then what 200 shows ???

with regards,
Abhishek Dixit

 

[xpress_embedo]

First of all you must know the difference between db and normal voltahe and power gaings.

Voltahe gain of 200. Means 20*log(200) which is equal to 46db.

While in case if power gains it is 10*log(power gain ratio)

Gain is the ratio of output to input

200 times gain means vout/vin = 200
Means if you are applying 0.01 volt input with gain of 200 times you will get output of 0.01*200

And maximun gain if lm386 is 200 times i.e 46db

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Have a look at this link also
http://www.learningaboutelectronics.com/Articles/How-to-connect-a-LM386-audio-amplifier-chip

 

[pebe]

hii pebe,
i saw the datasheet & you are right, the gain is 46dB, but can you tell me please what is meaning of 200 here??? if the my gain is 46 dB then what 200 shows ???

with regards,
Abhishek Dixit

It refers to the voltage gain of the IC. If you input a signal on 1mVRMS then you will get from the output a signal of 200mVRMS.

The ratio of input and output powers is defined as 10*log(Vout/Vin). The definition of voltage ratios on input and outputs, given as 20*log(Vout/Vin) is only valid when the input and output impedances are the same. With many amplifiers they are not. Usually the input impedance is fairly high while the output impedance is very much lower.

So the manufacturer is being strictly accurate in using the notation he does for stating the gain of the device. Some manufacturers (who should know better) give op-amp gains in dB, but unless refering to an RF or audio amplifier where both impedances are equal to, maybe, 50Ω, 75Ω or 600Ω then it is an incorrect use of the term.