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Design 2 stage amplifier

Jay2OR

Dec 7, 2014
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Design a dual stage amplifier circuit using a TL071, to form an active low pass filter with an upper cut-off frequency of between 18 and 25 kHz with and gain matched to the output for your MP3 player, and an OPA551 current amplifier with unity gain.

i don't know the gain of my mp3 player they gave me an mp3 file with a constant tone and volume

so i will just make one up and say 20
 

LvW

Apr 12, 2014
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Apr 12, 2014
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Any question?
Did you consult the data sheet for the "OPA 551 current amplifier"?
 

Jay2OR

Dec 7, 2014
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Dec 7, 2014
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Hi,
im speakers for my phone i need some help

its a dual stage amplifier circuit using a TL071, to form an active low pass filter with an upper cut-off frequency of between 18 and 25 kHz with and gain matched to the output for my phone, and an OPA551 current amplifier with unity gain.

I checked the datasheet for the OPA551 and found Max current out is 200mA
Speaker impedance is 8 ohms
i loaded a continuous tone on my phone and measured,
Max volume = pk-pk 2.68V
Medium volume = pk-pk 100mV
Low volume = pk-pk 10.30mV

how will i work out the gain? i tried this
200mA x 8 = 1.6V

max volume gain = 20xlog(1.6/2.68) = negative 4.4dB
im not sure if this is right

and then on medium and low i did the same thing and got 24dB and 43dB
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Nov 28, 2011
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Hi there and welcome to Electronics Point :)

You've made a good start by measuring the signal levels from the phone.

The assignment hasn't told you the basis for working out the gain, but I would say one reasonable way would be to ensure that the OPA551 won't be damaged (by having too much current drawn from it) even when your phone is set for maximum volume with a continuous tone at 100% amplitude.

So you need to work backwards from the output. Assume the speaker is roughly resistive and has the specified resistance (unless you have been taught more detail about real world speakers). You need to make sure the speaker won't try to draw more than the rated current from the OPA551, right? So what voltage does that current correspond to? Make sure you use the right units (volts RMS, volts peak, or volts peak to peak).

Then, what gain will the OPA551 be operating at? And what is the maximum input voltage you want?

You know the maximum voltage available from the phone, so now you can calculate the gain of the filter stage. Do you know what configuration you will use for the filter? Does the filter stage have a limit to the minimum passband gain? If it does, is there a simple way to adjust the input signal to suit it?
 
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