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Basic Electronics » Using LM741 amp in the Piezoelectric Sound Meter Project

July 01, 2010
by CyberGod
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I am learning about amplifiers and was curious if it's possible to substitute the amplification 2N3904 NPN BJT for LM741 in the Piezoelectric Sound Meter Project, and if yes how it can be done.

July 01, 2010
by mongo
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2N3904 is just an NPN general purpose transistor. I am not sure I quite follow the idea here. What are you trying to swap?

July 02, 2010
by CyberGod
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In the Piezoelectric Sound Meter Project we use the 2N3904 transistor to amplify the signal from the buzzer so that the MCU can read it. My question is can this be done with the LM741 Operational Amplifier instead of the transistor, and what does it take to be done.

July 02, 2010
by mongo
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OK, I see now.

There is no reason it cannot be done. It is a bit more complicated to do it that way but it can be done.

741's have been around for a long time and tend to work best on dual ended power supplies or something between 12 and 18 volts.

Another chip I have used in the past in place of 741's is the LF355. Pin for pin, I think is the same but the input impedance it really high, making it ideal for piezoelectric inputs. (FET inputs)

There is also the OP07 chip, which works nicely on a single-ended power supply at voltages as low as 3V.

Now that I have it all confused up for ya... Have fun with it.

July 02, 2010
by CyberGod
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What is dual-ended power supply ? I apologize for the type of beginners questions and for my grammar, English is not my native language.

July 02, 2010
by bretm
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It means that the op-amp needs +V and -V power-supply voltages, that is, voltages that are both positive and negative with respect to the ground level 0V. The intended use is for amplifying AC signals centered around 0V.

This in contrast to most battery-powered circuits like the TTL-oriented Atmega-based Nerdkit where there is a GND and a positive supply at 5V above ground.

If the rest of your circuit is designed around single-ended supply, adding a dual-ended supply component adds complexity to the circuit since you need to either add a negative supply or else add a bias voltage to the signal being applied to the dual-ended part of the circuit. For example, instead of running a dual-ended op-amp with +5V and -5V supply and amplifying signals that are near 0V, you could instead run it with +9V and 0V and amplify signals that are near 4.5V.

But a 5V supply range is a bit narrow for a LM741 to give great results. If I recall correctly, it's "negative" output won't go within about 2V of the negative supply voltage.

July 02, 2010
by mongo
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OOPS, Sorry about the terminology. I forget some times that some folks are newer than others. bretm is correct. A dual ended power supply has a + supply and a - supply in reference to a common ground. Picture two batteries in series. Between the two would be the common ground and the typical reference for most circuits.

There are ways of getting around the dual end thing but it does add more to the circuit. That is one reason I like the OP07, as it can be referenced across a voltage divider and the output can be amplified to cover almost the entire swing of the power supply. All that needs to be done with the output is to de-couple it for an AC circuit. (through a capacitor). The gain is also easier to control to reduce noise and distortion.

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