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How to amplify ac sqaure wave

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mbird

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Hi --

I have a CMOS chip and I generate a square wave with changing frequency. It is LVTTL (3.3Volt) and max current from the device is 24mA.

I want to drive a speaker in both directions with it so I added a capacitor such that RC>>Frequency of my square wave (I think this method is called coupling capacitor?). That worked and produces a nice AC square wave -3 to 3 volts.

Now I want to amplify this AC square wave so I can get up to 250mW for my speaker.

I can see how to amplify current for a DC square wave with a single transistor but how do you amplify an AC square wave? Can it be done with simple transistor amplifier? Do I need an Op Amp (but looking at those it seemed like they are not the right choice for driving a speaker?)

I did a lot of searching on the Internet for how to do this but nothing seemed right so I am posting here.

Thank you!
 
A 4V p-p sine-wave in an 8 ohm speaker is 250mW.
A 2V p-p square-wave in an 8 ohm speaker is 250mW.

An LM386 little power amplifier produces a 250mW sine-wave at clipping into an 8 ohm speaker when it has a 6.5V supply.
 
A 4V p-p sine-wave in an 8 ohm speaker is 250mW.
A 2V p-p square-wave in an 8 ohm speaker is 250mW.

Hi --

Thanks for your reply. I was thinking the following -- which may be wrong so please correct if it is...

I have 3.3V from my CMOS chip. The chip is limited to 24mA output. So the most power I can get out of the chip is 3.3 * 0.024 = 80mW. I want to get 250mW power to my speaker.
(I realize I may be incorrect in assumptions here so any correction would be appreciated!)

Thank you!
 
If you load the output of the chip to 24mA then its voltage will drop. It might drop to 2.5V. Then the minimum load resistance is 104 ohms without a coupling capacitor or 52 ohms with a coupling capacitor.

What is the impedance of the speaker? Use an amplifier IC that is made to drive a speaker.
 
What is the impedance of the speaker?

The speaker is 8 Ohms and it says its operation is nominally 0.2W.

Use an amplifier IC that is made to drive a speaker.

What is a good one to use?


I am really also interested not just in this particular speaker thing but in understanding how one would amplify an AC square wave in general. I have played with amplifying DC with a transistor so I thought I'd explore this next step in the hobby. I read some stuff about push-pull amplifier with two transistors and that seemed interesting.

Thanks for you time and help.
 
An amplifier IC like an LM386 has an output at clipping of 250mW when it amplifies a sine-wave, has an 8 ohm load and has a 6.5V supply.

Of course it is push-pull and has an output capacitor that keeps DC out of the speaker.
It can amplify speech, music or square-waves.
Because a square-wave is half the power in one direction then the other half of the power in the other direction its power is double that of a sine-wave.
 
That doesn't sound right. A square wave has √2 the power of a sine wave of the same peak-to-peak voltage.
 
That doesn't sound right. A square wave has √2 the power of a sine wave of the same peak-to-peak voltage.
Yeah, 2V p-p square wave across 8Ω is only 1/8W.

EDIT: A square wave delivers twice the power of a sine wave with the same p-p voltage.
 
Last edited:
Hang on AG was right.

A 2V p-p squarewave is 1V peak and the RMS voltage is 1V

P = 1²/8 = 0.125W


A 2V p-p sinewave is 1V peak which is just 1/√2 = 0.707V RMS.

P = 0.707²/8 = 0.0625W

0.0625/0.125 = 0.5
 
Of course I was right.
The amplifiers and speakers that are rated at "peak power" or "maximum power" have their RMS power number simply doubled because their output is severely distorted into square-waves.
 
We were talking about an amplifier that has the same p-p voltage with a sine-wave at clipping and a square-wave distorting like crazy. The square-wave has many harmonics that have the same amount of power as the fundamental frequency so the total power is double the power with a sine-wave at clipping.
 
We were talking about an amplifier that has the same p-p voltage with a sine-wave at clipping and a square-wave distorting like crazy. The square-wave has many harmonics that have the same amount of power as the fundamental frequency so the total power is double the power with a sine-wave at clipping.
I feel like I'm reading the old Abbott & Costello routine, "Who's on First?".:confused:
My original post was just to correct the statement that a 2V p-p square wave across 8 ohms would generate 250mW, when it will actually only generate 125mW. I didn't want to see our OP led down the proverbial garden path.
 
Yeah, I made a mistake about the power in my first post.
When the voltage is halved then the current is also halved so the power is 1/4.
 
Can you make the power spec. by paralleling CMOS chips and using a bridge circuit to drive the speaker?
 
You'd need a lot of CMOS ICs.
 
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