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Jmac222 11th December 2012 09:07 PM

LM1875 Single Supply Bandwidth
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I am putting together an amplifier as a final project for a class that is required by my associates degree in Electrical Engineering Technology. I have included an image of the circuit I am using with this post. My measured upper and lower frequency cutoffs are 20Hz and 76KHz. It would be amazing for me to theoretically determine the frequency response of this circuit. I am pretty sure the 1 ohm resistor and the .22uF capacitor are responsible for the upper frequency cutoff. I have absolutely no idea how to organize the portion of the circuit that provides the input bias. I have a feeling that it is responsible for the lower frequency cutoff. Can someone please steer me in the direction of an online resource or a technique that I can use to match my experimental results to my theoretical results.

Thank You,


Fast Eddie D 11th December 2012 09:28 PM

You've got it. the 1 uF cap and 22K resistor form a high pass filter, and the 1 ohm resistor and .22 uF cap form a high frequency shunt.

By the way that's a very easy circuit for a EE project.

Jmac222 11th December 2012 09:45 PM

Thanks for your reply. I am getting a technical degree. The university is putting together a bachelors program but its taking forever. I have very little experience so the circuit is fine for me.

sreten 11th December 2012 09:53 PM


For the high pass its 1uF in series with all the loop resistances to virtual or real earths.
That looks like (22K + 22K||22K)||1M = 32Kohm with 1uF for the high pass function
at the input, but the feedback loop has another, 10uF and 10K||200K = 9.1kohm.

Still I can't get to an estimate of -3dB at 20Hz. Try a sim like the great free TinaTi.

rgds, sreten.

Jmac222 11th December 2012 10:08 PM

The 10uF capacitor C2 doesnt short R1.

Then what about the 0.1uF capacitor?

Jmac222 11th December 2012 10:10 PM

When i put the 1uF capacitor in series with the .1 capacitor and use the resistance you gave me I get 50 Hz.

Jmac222 11th December 2012 10:15 PM

What is the virtual earth in this case?

Jmac222 11th December 2012 10:19 PM

When you said virtual earth I thought maybe you were referring to including the 10Kohm resistor from the feedback portion in parallel. With the bias resistors. Including that matches perfectly.

Fast Eddie D 12th December 2012 12:02 AM


Originally Posted by Jmac222 (Post 3280935)
The 10uF capacitor C2 doesnt short R1.

Then what about the 0.1uF capacitor?

The 10 uF cap shorts R1 and R2 at audio frequencies, theoretically ;) .

The junction of R1, R2, and R3 is at ground potential at audio frequencies, theoretically.

The 0.1 uF cap is a power supply pin bypass.

The input impedance at audio frequencies is 22K, theoretically.

Jmac222 12th December 2012 12:17 AM

@ Fast Eddie

I agree with your assessment of the input impedance at audio frequencies. The lower frequency cutoff I measured was definitely between 15Hz and 22Hz. If my measurements are not off I'm in error by about 10Hz. When I use 22k compared to 1uf for calculation of frequency attenuation. it's a 1 credit course atleast.

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