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Jmac222 11th December 2012 08:07 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 08:28 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 08:45 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 08:53 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 09:08 PM

The 10uF capacitor C2 doesnt short R1.

Then what about the 0.1uF capacitor?

Jmac222 11th December 2012 09:10 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 09:15 PM

What is the virtual earth in this case?

Jmac222 11th December 2012 09:19 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 11th December 2012 11:02 PM


Originally Posted by Jmac222 (
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 11th December 2012 11:17 PM

@ 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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