# amplifier BW design purpose

Discussion in 'General Electronics Chat' started by donut, Feb 4, 2013.

1. ### donut Thread Starter Member

May 23, 2012
51
0
I need help understanding how C125 and R178 are used in the attached circuit.

Does C25 and R178 just specifies the BW of the amp?

Are the components selected for C125 and R178 considered to be a good design?

With a GBW of 90MHz, RF of 200K, Ctot of 4pF, -3dB BW is 4Hz?

(√90MHz/2pi*RF*Ctot) page 11 data sheet http://www.ti.com/lit/ds/symlink/opa380.pdf

• ###### amp circuit.jpg
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Last edited: Feb 4, 2013
2. ### Audioguru New Member

Dec 20, 2007
9,411
896
Why is your schematic a hard-to-see negative image? It should have black lines and black text on a white background.

There is nothing in the circuit that cuts frequencies below 4Hz. The gain is the same at DC.

3. ### kubeek AAC Fanatic!

Sep 20, 2005
4,687
805
You sure you are using the equation correctly? I got 1.23MHz. Put this in excel or something similar, it will give you your answer in MHz. =SQRT(90000000/(6.28*200000*0.000000000047))*0,000001

Last edited: Feb 4, 2013
4. ### donut Thread Starter Member

May 23, 2012
51
0
You are right. I did not apply the order of operations correctly.

The data sheet of the AMP has multiple charts showing freq vs. gain relationships.My circuit has C125= 47pF. with Cf = 47pF and -3dB BW of ≈ 1MHz what is my Gain? I dont see a chart that matches Cf = 47pF.