OpAmp Amplifier RC circuit

[moe_3_moe]

Hello, I am working on the design of an amplifier. Here are the requirements.

Design an amplifier with a gain of 10 that is to operate from a single power supply and deliver 2 volts peak to a 1K load at 1 KHz. The input resistance of the amplifier must be greater than 100K and the frequency response is to be from 100 Hz to 5 KHz +0/-3dB, the low and high frequency cutoffs.

In this schematic, i do not understand the relation between , C1 - C2 and C6? and how do they form 3 high passes filters?

Thank you.

 

[MikeMl]

Some thing wrong with R5. C1 and C2 both contribute to the rolloff at low frequencies. C6 is the power supply bypass, and just keeps the 741 from oscillating. C3 contributes to the high frequency rolloff. I don't think the lowly 741 will drive a 1K load; look up the data sheet to see how much current it can source and sink...

 

[audioguru]

Your circuit has many errors:

1) You have pin 3 of the opamp at the positive supply voltage so its output will be saturated high and it will not work. It is because you connected R2 to the battery instead of to pin 3 of the opamp to make it +7.5V.

2) The voltage gain of your circuit at very high frequencies is (90k/10) + 1= 9001 because the value of R5 is much too low.

3) The input resistance of your amplifier will be slightly less that 100k ohms (not more than 100k ohms) if the position of R2 is corrected.
Use 220k for R1 and R2.

4) C6 is not a highpass filter, it is a power supply filter but its value is too low. Use 0.1uF to 10uf.
5) C2 with R1 and R2 (if the position of R2 is corrected) form an input highpass filter. Yours is at -3db at 1600Hz.
6) C1 and R6 form an output highpass filter. Yours is at -3dB at 160kHz but a 741 opamp cannot go that high.
7) C4 and R5 form a feedback highpass filter. Yours is at -3dB at 16MHz but a 741 opamp cannot go that high.
8) C3 and R4 form a feedback lowpass filter. Yours is at -3dB at 1778Hz.

EDIT: corrected.

 

[moe_3_moe]

Hey MIke, Thank you, regarding the Rload, we don't have to worry about it in our project. Audioguru, Thank you for your help. here is the updated version of the circuit, i fixed the R input and worked on getting the right values (i guess) for the capacitors. can you explain how C1 and R6 are a low pass filter as you mentioned before? btw, for C4 and C2 I used 10/WR instead of 1/WR, it is required by the professor but i didn't get why we should do that and eh couldn't explain it well.

**broken link removed**

 

[audioguru]

C1 and R6 form a highpass filter because at lower frequencies the reactance of C1 forms a voltage divider with R6. C1 passes high frequencies without any voltage loss. Your wrong values cause 10Hz to be at -3dB, not 100Hz. (I corrected my previous post).

Your value for C3 is ridiculously too low. It should be 355pF for 5kHz to be at -3dB.

Your professor wants the output to be down only -3dB at 100Hz. Then it is provided only by C1 and R6. If C2 and C4 also provided a cutoff at 100Hz then the output would be down -9dB instead of -3db.

 

[moe_3_moe]

using the Cutoff equation to find the capacitor value for C3=1/W.R4 with R=90K and W=5KHz . I got why it is multiplied by 10, thank you.