can anyone help me design a "noisy" feedback amplifier with amplifier gain of 100 and signal to noise ratio of 60dB

 

Use a 741 op amp with badly filtered supply rails.

Is this an assignment?

 

yea...could you please elaborate on that

 

Most people simply use a low noise audio opamp when they need low noise, not a noisy 43 years old 741 opamp.

Audio opamps have a signal to noise ratio of up to 120dB.

 

Oddly enough, he indicated that he wants a noisy amplifier. School projects are getting quite weird

 

Use long cables not sheiled for the signal and power supply leads. And do not twist them them together. Use an unregulated power supply.

 

If you can get your hands in one, you can also use a 301 OpAmp. It doesn't have internal compensation and is particularly noisy.

 

And make sure that your circuit is totally devoid of decoupling components.

 

i simulated this circuit in LTSPICE but the results arent right...could anyone tell me whats wrong with this design and suggest a possible solution

 

i simulated this circuit in LTSPICE but the results arent right

Could you say what results were not right?

 

Another though, you could deliberately introduce noise into the front end of the schematic. Bias up a zener diode with 10ma and have it feed in somewhere, for example. It is an old trick used to generate white noise.

 

The polarity of V2 is backwards because it is supposed to be a negative supply. You have its negative terminal connected to ground so it is actually a positive supply.

Why do you take the input signal and attenuate it down to almost nothing, then amplify it a lot?

your negative feedback around both opaps looks odd and might cause oscillation.

 

the frequency response doesnot give a gain of 40dB and the transient analysis gives out an output voltage of 700uV :-/ so clearly the design is not right...plz tell me what changes should i make

 

The polarity of V2 is backwards because it is supposed to be a negative supply. You have its negative terminal connected to ground so it is actually a positive supply.

Why do you take the input signal and attenuate it down to almost nothing, then amplify it a lot?

your negative feedback around both opaps looks odd and might cause oscillation.

i have given V2 a value of -15V i dnt think it would make much of a difference... and plz could you tell me explicitly wat changes i should make

 

I don't know why:
1) You have an attenuator at the input of the first opamp.
2) You have two opamps when the second opamp by itself will work.
3) You feed the output of the second opamp back to the input of the first opamp as some kind of feedback.

 

Another though, you could deliberately introduce noise into the front end of the schematic. Bias up a zener diode with 10ma and have it feed in somewhere, for example. It is an old trick used to generate white noise.

Yes, great idea. Something similar to this was/is used by hams to make a noise bridge used for measuring 50Ω Z.