Cascading Op-Amps

[canadianpoet2012]

I'm looking to cascade two non-inverting op-amps circuits. I want then to run in series, but with only one feedback loop.
However, I cannot find anything on the net to tell me how to do this!!! Attached is how I think it is connected, can anyone tell me if I'm right or wrong?!

 

[Styx]

why would you want to do that?

 

[Nigel Goodwin]

I agree - it seems a totally bizarre idea?.

Certainly the diagram looks horrible, connecting the two inverting inputs together isn't going to work!.

For a start try telling us what you are trying to do, are you trying to achieve some specific aim?.

 

[canadianpoet2012]

I am investigating ways of increasing the bandwidth! I don't want to change the resistor values and affect the gain that way.

I'm aware that just cascading two op-amps will square the gain, and thereby, reduce the bandwidth. But I was wondering if cascading two and having a common feedback would have a more positive affect.

 

[tavib]

cascading two op-amps like 741 with a single global negaive feedback isn't used because the high probability for oscillating on high freq. This is happen because the 741 have internal a capacitor for freq compensation. With a single 741 loop the max phase shift will be 90 degree because of presence of a single compensation capacitor. But with two 741 cascaded the loop will have two compensation capacitors cascaded and a 180 degree phase shift can be touched on high frequency. At this point of frequency the feedback become positive from negative and if total loop gain is over 1 you will have a oscillator on this frequency.

This preblem of oscillation on high frequency is common to all amplificators with more than one single stage and with a global feedback loop. The phase shift is produced usually by parasitic capacitance from every stage input, or by any deliberated added capacitor. With two stage amplifier the oscillation is produced only if the open loop amplifier is very high (like in case of op-amps), but with three or four stage amplifiers the oscilation condition is riched very easy because of multiple points of phase shifting.

 

[Nigel Goodwin]

I am investigating ways of increasing the bandwidth! I don't want to change the resistor values and affect the gain that way.

I'm aware that just cascading two op-amps will square the gain, and thereby, reduce the bandwidth. But I was wondering if cascading two and having a common feedback would have a more positive affect.

Just use two seperate amplifier stages, and split the gain between them, as you're using two amplifer stages you could use them in inverting mode, and the final output will be non-inverted, assuming that's a requirement?.

I don't see why having two amplifiers would reduce the bandwidth, if they are both identical the bandwidth should be the same as one of them.

Also, as you used 741's in the diagram, try using a decent opamp, the 741 is the lowest spec one available, and has very low bandwidth.