Op-amp design specification for certain application

Dear friends,

if my application requires an amplifier that can amplify a signal with frequency of 1 MHz with variable adjustable gain from unity to 100. How much should be the Gain bandwidth product of my op-amp to have accurate closed loop gain.
Generally books refers to the DC gain for specifying high loop gain accuracy requirement but in my case the frequency is at 1 MHz, how much should be the open loop gain


Thank you very much in advance

The GBW needs to be at least 100MHz (1MHz x Gain=100). The open-loop gain will determine the accuracy of your closed-loop gain. For Ac=closed-loop gain and Ao=open-loop gain:


Ac=Ao/(1+AoxFeedback)

As Ao gets very large, the above equation reduces to

Ac=1/feedback

i.e., open-loop gain has less effect the larger it is.

Dear Barry

Thank you for your reply

indeed Ao is the problem, how can I still have high Ao of my operational amplifier at frequency of 1 MHz ??

and by the way if I would follow the formula 1MHz x Gain=100, and if you look to the op-amp frequency response you will see that the Ao becomes equal to the Ac at this point as shown in figure below as an example, this point makes me confused regardless it is very basic thing in electronics

"Accurate" closed loop gain wants some care, as the
GBWP is usually the -3dB point - where gain has rolled
off by 30% or so. That's not "accurate" really. You will
want some excess GBWP if you (say) expect to hold a
+/-5% or 1% total accuracy at the gain-block level.

If you had such a spec (gain error @ freq) you could
work back through the error budget to see what the
real -XdB has to be, and then relate that (by gain
slope) to your manufacturer's conventionally rated
GBWP needed.

Junus2012 said:

Dear Barry

Thank you for your reply

indeed Ao is the problem, how can I still have high Ao of my operational amplifier at frequency of 1 MHz ??

and by the way if I would follow the formula 1MHz x Gain=100, and if you look to the op-amp frequency response you will see that the Ao becomes equal to the Ac at this point as shown in figure below as an example, this point makes me confused regardless it is very basic thing in electronics

View attachment 153994

Click to expand...

You have no control over Ao of the opamp-it's an intrinsic parameter of the device. If you need a higher Ao, you'll need a different device.

Look again at the first formula I posted. For high frequencies the "1" in the denominator has more effect. Just as a thought experiment: lets say you have an amplifier with gain=100 (feedback=.01). The low frequency CL gain (OL gain very high) will be 1/.01=100. Great. Now, at some higher frequency lets say the open-loop gain drops to 1000. Using the first equation your CL gain is now 1000/(1+1000*.01)=90.9

The GBW of your amplifier really depends on the application you target. GBW of the loop gain being more or less the -3dB frequency of the closed loop response. For example, if you are using the opamp in a filter, where you want it to perform like an integrator, then the GBWP is in the order of 50 to 100x the filter corner frequency and if the Q of the biquads you are realizing with the integrators is high, then the requirement becomes more stringent.
If for example your target application is switched capacitor or something that you want accurate settling within certain amount of time, then you design the GBW of the loop so that tau=1/(2*pi*GBW) is small enough to allow for accurate settling. Accurate here means the equivalent number of bits to which you want to settle and for different values you get different tau, respectively different values for the GBW product.

Thank you guys for your help

Now understand that if I want to work at some gain I should not follow exactly the formula of GBW as it, because it define the frequency at the value of -3dB or when the gain is already dropped by 30%. Therefore if I want precise closed loop gain I should design the circuit with higher GBW,

for example in my example instead of GBW = 100 MHz it must be 1GHz or even more,,, if I can not reach this frequency in my design then I must believe that I have to reduce the amplifier closed loop gain requirement

Junus2012 said:

Thank you guys for your help

Now understand that if I want to work at some gain I should not follow exactly the formula of GBW as it, because it define the frequency at the value of -3dB or when the gain is already dropped by 30%. Therefore if I want precise closed loop gain I should design the circuit with higher GBW,

for example in my example instead of GBW = 100 MHz it must be 1GHz or even more,,, if I can not reach this frequency in my design then I must believe that I have to reduce the amplifier closed loop gain requirement

Click to expand...

You can always use multiple stages, so that the gain of each op amp is less.