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Gain Bandwidth Product

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jonnybgood

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I am asked t find a suitable op amp from a choice of 5. I have the closed loop gain, maximum operating frequency and Vin. I am refering to the datasheets but only one datasheet give the gain bandwidth product. is there someway I can calculate it with other values?
 

WimRFP

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Hello,

Frequently they show an open-loop gain versus frequency curve. With increasing frequency, the gain starts to drop with 6 dB each doubling of frequency (6 dB/oct). If you multiply frequency with the gain for that region, you have the GBW.

You may know that besides GBW, many other things are of importance during the selection process (offset effects, noise, bias current, slew rate, rail to rail, current capability, PSRR), etc.

What is your frequency of interest?
 

jonnybgood

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Hi, thanks for your help. My frequency of interest is 750kHz, and I was given the op amps : LM741, LM358, OP177, LT1078 and LF411 to choose from. When I referred to their datasheet none of them seemed suitable at that frequency, unless I'm doing something wrong or there is a small mistake in the tutorial Problem. It specifies to ignore slew rate and more than one op amp is suitable. After that, it asks for the input bias current of the op amp chosen and to add any components necessary to minimize the input bias current.
 

WimRFP

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Hello,

To allow the use of the standard formulas for opamp amplifiers, the GBW need to be far above the gain*frequency product that you want to obtain. To be more precise, (opamp gain)*feedback >> 1. So if you want a voltage gain of 10, the GBW of the opamp should be 50 MHz or above.

You may know that the opamp has input capacitance also, when you make the resistors too large, you get an additional reduction of the feedback and an additional phase shift in it, this may result in instability or peak in the gain versus frequency curve.

Other things are slew rate. If you want several volts of output (let us say 4Vpp), the slew rate should be >> 2*pi*f*vtop = 9V/us

It is a good exercise to evaluate a complete opamp circuit, including the internal frequency dependency and external frequency dependency (for example due to input capacitance or capacitive load). It may cost you some A4 sheets. Other thing is to do this in simulation where you can play with the opamp characteristic.
 

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Hello,

To allow the use of the standard formulas for opamp amplifiers, the GBW need to be far above the gain*frequency product that you want to obtain. To be more precise, (opamp gain)*feedback >> 1. So if you want a voltage gain of 10, the GBW of the opamp should be 50 MHz or above.

You may know that the opamp has input capacitance also, when you make the resistors too large, you get an additional reduction of the feedback and an additional phase shift in it, this may result in instability or peak in the gain versus frequency curve.

Other things are slew rate. If you want several volts of output (let us say 4Vpp), the slew rate should be >> 2*pi*f*vtop = 9V/us

It is a good exercise to evaluate a complete opamp circuit, including the internal frequency dependency and external frequency dependency (for example due to input capacitance or capacitive load). It may cost you some A4 sheets. Other thing is to do this in simulation where you can play with the opamp characteristic.
Your right, I need to get down to basics again.. and in fact I find A3 papers more suitable for taking notes and diagrams on op amps ;) Thanks for your help I will get into the points you mentioned further.

thanks
 

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