Hi ,
Customer's product is a high accuracy ac power line quality analyse equipment. They want to know whether the GBW is always constant in any condition?
They sch have 4 stage amp,every stage gain is 10. The OPA228 GBW is 33Mhz ,Dose it mean if the frequency less then 3.3Mhz the gain can be guaranteed by 10?
Or can I using the formula as below picture and the gain is 5 at input frequency equal to 3.3Mhz? Which is right?
Thanks
Hi Jeff,
The OPA228 is a decompensated Op amp and its open-loop gain-phase response deviates from a perfect first-order gain roll off and phase response. When you examine the datasheet open-loop gain/phase plot you can see an obvious ripple in the phase between 1 and 2 MHz, and there appears to be another pole breaking around 2 to 3 MHz, and a zero between 10 to 20 MHz. A ripple in the phase such is often an indicator of gain peaking in the response at that point.
When I construct four stages of OPA228 each operating with a gain of 10 V/V using TINA Spice, the following gain vs frequency response of obtained.
You will note the gain peaking just above 2 MHz, that becomes increasingly higher with each additional stage (blue is stage 1 output, olive is stage 2 output, etc.). Therefore, the peaking makes it more difficult to determine the -3 dB bandwidth. Using the brown curve it looks to be about 3 MHz. However, because of the overall gain of 10,000 V/V the output swing of the final stages is likely going to be large signal - volts peak-to-peak. If that is the case, the OPA228 slew rate of 11 V/us will likely become the limiting factor.
Some of the High-Speed Op amps that Michael suggested would likely provide more satisfactory performances if speed and bandwidth are required.
Regards, Thomas
Precision Amplifiers Applications Engineering
