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Design >> Analog Design >> High speed opamp -depends on GBW or BW?
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Message started by sylak on May 16^th, 2006, 9:50pm

Title: High speed opamp -depends on GBW or BW?
Post by sylak on May 16^th, 2006, 9:50pm

Wondering what causes High speed in an opamp(closed loop).. I derived the step input for an closed loop single pole system to be
V0(1-e(-wf))
where wf is the closed loop bandwidth= wo(1+βAo)

where wo is open loop BW , Ao is open loop gain and β is feedback factor..

Is it good to approximate this wf to wo*β*Ao... considering wo << wo*β*Ao.

If I do this then the speed of the system depends on GBW and not BW.? But most of the books say speed depends on the BW(Rise time =0.35/BW).

Put it in another way
If I have an opamp1 which has say a BW of 300MHz and GBW of 1MHz and
and another opamp2 which has a BW of 500Mhz and GBW of 800Hz. Which is is faster :opamp1 or opamp2?

Would appreciate enlightment on a simple (bothersome ) question? :o

Thanks

Title: Re: High speed opamp -depends on GBW or BW?
Post by Marc Murphy on Jun 7^th, 2006, 9:58am

Consider the GBW to be f3dB*OpenLoopDCgain. In a closed-loop configuration you directly trade-off bandwidth and gain.

As you've pointed out, the bandwidth is extended by (1+A*B), which is the same amount the gain is reduced by.

Consider one amp (single-pole rolloff) with a DC gain of 100 and f3dB of 1 MHz and another with 10 and 10 MHz. You'll note that the Bode plots overlap at 10 MHz. They both will allow the same closed-loop bandwidths, given the same gains (<=10).