[SOLVED] Design of High-speed opamps

[rmanalo]

Hello everyone,

[1] When designers say high-speed opamps, do they mean high-slew rate or high bandwidth?
[2] How do we increase bandwith for a given GBP (gain-bandwidth product)?
-implement it in feedback right? but GBP is based from open-loop gain (correct?), if we reduce the open-loop gain, would the unity-gain frequency (hence GBP) remain the same?
(Premise: I have a single-ended op-amp with differential inputs that need 100 Mhz bandwidth, so far my open-loop gain is 45 dB and rolls off at 200 Khz, my UGF is 150 MHz, without capacitive loading, I'd like to have 100 Mhz bandwith by reducing the open-loop gain)

 

[dick_freebird]

You can't separate full-power bandwidth from slew rate,
but small signal bandwidth can be somewhat decoupled
from SR by architecture (see for example slew enhancing
resistor networks in op amp front ends, etc.).

One common trick is to make a family of op amps with
different compensation values - for example a unity gain
stable op amp will have a lower unity gain bandwidth
than a "10X gain stable" opamp of the same schematic
other than comp cap value. Presuming you want A=10,
you'd get better application bandwidth. But at A=5 the
unity-gain-stable part will be stable and the A=10-stable
one will oscillate (or be about to).

AVOL in single stage op amps tends to come from one
high gm, high Rout stage. But this means low current
high impedance, meaning low frequency corner. Very
high speed op amps may substitute a pair of lower-gain
gain stages (likely a second differential stage, as front
gain is key to low-input-overdrive slew rate and full
power bandwidth, so you want some gain early in the
chain). Both stages will be higher BW than the single
high gain stage; however when they do peter out, the
rolloff may be more "brick wall" abrupt and the comp
scheme more difficult.