gsensor
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Hi,
I'm working with 0.13 um CMOS (IBM) technology with a 2.5V supply. I have designed successfully a one-stage folded cascode amplifier, providing low noise and low power by using large length device for cascode mirrors. This op-amp as the following characteristics:
Open-loop gain: 75 dB PSSR+: 80 dB PSSR-: 75 dB Output swing: 0.4V-2.1V (Gnd+Vth ,Vdd-Vth) Input voltage: 0.3V-1.25V (I will use this op-amp only with this range of input vcm..) Power: 32 uW Bandwidth: 3 kHz (limited by C load) C load=2pF
As I need a rail-to-rail output swing, low power dissipation, a bandwidth of 4 kHz and an open-loop gain of at least 80 dB, I have tried, unsuccessfully to add a second stage to this op-amp, using a simple indirect feedback compensation (see below).
The problem is that I keep getting a huge offset no matter how I size the 2 transistors of this second stage....What I'm I doing wrong ? Is there a proper way to design this stage ?
If this is not possible with my specifications, what other second stage could provide me the rail-to-rail output swing I'm seeking, without an offset voltage bigger than 200 uV (without mismatch variations) for an input common voltage of 0.3V to 1.2V and without increasing dramatically power consumption ?
Thanks a lot for your help ! gsensor
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