# questions about OP design procedure in TSMC018 technology

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#### airboss

##### Member level 3
Hi,

i'm given a spec and i have to achieve the gain, in TSMC018. after giving current and allocating the overdrive voltage, i did the simulation. i extracted gm and ro for each transistor, and plug the numbers into the gain equation to calculate the gain. however, the order of the magnitude of the gain i got from calculation is far below that of the desired gain. i don't think i can solve this by simply increasing the length or the current.

on the other hand, i simulated an folded cascode amplifier. the AC response simulation shows the gain is 52 dB. then i found ro and gm of each transistor (based on dc operating point simulation), and then plug them into the folded cascode gain equation, and then i got a number of 8.1xxx, which is far below 52 dB.

why is there such a discrepancy in the latter case? does that mean, to see if the design meets the required gain, we cannot do any hand calculation but simply simulate until we get the correct gain? (in razavi's textbook, he did calculate if his "guess" for vov and allocation of current gives him enough gain)

or, can anybody share your design procedure, after you allocate overdrive voltage, and find the current?

thanks

#### rush3k

##### Member level 1
usually i define the ICMR and the output output rail-to-rail voltages and use this specifications in helping me calculate my Vdsat/Vov. the hand calculation should give a close match to the simulator, recheck your calculations especially the lambda effect on L

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