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https://designers-guide.org/forum/YaBB.pl Design >> Analog Design >> Folded Cascode Design https://designers-guide.org/forum/YaBB.pl?num=1099935616 Message started by venusgarg on Nov 8th, 2004, 9:40am |
Title: Folded Cascode Design Post by venusgarg on Nov 8th, 2004, 9:40am Hi i am designing a Double folded cascode amplifier, but i am not getting a positive Gain plot and i have checked the operating points of my transistors they are all saturated. Can someone please help me how to increase the current through design and also what will be a Vt of the transistors if i am stacking 4 transistors from Vdd to Gnd if someone knows anything about this please help me Venus |
Title: Re: Folded Cascode Design Post by sheldon on Dec 10th, 2004, 6:18pm Venus, Could you provide more information about your design? In general, the issue when stacking cascoded devices is the Vds not the Vt. There are techniques for designing low overhead current mirrors, see Johns and Martin's book for more details. It was not mentioned, are the input devices included in the cascode stack? It may also help to separate the input stage from the gain stage, for example use a differential input stage with a folded cascode gain stage. Sheldon |
Title: Re: Folded Cascode Design Post by venusgarg on Dec 10th, 2004, 8:05pm Hi thanks for your reply I am done with my design of double folded cascode amplifier design but i am not satisfied with gain of an, can you please guide me how to increase the gain of an amplifier, i am using differential cascode input stage and CMFB at one output and taking output from other leg of the amplifier and also my Vout/Vin vary with input signal when it varies from 1mA to 100mA, is it OK or something is wrong and please also tell me how to increase the gain of an amplifier Venus |
Title: Re: Folded Cascode Design Post by sheldon on Dec 18th, 2004, 6:52pm Hi, Still can't quite understand the circuit architecture, so it is difficult to answer in detail. One comment, is that open loop gain is relatively insensitive to bias current. For example, for a bipolar transistor: transconductance is gm = Ic/Vt and output resistance is ro= Va/Ic so the gain is Av = Va / Vt. To increase gain, alternative architectures are required. For example, using a cascode increases the output resistance, ro~Bo*Va/ Ic. So gain becomes Av = Bo * Va / Vt. MOS transistors have similar characterisitics, that is, increasing bias current does not usually increase gain. Since you are already using cascodes, the next step is to use regulated cascodes(gain boosting). Sheldon |
Title: Re: Folded Cascode Design Post by venki_vlsi on Mar 18th, 2005, 4:53am hi inrease the lengh of m1 and m2(input applying transisitor) by 3-5 times to increase gain.u can also increase by increasing the width of load transisitors which r near vdd if u use n-type differential ckt input otherwise near gnd for p-type differential ckt.u can also do two stage cascode but u have to apply compensation. ok bye |
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