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Design >> Analog Design >> Help Identifying the 2 transistors on the +ve feedback path
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Message started by Larry_80 on Dec 6^th, 2011, 10:52am

Title: Help Identifying the 2 transistors on the +ve feedback path
Post by Larry_80 on Dec 6^th, 2011, 10:52am

Hello All,
Please i need some help in identifying the positive feedback path (with explanation) in the attached circuit. I will really appreciate any help....please identify the transistors in the +ve feedback path.
Regards.

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by raja.cedt on Dec 6^th, 2011, 10:57am

hello,
how you are biasing M5,M7? if you are generating through some constant voltage then there is no +ve feedback.

i think you forgot to connect m7 as diode connected, if so m3,m1,m5,m7 forms +ve feedback loop.

Thanks,
Raj.

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by raja.cedt on Dec 6^th, 2011, 11:03am

one more thing,
M6 provides -ve feedback ..
Thanks,
Raj.

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by Larry_80 on Dec 6^th, 2011, 11:05am

Sorry, yeah M7 is diode connected and Vb is some constant biasing voltage.

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by Larry_80 on Dec 6^th, 2011, 11:07am

Thanks Raja, i came up with same answer as you did, just wanted to be sure....i'm afraid of screwing up...thanx a lot!

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by loose-electron on Dec 6^th, 2011, 4:05pm

looks like a wide swing cascode bias circuit?

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by thechopper on Dec 6^th, 2011, 5:26pm

Actually thanks to m6 local negative feedback the positive feedback loop is stable, since gain is less than 1

Best
Tosei

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by raja.cedt on Dec 7^th, 2011, 4:26am

but i doubt about the amount of degeneration because of it's huge value. any how i would like to ask now a days is any one using gm=1/r ckt for biasing?, because in this lower technology it may not work as we expect in theory due to lack of square law.

Thanks,
raj.

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by RobG on Dec 7^th, 2011, 8:03am

raja.cedt wrote on Dec 7^th, 2011, 4:26am:

I feel like I'm part of a conversation where everyone but me can see another poster's responses as I can't figure out what you folks are talking about ;).

Raj - what sort of bias should they use? If we assume Larry's circuit is a constant gm bias (M5 is a multiple placement of M7) then it should properly bias a PMOS diff pair operating at the average current density of M5 and M7. Under these conditions the gm will remain constant over temp (well it would if a zero-tc resistor was used). I think this will be true from subthreshold to velocity sat, and to different devices such as bipolar.

Regarding the "positive" feedback path, the signal path is counter-clockwise. Assuming M5 is multiple placements of M7, at low currents the path is positive feedback. This is what you want at startup since this keeps increasing the current in the devices until the gain from M7 to M5 is reduced to 1 by the degeneration of M6. At this point the feedback isn't really positive or negative.

rg

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by analogbeginer on Dec 10^th, 2011, 5:46am

Hi RobG,

Isn't at that point, the -ve f/b takes control of the loop, finalizing the current. So can it not be called as -ve f/b at the time, when current stabilizes? May be we cannot say about the f/b involved, when M5 looses control, say it becomes linear (but this will never happen--i guess).

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by RobG on Dec 10^th, 2011, 6:52am

Just curious, are -ve and +ve new terms being used in the textbooks? I haven't seen them before.

Analogbeginner, I suppose it can be viewed a few ways depending on your terminology. At "low" currents the loop would be positive feedback because the gain from M7 to M5 is greater than one (if M5 is multiple placements of M7 and the votlage drop across M6 is negligible). At "high" currents the voltage drop across M6 will make the gain less than one. I guess you could call that negative feedback.

The circuit will stabilize at a current where the gain from M7 to M5 is exactly 1. If a gain of more than 1 is positive FB, and if a gain of less than 1 is negative feedback, then what is the gain when it is exactly 1? It is sort of a philosophical question in my mind. Positively negative I guess ;D

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by analogbeginer on Dec 10^th, 2011, 6:12pm

RobG, I was thinking that at every current value, there exists +ve & -ve f/b, but whichever is predominant takes control & decides the current--> whether it is going to settle or increase, but I'm not sure, whether the point where the current stabilizes will be at a point were the gain from M7-->M5 will exactly be less than 1. I was thinking it was a relative factor, between the +ve & -ve component, which decides the current.

Title: Re: Help Identifying the 2 transistors on the +ve feedback path
Post by raja.cedt on Dec 11^th, 2011, 7:22am

hello analogbegnir,
may be you can think in this way to get good idea.

The final bias point will be some where else, so whenever you are at higher bias current degeneration resister reduces the mirroring ratio and hence loop will move towards current reduction direction and if you are at lower than the final bias current, degeneration will increase the mirroring ratio hence loop will get you the final value.

Thanks,
Raj.