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.

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

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!

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

Best
Tosei

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

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

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.