Comments on: A compact common-mode feedback loop | using a PMOS triode device for CMFB http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/ Tutorials and Insights in Electronics and Circuit Design Fri, 19 Mar 2010 03:10:53 +0000 hourly 1 By: Hassan http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/comment-page-1/#comment-111 Hassan Tue, 18 Nov 2008 23:37:44 +0000 http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/#comment-111 By cascading I meant stacking transistors which you already calculated the limits. But the other point was that how much common feedback limits differential swing at the output. For this structure to work, two PMOSs on top should be kept in triode, in case differential swing is higher than vdd-Vth one of the two will be off while the other is still in triode. Second limit should be compared to what you calculated then smallest defines the differential swing. not matter what number we are targeting for diff. swing. Regards, Hassan By cascading I meant stacking transistors which you already calculated the limits. But the other point was that how much common feedback limits differential swing at the output.
For this structure to work, two PMOSs on top should be kept in triode, in case differential swing is higher than vdd-Vth one of the two will be off while the other is still in triode.
Second limit should be compared to what you calculated then smallest defines the differential swing. not matter what number we are targeting for diff. swing.

Regards,
Hassan

]]> By: Poojan Wagh http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/comment-page-1/#comment-104 Poojan Wagh Tue, 18 Nov 2008 02:00:26 +0000 http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/#comment-104 @Hassan: Thanks for the comment. Could you explain a little what you mean by cascoding effecting the differential swing? Indeed, as you stated, the topology does not meet a full rail-to-rail swing. As far as I can tell, the highest voltage on each of the outputs will be Vdd - Vtriode - VDSAT1 - VDSAT2, where VDSAT1 is the current source |VGS - Vt| and VDSAT2 is the cascode |VGS - Vt|. Depending on bandwidth and sensitivity to threshold shift, one can design VDSAT's to less than 200 mV. In addition, Vtriode can be (in fact, should be) made much less than the |VGS - Vt| of the triode device. Adding this all up, we're talking about a peak voltage about 250 mV from supply--which isn't bad. @Hassan:
Thanks for the comment.
Could you explain a little what you mean by cascoding effecting the differential swing?
Indeed, as you stated, the topology does not meet a full rail-to-rail swing.
As far as I can tell, the highest voltage on each of the outputs will be Vdd – Vtriode – VDSAT1 – VDSAT2, where VDSAT1 is the current source |VGS – Vt| and VDSAT2 is the cascode |VGS – Vt|. Depending on bandwidth and sensitivity to threshold shift, one can design VDSAT’s to less than 200 mV. In addition, Vtriode can be (in fact, should be) made much less than the |VGS – Vt| of the triode device. Adding this all up, we’re talking about a peak voltage about 250 mV from supply–which isn’t bad.

]]> By: Hassan http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/comment-page-1/#comment-103 Hassan Mon, 17 Nov 2008 23:38:41 +0000 http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/#comment-103 Hi Poojan, one main issue is that this will limit differetial output swing it two ways, 1 cascoding 2. peak more than vdd-Vt will trun off one side of two PMOSs in triode. in low voltage design is not acceptable unless where swing is not a problem. Regards, Hassan Hi Poojan,
one main issue is that this will limit differetial output swing it two ways, 1 cascoding 2. peak more than vdd-Vt will trun off one side of two PMOSs in triode. in low voltage design is not acceptable unless where swing is not a problem.

Regards,
Hassan

]]> By: The case for the trans-conducting LNA | Circuit Design http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/comment-page-1/#comment-80 The case for the trans-conducting LNA | Circuit Design Sat, 01 Nov 2008 20:08:32 +0000 http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/#comment-80 [...] high output impedance of the LNA, a common-mode feedback circuit is necessary. I have detailed two ways of doing [...] [...] high output impedance of the LNA, a common-mode feedback circuit is necessary. I have detailed two ways of doing [...]

]]> By: Poojan (Wagh) Blog :: The case for the trans-conducting LNA http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/comment-page-1/#comment-79 Poojan (Wagh) Blog :: The case for the trans-conducting LNA Sat, 01 Nov 2008 20:07:13 +0000 http://www.circuitdesign.info/blog/2008/10/a-compact-common-mode-feedback-loop-using-a-pmos-triode-device-for-cmfb/#comment-79 [...] high output impedance of the LNA, a common-mode feedback circuit is necessary. I have detailed two ways of doing [...] [...] high output impedance of the LNA, a common-mode feedback circuit is necessary. I have detailed two ways of doing [...]

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