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Design >> Analog Design >> DETERMINING POLARITY OF FEEDBACK OF A CIRCUIT
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Message started by Larry_80 on Aug 3^rd, 2011, 4:00pm

Title: DETERMINING POLARITY OF FEEDBACK OF A CIRCUIT
Post by Larry_80 on Aug 3^rd, 2011, 4:00pm

hello Guys,
I am having trouble on how to determine the polarity of feedback of a circuit. I have attached 2 circuits that i need explanation of. Basically, with an emphasis on explaining to me why an increase in current will cause deduction in drain voltage for example especially in cases where the Load is a current source.

Title: Re: DETERMINING POLARITY OF FEEDBACK OF A CIRCUIT
Post by AnalogDE on Aug 3^rd, 2011, 5:33pm

Both look like negative feedback to me.

You need to be more clear on your second question -- which current and which drain?

Title: Re: DETERMINING POLARITY OF FEEDBACK OF A CIRCUIT
Post by Larry_80 on Aug 3^rd, 2011, 5:59pm

For example for circuit a.) I was trying to find out the polarity, and this was how i went through it...
Vin increases, Id1 increases, id2 reduces,this makes Vd2 increase?, since vd2 is connected to gate of M3(PMOS), this makes Id3 reduce?, reduction in id3 means drop across R1 reduces as a result vout reduces which makes Id2 reduce=> Positive feedback. I might be doing something wrong somewhere as u said it was negative feedback.

Title: Re: DETERMINING POLARITY OF FEEDBACK OF A CIRCUIT
Post by aaron_do on Aug 3^rd, 2011, 6:33pm

Hi,

where did you get the circuit from?

You weren't doing anything incorrect...the first example is positive feedback.

regards,
Aaron

Title: Re: DETERMINING POLARITY OF FEEDBACK OF A CIRCUIT
Post by Larry_80 on Aug 3^rd, 2011, 6:37pm

I got the circuit from Razavi Fundamental of microelectronics. I am just trying to go back to the basics and really understand some of the fundamentals easily overlooked.

Title: Re: DETERMINING POLARITY OF FEEDBACK OF A CIRCUIT
Post by buddypoor on Aug 4^th, 2011, 12:54am

Yes, the first circuit has positive and the second circuit has negative feedback.

Title: Re: DETERMINING POLARITY OF FEEDBACK OF A CIRCUIT
Post by AnalogDE on Aug 4^th, 2011, 8:32am

My bad, the first one is indeed positive feedback.

It's simple to look at it in terms of voltages. Let's assume the output moves up in voltage for some reason -- the NMOS M2 gate increases, pulling its drain node down, which increases Vgs of PMOS M3, turning it on harder, which pulls its drain up => feedback acts to enforce the voltage change on the output node => this is positive feedback.