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why offset voltage and current source both for diff pair?

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leohart

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Gray p231, it is said both offset voltage and current source are required to represent the effect of mismatch in general so that the model is valid for any source resistance...

Why only a offset voltage source is insufficent?Anyone konw this?
 

when you are using only one source say current source or voltage source you are linearizing noise... i mean say you are using the voltage source to simulate the effect of noise and it would be a linear one which would give the same current contribution for a particular value of voltage noise.... but there would be variation in voltage as well as current which are totally independent of each other....
 

sorry,I dont quite understand...
 

Re: why offset voltage and current source both for diff pair

I did not have the Gray/Meyer in front but I think mismatches in bipolar manufacturing result in a mismatch of many model parameter.

The obvious one is the IS of the spice bipolar model. But others are current gain, emitter and base resistance, for some RF circuits still mismatches of the depletion capacitances are important.

For DC at lower current density mismatch in base current gain is the next important. That is historical related to the most demanding application of opamps. The source resistance and the base current matching could have similar offset effects like the IS.
 

I think the purpose to have two typies of sources(both for offset and noise situation) is counting for circuit thoery, but I just cannot see the obvious reason for this... ask for a simple but in depth explaination why only just one of them is not self consistent in circuit thoery...

ps: I think the problem roots from counting offset for input source with arbitary source impedence...if we use ideal voltage source, only voltage offset source is needed then,,,
 

let me explain with a example.... suppose you have 1volt input and at that moment of time a 20uV noise is present and there would a current component of noise independent of 20uV noise.... i mean take two instants when the input is 1V and the noise is 20uV... the current at both instants wont be the same to model this current variation only current noise source is introduced...
 

Re: why offset voltage and current source both for diff pair

A.Anand Srinivasan said:
let me explain with a example.... suppose you have 1volt input and at that moment of time a 20uV noise is present and there would a current component of noise independent of 20uV noise.... i mean take two instants when the input is 1V and the noise is 20uV... the current at both instants wont be the same to model this current variation only current noise source is introduced...

Can we relate the problem to input source impedence matching,because in Gray it is said this is all about input source impedence matching?

and let's forget noise and focuse on offset...

these two sources are actually input referred offset, so I'm wondering why only a voltage offset source cannot compensate for all offset so that the output can return zero...Can you give a example in this?
 

i think i got what Gray might have meant... he must have spoken with respect to the input.... i mean the voltage source would be in series with the input and the current source in parallel.... consider the case where you model only using a voltage source... suppose the input is open circuited then then circuit would look as if the noise source has also been open circuited... well you would know that shouldnt be the case...
 

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