Diff-amp question

[Samuel Groner]

Hi,

I've got a question regarding the basic diff-amp config with one opamp
and four resistors. Does the resistor connected output and "-"-input
load the opamp?

Thanks
Samuel

 

[John Larkin]

On 20 Jun 2004 12:30:57 -0700, micpreamp@yahoo.de (Samuel Groner)
wrote:

Hi,

I've got a question regarding the basic diff-amp config with one opamp
and four resistors. Does the resistor connected output and "-"-input
load the opamp?

Thanks
Samuel

Sure. And it can self-heat and mess up the CMRR.

I've added a follower opamp before the feedback resistor, inside the
loop, to reduce the heating of the first amp, to reduce thermal
offsets and transient hooks.

John

 

[Ian Bell]

Samuel Groner wrote:

Hi,

I've got a question regarding the basic diff-amp config with one opamp
and four resistors. Does the resistor connected output and "-"-input
load the opamp?

Thanks
Samuel

Yes, because it at least provides bias current for the input and depending
on the configuration (such as a diff amp) it may be a virtual earth. Is
this a problem. usually this resistor value is high enough for it not to
significantly load the output.

ian

 

[Samuel Groner]

Thanks for the answers! Thought that, but nowhere read about it, so was not sure..
Samuel