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[SOLVED] Amplifier gain dependence of Common mode value

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andhov

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Hi Guys,

I have simulated a sample diff amp/op amp and found a difference of DC gain for various CM(common mode) voltage.
All transistor work in saturation region. If the variation is small(like a ±5dB) I will not concern and will think that is something technology dependence but it varies in ±15dB range.
Has someone experienced this kind of dependence????

Thanks.
 

You must be operating near the supply rails.
Otherwise, Assuming you are using gain-controlled by negative feedback ratio, It should not do that.
But if you are using open loop, then yes it is possible Hfe varies by that much.
 

SunnySkyguy said:
You must be operating near the supply rails.
Otherwise, Assuming you are using gain-controlled by negative feedback ratio, It should not do that.
But if you are using open loop, then yes it is possible Hfe varies by that much.
Click to expand...

I want to understand the reason of variation. Why it is possible???
 

You didn't give any details yet. Are you talking about open loop gain? What's the absolute gain? How much is the variation?

Generally speaking, some open loop gain variation can be expected near the common mode range limits. If it's severe, you're either exceeding reasonable common mode range or the amplifier isn't well designed. In any case, you should show a circuit.

P.S.: +/-15 dB is severe.
 

i like to say "The best answer can only be given with the best described question"

perhaps avoid false assumptions for the reader and give more details. Did you balance input source resistance? How do you know "All transistor work in saturation region."
 

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