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Output of Variable gain amplifier?

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poojadholam

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Hello,

I have designed VGA using a rail-to-rail amplifier. My common mode voltage is 1.65 V.
If I am providing 1.65 V DC to both inputs and 1000 as a DC gain. I am getting 1.634 V as an output.
Is it the right value that i am getting?
thanks in advance
opamp.png
 

Hi,


V_out = V_CM + V_offset * gain
(Where V_offset is the combination of V_offset (datasheet) + I_bias * R_input)

***
Btw:
Why do you design a "VGA" and then use external feedback resistors? This will kill the selected gain.
It may be correct... I don`t know...

Klaus
 
It is about correct. Suppose your opamp has infinite gain. The non-inverting input is at 1.65v. So, the feedback and the infinite gain of the opamp will force the inverting input to be at 1.65V, which means there is no current flowing through Rn, because it sees 0V across. Consequently, no current in the feedback resistor Rf. In this case your amplifier works as a follower and the output is also at 1.65.
With finite gain you can expect a bit of difference from 1.65V.
 
Last edited:
Hello,
Thank you.
Now can you tell me if i provide 1.65 V to both inputs and extra 1 uV as a differential voltage to the inverting input and gain is 1000. So output will be 1.649V ?

Sorry for my mistake. That VGA block is Rail-to-Rail Amplifier and then i gave feedback.
 

Yes, if we call Vadd the added voltage to the input of Rn with respect to Vin (that is the voltage applied to the non inverting pin), then

Vout = Vin - Rf/Rn*Vadd

Ideally if Vadd=0 then Vout=Vin regardless the gain.

I don't understand why you call this configuration "VGA" that stands for "Variable Gain Amplifier".
 

A more complete expression for this situation is:

Vout = 1.65[1/(1+(1+Rf/Rn)/A)] - dVi(Rf/Rn)

where dVin is your 1uV above 1.65
 

A more complete expression for this situation is:

Vout = 1.65[1/(1+(1+Rf/Rn)/A)] - dVi(Rf/Rn)

where dVin is your 1uV above 1.65

Thank you.. using this formula and simulation i am getting almost same answer.

- - - Updated - - -

Yes, if we call Vadd the added voltage to the input of Rn with respect to Vin (that is the voltage applied to the non inverting pin), then

Vout = Vin - Rf/Rn*Vadd

Ideally if Vadd=0 then Vout=Vin regardless the gain.

I don't understand why you call this configuration "VGA" that stands for "Variable Gain Amplifier".

Thank you...
I called it VGA because what i have designed its VGA. I used combinations of different switches and resistors to realize different gains in the feedback. loop.
 

Hi,

The confusing is:
You named the "OPAMP symbol" = "VGA".
Thus we assumed this (just the symbol) is the VGA.

But the "whole circuit including the resistors" is the VGA.

***
Next time you could avoid this confusion by:
* Using a headline on the whole circuit "VGA"
* Naming the symbol "OPAMP"
* tell us that Rn and/or Rf is variable. Or - more realistic - show the switches and different resistors.

Klaus
 

I agree with confusion regarding the terminology. You haven't explained what's "Variable" about the VGA, are you varying the resistors? Gain of 1000 is a lot and will take care in many respects to get it to perform well.


As to your specific question I wouldn't be surprised at all by the result your seeing. Offsets can easily explain that. Keep testing it and see if it performs in the other ways you expect.
 

I agree with confusion regarding the terminology. You haven't explained what's "Variable" about the VGA, are you varying the resistors? Gain of 1000 is a lot and will take care in many respects to get it to perform well.


As to your specific question I wouldn't be surprised at all by the result your seeing. Offsets can easily explain that. Keep testing it and see if it performs in the other ways you expect.


Sorry for the confusion. I am varying feedback resistors. I have used 7 resistors and they all are connected parallel, with these 7 resistors I am getting 7 different gains. and this resistor combination depends upon switches. see the image.View attachment logic.pdf
D1 to D7 are voltages which will switch on the TG.
i achieved 109.7 dB open loop gain by using rail-to-rail amplifier.

@KluasST: Sorry for that...
I will keep in mind from the next time..
 

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