Differential amplifier - Coupling capacities needed?

[arefeidi]

amplifier coupling

Hi,

I'm building up a differential amplifier like this one: http://www.ecircuitcenter.com/Circuits/adcin_diff_se/adcin_diff_se1.htm (With Vref=0V, so no additional Offset voltage)

Vpos is an AM-modulated 50kHZ sine wave, Vneg is exactly the same signal with 180° phase shift. Someone said to me that I need to have coupling capacities at the input, though - I don't understand why.

If there is a DC offset in the input signals, doesn't it get removed by the differential amplifier anyway? Vout=(R2/R1)*Vpos-Vneg
So what would be the reason to add coupling capacities at both inputs?

 

[rameshchand]

amplifiercoupling

hi
here i have attached ur request

 

[arefeidi]

ac coupling differential amplifier

Thanks for the link, but that doesn't really help, as I'm not searching for an IC.

I'm builing my own differential amplifier with an op amp (TL071). What I need to know is, does it make sense to use coupling capacities at its input - and if so, why does it make sense?

 

[arefeidi]

Noone out there who could help me?

 

[Old Nick]

I assume you're designing a PCB for this, so why don't you just include the capacitors in your design, and then you have the choice to implement them in your cicuit, or short across them.

What the capacitors will do is give you a greater dynamic range at the input of the amplifier. The op-amp you're using will have a Vin min and max, a DC offset could place your singmal close to one of the input rails, thus giving you little dynamic range. Using AC coupling the inputs will give you the full input rail-rail range for your signal to swing across.