[mendiola]

Dear All:

To achieve a gain of 5 Can I use a Rf=5000 ohm and Ri=1000 ohm or Do I have to use Rf=1000 Ohm and Ri=250 Ohm.?

I will use the R9 resistor = 1.6K to reduce the DC offset voltage.

Schematic: (TPA6120A2 Evaluation Board)

Gain = ( 1 + (Rf/ri))

Regards.
Alfredo Mendiola Loyola
Lima, Peru

 

[jcx]

for CFA op amps compensation, "speed" depends on the value of the feedback R
 
using 5k Rf will slow the chip, make it a little more stable, but applies less feedback possibly giving a little more distortion
 
the +,- inputs of CFA op amps are not the same, balancing Z seen at +,- inputs doesn't help offset as much as it does with VFA bipolar input op amps
 
lowering the + input R to gnd value does reduce Voffset - but is a heavier load for your source
 
raising the feedback R increases Vos from the - input bias current
 
the THS6012 data sheet is invaluable too - more DC specs - they are the same chip
 
 
for headphones that the TPA6120 is a good choice to drive the Vos shouldn't be a problem but it isn't the chip's best performance spec - a DC servo, output blocking cap or a better DC spec input op amp in multiloop around the TPA as output stage all would be better on Vos

 

[jasonhanjk]

1. mendiola
2.  

No.
 
You should gain up using opamp at the front stage.

 

[peranders]

When you increase the feedback resistor you will get more instability. Check also the datasheet.

 

[jcx]

simply wrong there - for CFA the feedback R directly enters the GBW product equation - the larger  the R the slower and more nearly single pole the open loop gain - increasing feedback R overcompensates CFA op amps
 
the effect you may see and misinterpret is that CFA are very sensitive to parasitic C on the -input, particularly any output to -input parasitic C
 

http://www.ti.com/lit/ml/sloa080/sloa080.pdf

The manufacturer determines the optimum value of RF during the characterization of the
IC. Referring to Figure 8–6, it is seen that when RF exceeds the optimum value recommended
by the IC manufacturer, stability increases. The increased stability has a price
called decreased bandwidth.