OpAmp Pre-Amplifier Design

Dear all,

Would you please advise the design criteria for using Op-Amps as Pre-Amp to maintain min. output offset.

I raise this question because I have encountered one problem. I have used a 1M res. as feedback res. and also a 1M res. to connect to Gnd. So, it is a unity gain Amp. When I install the OPA627AP, there is no problem, 0mV DC offset, OK ... Amen ...

But, when I changed all 1M to 10K, the gain is equal .... but the most terrible things happened.... The DC offset is 300mV! It also change when I turn the volume control (100k) in front of the +ve input of the OpAmp.

Also, when I changed the OpAmp from OPA627 to AD847... The DC offset go to 2.8V!!! Oh My God.

I guest it is because of the Input Bias Current of the OpAmp. However, how can I overcome this problem ??? Which values of resistors should I use? What's the design criteria should be adopted?

Thank you for your kind attention... Thx.
 
A general rule to remember when using op-amps is
to make sure that the total resistance seen by the
two inputs is more or less the same - if the
leakage currents into the two inputs are the same,
then the DC voltages at both inputs will be the
same and the output offset will (in principle) be
zero. You need to put in parallel the resistance
you add yourself between inputs and ground and
between inputs and the op-amp output, as well as
taking into account the current path through the
output of the preceding stage (normally you assume
that this is a short circuit to ground, as is the
op-amp's output pin). You may need to add a series
resistor to the appropriate input to get the
effective resistance right.

I hope this makes sense!

Alex
 
You should probably use values of feedback resistors far lower than Meg-ohm. The large resistors' 4KT will cause the current noise of the op-amp inputs to be higher than normal. If you want unity gain, try connecting the - input directly to the output to form a unity gain follower. Offset should not be much of a problem in this situation.

If you want to use feedback resistors R1 and R2 connected to the - input, make sure you also connect a resistor from the + input to ground. This resistor is known as Rp and should be equal to R1 and R2 in parallel. If R1=R2 (as in your case) simply use half -- if R1 and R2 are 10k, make Rp 5k.
Rachel said:
Dear all,

Would you please advise the design criteria for using Op-Amps as Pre-Amp to maintain min. output offset.

I raise this question because I have encountered one problem. I have used a 1M res. as feedback res. and also a 1M res. to connect to Gnd. So, it is a unity gain Amp. When I install the OPA627AP, there is no problem, 0mV DC offset, OK ... Amen ...

But, when I changed all 1M to 10K, the gain is equal .... but the most terrible things happened.... The DC offset is 300mV! It also change when I turn the volume control (100k) in front of the +ve input of the OpAmp.

Also, when I changed the OpAmp from OPA627 to AD847... The DC offset go to 2.8V!!! Oh My God.

I guest it is because of the Input Bias Current of the OpAmp. However, how can I overcome this problem ??? Which values of resistors should I use? What's the design criteria should be adopted?

Thank you for your kind attention... Thx.
 
Just to add to what has been said already, with a bipolar input op-amp there are two sources of output offset. One is input bias current, which can be different on the two inputs and the other is input offset voltage.

The maximum input offset voltage is typically a few mV. This is the difference in input V that is needed to cause the output V to be zero. This arises because of slight differences in the characteristics of the input transistors. If you set your feedback up so the dc gain is A, then when the op-amps inputs are equal there will be a dc offset on the output of Voffset x A. With JFET input op-amps this offset tends to be higher than for bipolars.

The input bias current is typically a few 10s of micro-amps and is mostly a problem when you have large dc resistances on the inputs. The difference between inputs tends to be very small, typically less than 1uA or so. To calculate the output offset you need to multiply the input bias current by the dc resistance "seen" at the input pin to ground (or op-amp output which should be considered as a ground for this calculation). Do this for both inputs and you will get two input offset voltages. These will have the same effect as the input offset voltages mentioned earlier. In general, you usually minimize your offset voltage by ensuring the dc resistance seen at both inputs is identical (thus making the voltage offsets almost the same so they cancel out). JFET op-amps tend to have near zero input bias current and this can be ignored.

The total output offset will be a combination of the two sources of input voltage offset. Simply add the effects up using worst-case values from the op-amp data sheet and this will give you the worst case output offset.

As Randy points out it is more usual to use smaller value resistors for the feedback loop to reduce offsets, usually in the range 1k to 100k. Note that connecting the volume pot directly to the op-amp input will cause the dc offset to vary with volume setting as the dc resistance to ground varies. You can overcome this effect most easily by using a JFET input op-amp. Otherwise you must isolate the pot with a capacitor and add a resistor from +'ve input to ground.