From: Winfield Hill
Subject: Re: Voltage error caused by input bias current
Date: 1 Jan 2003 07:17:12 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.20
Bill Sloman wrote...
> Stefan Salewski Salewski@PHYSnet.Uni-Hamburg.de wrote
>> I try to understand the effect of input bias current
>> to the output voltage of an inverting OpAmp amplifier.
>> I have a simple noninverting OpAmp amplifier as shown
>> on page 193 (Figure 4.30) in "The Art Of Electronics":
>> Noninverting input is connected to ground, signal is
>> connected with R1 to inverting input, and inverting
>> input is connected to output with R2, so gain is -R2/R1.
>> On page 194 Mr. Horowitz and Mr. Hill writes:
>> In this circuit the inverting input sees a driving
>> impedance of R1 || R2, so the bias current produces a
>> voltage Vin = IB (R1 || R2), which is then amplified by
>> the gain at dc, -R2/R1.
> You've used the wrong gain - the "noise gain" of an inverting
> op amp is (R2+R1)/R1 - the gain of the circuit seen as a
> non-inverting amplifier, and this should be applied to all
> voltage changes appearing at the op-amp inputs, as opposed
> the free end of R1.
Thanks Bill, looking at our sentence (written many years ago),
I see it's not only misleading, but wrong. Properly applying
the non-inverting gain yields Vo = Ib R2, the correct answer.
>> I am not absolutely sure if I have understood this correct.
>> In my simple mind the inverting input of the OpAmp should
>> be at ground potential too because of the large differential-
>> mode voltage gain of the OpAmp. So nearly all bias current
>> have to flow through R2, and the voltage offset at the output
>> should be just Vout=R2*IB. Indeed I have found a less popular
>> german book with this expression.
That's correct, and an easier way to view the issue.
> This is an easier way of getting the right answer, but Win and
> Paul Horowitz have other pedagogic points to make, and they need
> the concept of "noise gain" for predicting other non-idealities.
The use of noise gain wasn't a popular approach 20 years ago (it
appears only once in our book) but it's become widely used in the
last 10 years, especially for analyzing transimpedance amplifiers,
and we'll introduce it earlier in the 3rd edition.