I'd use an instrumentation amp chip, but have a look at http://www.hallikainen.org/cuesta/et113/InstrumentationAmpAnalysis.pdf . The basic diff amp is from points F and J to the right. Adding the stuff to the left makes it an instrumentation amp. Looking only at the circuitry to the right of F and J, the 1K resistors can be called R1 and the 2K called Rf. The gain is then Rf/R1. Note that the differential amp is a single amp being used as both an inverting and non-inverting amp. The gain of the inverting side (from J to the output) is -Rf/R1. The gain from the non-inverting input of the op amp to the output is 1+(Rf/R1). The voltage divider on the non-inverting input drops this gain down to Rf/R1 so it matches the inverting side. Looking to the left side of the circuit, if we call the 2K resistors Rf and the 1K R1, the gain is 1+(Rf/(R1/2)) or 1+(2Rf/R1). This gain gets multiplied by the Rf/R1 of the right side to get the overall gain. If you use JUST the right side of the circuit (the diff amp instead of the instrumentation amp), the input resistance is low and unbalanced. The instrumentation amp has a high input resistance, the input is balanced, and the CMRR is higher. Due to the precision of resistor matching required to get a reasonable CMRR (CMRR=Ad/Acm), again, I'd go with an instrumentation amp on a chip. Harold On Thu, 25 Oct 2001 14:55:52 -1000 Jay Hanson writes: > Can anyone point me at a simple differential amplifier circuit using > a > common operational amplifier chip and a single +12 volt supply? I > want to > amplify the difference between two photodiodes. Frequency in the > kilohertz > range. > > Tnx, > Jay > > -- > http://www.piclist.com hint: The list server can filter out > subtopics > (like ads or off topics) for you. See http://www.piclist.com/#topics > > FCC Rules Online at http://hallikainen.com/FccRules Lighting control for theatre and television at http://www.dovesystems.com ________________________________________________________________ GET INTERNET ACCESS FROM JUNO! Juno offers FREE or PREMIUM Internet access for less! Join Juno today! For your FREE software, visit: http://dl.www.juno.com/get/web/. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.