You should get app note AN-72 from National Semiconductor. It's title is "the LM3900: A New Current-Differencing Quad of +/- Input Amplifiers". The first important thing to know is that the inputs are never connected directly to a voltage source. Second, they are Single supply devices with both inputs one diode drop above ground.
In the simplest version, a resistor from the positive supply is connected to the + (plus) input to Set the input operating current. Then a resistor is connected from the ouput to the - (minus) input. For operation with the output at one-half the supply voltage, this resistor is half the value of the resistor connected to the + (plus) input. The DC operating point is set when the currents into the inputs are the same. To connect a signal to one of the inputs, it must be a current source. This can be as simple as a voltage driving a resistor that is connected to one of the inputs. The current that is generated thru the resistor creates a current difference between the inputs which is reflected in the output as a voltage change. I'll say it again, the inputs are never connected directly to a voltage source because both inputs are one diode drop from ground. There are two transistors connected as a current mirror and a third one following them to sense the voltage at the output of the current mirror. A matched triplet of transistors. I hope this helps. -=-=-=-=-=- "http://www.dibsplace.com/design/"; -=-=-=-=-=- DIBs Electronic Design | Dave Baldwin Adapt and overcome. | Buy a book and read it. -=-=--=-=--=-=--=-=--=-=-=-=--=-=--=-=--=-=--=-=--=-=-=-=- -- Author: Dave Baldwin INET: [EMAIL PROTECTED] Fat City Hosting, San Diego, California -- http://www.fatcity.com --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
