> From: Lindy Mayfield[SMTP:Lindy.Mayfield@EUR.SAS.COM] > Sent: Sunday, June 13, 2004 2:59 PM > To: PICLIST@MITVMA.MIT.EDU > Subject: [EE]: Op-Amp question: what is "ac-coupled"? > Hello. I'm trying to learn about op-amps and am reading Chapter 4 > from The Art of Electronics, 2nd Ed., 4.05 where it says, "If the > signal source is ac-coupled, you must provide a return to ground > the input current." > I cannot seem to find anywhere where it is defined what ac-coupled > means. Could someone please clarify this for me? > Thanks, Lindy The term "AC coupled" can be defined in two ways. First, it means that the circuit does not pass direct current. Only alternating current is passed ("coupled") into the amplifier. The second way to define it is in terms of how it is implemented. That is, an AC coupled circuit requires all input to pass through a capacitor (mostly, that is, since AC coupling can be done with parallel inductors, but that is rare). Many amplifiers, especially high gain ones, are AC coupled. Having the gain be low at zero frequency reduces problems caused by amplifying the inevitable DC drift present at the input of all amplifiers. If the input offset voltage of the amplifier changes by 1 millvolt over time, and the gain at DC is 1000, then the output will change by 1 volt. Rolling off the gain at DC to 1 will reduce the change at the output to an unamplified 1 mV while allowing the gain to remain 1000 at all frequencies above the low frequency cutoff. Using a capacitor to reduce output drift in this way causes other problems in some cases. If the amplifier is an op amp, its inputs require a certain DC current to flow into them (the "input bias current") in order for the amplifier to work. A capacitor in series with the input may block this essential current. You will have to make sure that the AC coupling scheme you use will still allow DC current to flow to the inputs without amplifying the DC part of the input or the amplifier's offset drift. John Power -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body