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Berkeley ELENG 105 - Lecture 39 Intro to Differential Amplifiers

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1Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39Lecture 39: Intro to Differential AmplifiersProf J. S. SmithDepartment of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithContextNext week is the last week of lecture, and we will spend those three lectures reviewing the material of the course, and looking at applications of the material.Today we are going to look at the basics of differential amplifiers.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithReadingzAll of the reading assignments are donezTime to start reviewing for the final!Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithLecture OutlinezWhy differential signaling important, and becoming more importantzIntroduction to differential amplifiers2Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithDevice MatchingzOne of the things that we depend on in the design of analog integrated circuits is device matching. For example, if we make a current mirror, we are depending on the reference and the mirror device behaving is a very similar fashion. When a gate-source voltage is developed on the reference device, passing a given current, the same voltage appearing across the gate to source on the mirror device will allow the same drain current.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithVariationsz The transistors will generally vary due to several causes:z Temperature→very similar for devices on the same substratez Implant variations→important for small devicesz Variations in width→important for narrow devicesz Variations in length→important for short channel devicesz Layout variationsOften, analog devices will not be minimum sized devices, so that output resistances will be lower, short channel effects will be smaller, and the effect of variations is reduced.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithDifferential vs single ended signalszA voltage is only defined between two pointszCharge times voltage difference gives the energy needed to move a charge between two points.zA single ended signal is actually a misnomer, because all voltages are measured between two points, its just that single ended signals use “ground” as a reference. Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithWhat is “ground”zIn a low frequency circuit, with a big ground plane available, it is possible to approximate that ground is a stable reference which all voltages can be measured with respect to. zAt high frequencies, and at low voltage swingswhich are needed for reasonable power consumption, the variation of the “ground” can vary by more than the signal level.zThis is even a problem at low frequencies, if devices are separated by any distance. “ground” at one device can easily be several volts different than “ground” at another device. (DC and AC)3Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithDifferential signalingzOne solution to this problem is to use differential signaling, that is two wires, close to each other, and in a symmetric configuration, carry the signal as a voltage difference.zThey needed to be routed together and twisted to avoid coupling magnetic fields from other wires or sourceszThey should also be driven with the same impedance, so they pick up the same noise voltageszIf all of these things are done, then noise picked up by the wires is picked up by both in the same amount and with the same sign→”Common mode”Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithDifferential inputzIn order to use a differential signaling scheme, we need to produce an amplified version of the difference between the input voltageszThis is called a differential amplifier.zThe output of a differential amplifier can either itself be differential, or it could convert the signal into a single ended version with respect to ground.zIf the inputs change in voltage together, swinging in the same direction, this is called common modezA good differential amplifier is designed to cancel the common mode: called common mode rejection.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithDifferential AmplifiersDifferential amplifiers also solve some of the other problems that we have brushed aside so far- like how to bias the input to the right quiescentvoltage.Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithThe Differential Amplifier ConceptThe basic idea: amplify the difference between two inputs and reject the common component +_+_vin2vin1vout2 vout1 _ + +vout,diff= Av,diff(vin,diff)=Av,diff(vin1– vin2) … largevout, comm= Av,comm(vin,com)=Av,comm[(vin1+ vin2)/2]…small4Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithTwo single ended ampszOne way you could think of making a differential amplifier is to use two separate single ended amplifiers, as we have been studying for the last several weeks. If the devices happened to be identical voltage amplifiers, we would have:(If the gain is the same A= A1=A2)inoutininoutoutinoutinoutvAvvAvAvvvAvvAv∆=∆−=−==221121222111Department of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithA First try at a Differential AmplifierNotice that this is just two common single transistor amplifiers withresistor pull upsDepartment of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithCommon ModezThe common mode of a differential signal is the average of the two voltages.zIf we have a voltage v+and a voltage v-the differential voltage is:zAnd the common mode voltage is:−+−=∆vvv)(21−++= vvvCMDepartment of EECS University of California, BerkeleyEECS 105 Spring 2004, Lecture 39 Prof. J. S. SmithAmplify the difference, not the common modezSince the information is carried only by the difference between the two voltages, and the common mode voltage represents noise, we would prefer not to amplify it.zAmplifying the common mode voltage wastes


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Berkeley ELENG 105 - Lecture 39 Intro to Differential Amplifiers

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