EECS 105 Spring 2004 Lecture 39 Lecture 39 Intro to Differential Amplifiers Prof J S Smith Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 39 Prof J S Smith Context Next 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 Berkeley 1 EECS 105 Spring 2004 Lecture 39 Prof J S Smith Reading z z All of the reading assignments are done Time to start reviewing for the final Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 39 Prof J S Smith Lecture Outline z z Department of EECS Why differential signaling important and becoming more important Introduction to differential amplifiers University of California Berkeley 2 EECS 105 Spring 2004 Lecture 39 Prof J S Smith Device Matching z One 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 gatesource 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 Berkeley EECS 105 Spring 2004 Lecture 39 Prof J S Smith Variations The transistors will generally vary due to several causes z Temperature very similar for devices on the same substrate z Implant variations important for small devices z Variations in width important for narrow devices z Variations in length important for short channel devices z Layout variations Often 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 z Department of EECS University of California Berkeley 3 EECS 105 Spring 2004 Lecture 39 Prof J S Smith Differential vs single ended signals z z z A voltage is only defined between two points Charge times voltage difference gives the energy needed to move a charge between two points A 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 Berkeley EECS 105 Spring 2004 Lecture 39 Prof J S Smith What is ground z z z In 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 At high frequencies and at low voltage swings which are needed for reasonable power consumption the variation of the ground can vary by more than the signal level This 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 Department of EECS University of California Berkeley 4 EECS 105 Spring 2004 Lecture 39 Prof J S Smith Differential signaling z z z z One 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 They needed to be routed together and twisted to avoid coupling magnetic fields from other wires or sources They should also be driven with the same impedance so they pick up the same noise voltages If 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 Berkeley EECS 105 Spring 2004 Lecture 39 Prof J S Smith Differential input z z z z z In order to use a differential signaling scheme we need to produce an amplified version of the difference between the input voltages This is called a differential amplifier The 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 If the inputs change in voltage together swinging in the same direction this is called common mode A good differential amplifier is designed to cancel the common mode called common mode rejection Department of EECS University of California Berkeley 5 EECS 105 Spring 2004 Lecture 39 Prof J S Smith Differential Amplifiers Differential amplifiers also solve some of the other problems that we have brushed aside so far like how to bias the input to the right quiescent voltage Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 39 Prof J S Smith The Differential Amplifier Concept The basic idea amplify the difference between two inputs and reject the common component vin1 vout1 vout2 vin2 vout diff Av diff vin diff Av diff vin1 vin2 large vout comm Av comm vin com Av comm vin1 vin2 2 small Department of EECS University of California Berkeley 6 EECS 105 Spring 2004 Lecture 39 Prof J S Smith Two single ended amps z One 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 vout1 A1vin1 vout 2 A2 vin 2 vout1 vout 2 A1vin1 A2 vin 2 vout A vin If the gain is the same A A1 A2 Department of EECS University of California Berkeley EECS 105 Spring 2004 Lecture 39 Prof J S Smith A First try at a Differential Amplifier Notice that this is just two common single transistor amplifiers with resistor pull ups Department of EECS University of California Berkeley 7 EECS 105 Spring 2004 Lecture 39 Prof J S Smith Common Mode z z The common mode of a differential signal is the average of the two voltages If we have a voltage v and a voltage v the differential voltage is v v v z And the common mode voltage is vCM 1 v v 2 Department of EECS EECS 105 Spring 2004 Lecture 39 z z z University of California Berkeley Amplify the difference not the common mode Prof J S Smith Since 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 Amplifying the common mode voltage wastes the room we have between the rails Minimizing the common mode will also improve our ability to correctly bias the following stages Department of EECS University of
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