6.012 - Microelectronic Devices and Circuits Lecture 19 - Differential Amplifier Stages - Outline Announcements Design Problem - coming out tomorrow; PS #10 looks at pieces; neglect the Early effect in large signal analyses Review - Single-transistor building block stages Common source: general purpose gain stage, workhorse Common gate: small R in , large R out , unity A i , same A as CS v Source follower: large R in , small R out , unity A v , same A i as CS Series and Shunt feedback: we'll see in special situations Differential Amplifier Stages - Large signal behavior General features: symmetry, inputs, outputs, biasing (Symmetry is the key!) Large signal transfer characteristic Difference- and common-mode signals Decomposing and reconstructing general signals Half-circuit incremental analysis techniques Linear equivalent half-circuits Difference- and common-mode analysis Example: analysis of source-coupled pair Clif Fonstad, 11/17/09 Lecture 19 - Slide 1
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Announcements Design Problem - coming out tomorrow; PS #10 looks at pieces;
neglect the Early effect in large signal analyses
Review - Single-transistor building block stages Common source: general purpose gain stage, workhorse Common gate: small Rin, large Rout, unity Ai, same A as CSv Source follower: large Rin, small Rout, unity Av, same Ai as CS Series and Shunt feedback: we'll see in special situations
Differential Amplifier Stages - Large signal behavior General features: symmetry, inputs, outputs, biasing (Symmetry is the key!) Large signal transfer characteristic
Difference- and common-mode signalsDecomposing and reconstructing general signals
Half-circuit incremental analysis techniquesLinear equivalent half-circuits Difference- and common-mode analysisExample: analysis of source-coupled pair
Clif Fonstad, 11/17/09 Lecture 19 - Slide 1
IBIAS
-V
+V
1
2
3
IBIAS
-V
+V
1
2
3
Linear amplifier layouts: The practical ways of puttinginputs to, and taking outputs from, transistors to form linear amplifiers
There are 12 choices: three possible nodes to connect to the input, and for each one, two nodes from which to take an output, and two choices of what to do with the remaining node (ground it or connect it to something).
Not all these choices work well, however. In fact only three do:
Name Input Output Grounded Common source/emitter 1 2 3
Common gate/base 3 2 1
Common drain/collector 1 3 2 (Source/emitter follower)
(see text for details of analysis) Clif Fonstad, 11/17/09 Lecture 19 - Slide 10
Diff. Amps: large signal analysis of emitter coupled pairs, cont.
Results: The outputs only depend on the difference between the inputs, (vI1 - vI2):
Slope around origin = -gmRC
!
vO1 = VCC "#F RC IBIAS
1+ e"q v I 1"v I 2( ) kT[ ]
vO2 = VCC "#F RC IBIAS
1+ eq v I 1"v I 2( ) kT[ ]
vO = "#F RC IBIAS tanhq vI1 " vI 2( )
2kT
Symmetrical
Clif Fonstad, 11/17/09 Only the difference in the inputs matters!! Lecture 19 - Slide 11
_______________________________________
Differential Amplifier Analysis - difference-mode and common-mode signals
Any pair of signals can be decomposed into a portion that is the identical in both, and a portion that is equal, but opposite in both. For example, if we have two voltages, v1 and v2, we can define a common-mode signal, vC, and a difference-mode signal, vD, as: vC = (v1 + v2)/2 vD = v1 - v2 In terms of these two voltages, we can write v1 and v2 as:
v1 = vC + vD/2 v2 = vC - vD/2
In incremental analysis of linear amplifiers we will decom-pose our inputs into difference- and common-mode inputs:
vic = (vin1 + vin2)/2 and vid = vin1 - vin2. We will apply vid to the circuit and get vod (= Avdvid), and
then apply vic to the circuit to get voc (= Avcvic). Then we will reconstruct our outputs: