E3 238 Analog VLSI Circuits
Lecture 12: Emitter/Source Coupled Pairs
Gaurab Banerjee
Department of Electrical Communication Engineering,
Indian Institute of Science, Bangalore
banerjee@ece.iisc.ernet.in
Emitter Coupled Pair
From KVL around the input loop:
For each transistor, assume:
Identical
transistors
(1)
Emitter Coupled Pair
Also, let αF1= αF2 = αF. Then, from KCL at the emitters:
(2)
From (1) and (2):
=> output:
Emitter Coupled Pair: Linear Range
•
Linear input range limited to about +/- 3VT, about 75 mV
•
In practice, circuit behaves linearly when Vin < VT
•
Note: For Vid = 0, Vod = 0 -> important in DC coupling of stages
Emitter Coupled Pair: With Degeneration
Linear range increases as drop
across the emitter resistor
increases -> local negative
feedback
Source Coupled Pair
Apply KVL around the input loop:
Assume M1, M2 are in saturation
From the MOS I/V equations:
Source Coupled Pair
Apply KCL at the common source node -> Id1 + Id2 = ITAIL
After solving the Quadratic formula:
Source Coupled Pair
Range of input voltages over which M1, M2 operate in the active region
Large
overdrive =
Large input
range
Source Coupled Pair
•
Vov affects many other parameters in addition to input range: speed,
offset, output swing, etc. -> more later
•
For a fixed current, gm is maximized by minimizing overdrive
(analysis before) -> tradeoff between power and linearity
Differential Output Voltage:
Differential and Common Modes
Case 1:
Case 2:
•
Consider the simplified differential pair
•
Neglect gmbs, gds for simplicity
•
Assume R1=R2=R, gm1=gm2=gm
Differential and Common Modes
Now, assume mismatches, i.e., R1 ≠R2
Ideally zero
“mode conversion” products -> problematic in cascades of amplifiers