30
Differential Amplifiers Prof. Paul Hasler
Differential Amplifiers
Prof. Paul Hasler
Differential Transistor Pairs
MOSFET Diff-Pair BJT Diff-Pair
Differential Transistor Pairs
MOSFET Diff-Pair BJT Diff-Pair
The bottom transistor (the one with Ibias) sets the total current
Differential Transistor Pairs
MOSFET Diff-Pair BJT Diff-Pair
The bottom transistor (the one with Ibias) sets the total current
The upper two transistors compete for a fraction of this current
Analysis of Diff-Pair
Analysis of Diff-Pair
Analysis of Diff-Pair
Analysis of Diff-Pair
Analysis of Diff-Pair
Analysis of Diff-Pair
Source of Common-Mode Gain
Analysis of Diff-Pair
Analysis of Diff-Pair
Differential Pair Currents
-0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.40
0.5
1
1.5
2
2.5
3
Differential input voltage (V)
Outp
ut c
urre
nt (nA)
Iout+Iout
-
Transistor ‘Leg’ Currents
Transistor ‘Leg’ Currents
BJT Differential Pair Analysis
V1 V2
Vb
GND
Iout Iout+ -
BJT Differential Amplifier
V1 V2
Vb
GND
Iout Iout+ -
BJT Differential Amplifier
V1 V2
Vb
GND
VddVdd
Vout
Iout
BJT Differential Amplifier
V1 V2
Vb
GND
Fixed output voltage: Current output
Zero output current: Voltage output
VddVdd
Vout
Iout
Basic Differential Amplifier
Transfer Functions
Wide-Output Range Differential Amplifier
Simple differential amplifer
"Vmin" effect
Wide-Output Range Differential Amplifier
Wide-output range amplifier
Simple differential amplifer
"Vmin" effect
Wide-Output Range Differential Amplifier
Wide-output range amplifier
Simple differential amplifer
"Vmin" effect
A Unity-Gain Follower
A Unity-Gain Follower
A Unity-Gain Follower
A more ideal Follower / Buffer Circuit
Amp Gain = Av
Solving: Vout = VinAv
1 + Av
A Unity-Gain Follower
Amp Gain = Av
Solving: Vout ~ Vin
A Unity-Gain Follower
Amp Gain = Av
Solving: Vout ~ Vin
Differential Pairs / Amplifiers
• Differential Transistor Pairs• Differential Amplifiers• Unity-Gain Amplifiers
