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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
3
Iout-
2.5
Iout+
Output current (nA)
2
1.5
1
0.5
0
-0.4
-0.3
-0.2
-0.1
0
0.1
Differential input voltage (V)
0.2
0.3
0.4
Transistor ‘Leg’ Currents
Transistor ‘Leg’ Currents
BJT Differential Pair Analysis
V1
I+
out
Iout
Vb
GND
V2
BJT Differential Amplifier
I+
out
Iout
V1
V2
Vb
GND
BJT Differential Amplifier
Vdd
Vdd
Iout
Vout
V1
V2
Vb
GND
BJT Differential Amplifier
Vdd
Vdd
Fixed output voltage:
Current output
Iout
Vout
V1
V2
Vb
GND
Zero output current:
Voltage output
Basic Differential Amplifier
Transfer Functions
Wide-Output Range Differential Amplifier
Simple differential amplifer
"Vmin" effect
Wide-Output Range Differential Amplifier
Simple differential amplifer
"Vmin" effect
Wide-output range amplifier
Wide-Output Range Differential Amplifier
Simple differential amplifer
"Vmin" effect
Wide-output range amplifier
A Unity-Gain Follower
A Unity-Gain Follower
A Unity-Gain Follower
Amp Gain = Av
Av
Solving: Vout =
V
1 + Av in
A more ideal Follower
/ Buffer Circuit
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