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

curr

ent (n

A)

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 = Vin

Av

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