741 Op-AmpWhere we are going:

Source Degeneration

GND

Vout

Vin

GND

Vout

Vin

CircuitElement

Why do this?

• Higher Linearity• Possible Stability

Why not do this? gm

• Lower Bandwidth• Higher Noise / f

Source Degeneration

GND

Vout

Vin

GND

Vout

Vin

V1

Neglect VA of Q1 and assume matched devices:

Q1

II = Ieo e

V1 /UT = Ieo e(Vin - V1 + Vout/Av )/UT

2 V1 = Vin + Vout / Av

I = Ieo e(Vin + Vout/Av )/(2 UT)

A similar result for MOSFETs

Common Emitter

Amplifies the input signal at the output

Ibias 100A

Vdd

GND

Vout

Vin

Common Emitter / Common Source

Assuming an ideal current source:

Ibias = Ico eVin/UT eVout /VA

Vout = -VA ln(Ibias/Ico) + ( VA / UT) Vin

Common Drain

Amplifies the input signal at the output

Vout = ( VA / UT) Vin

Ibias = Ibias eVin/UT eVout/VA

100pA

Vdd

GND

Vout

Vin

Ibias

Input conductance = 0

Common Drain

We must account for the other current source:

Vout = ( (VAn // VAp) UT) Vin

Id = Ibias e-Vout/VAp

= Ibias eVin/UT eVout/VAn

Vb

Vdd

GND

Vout

M6

M7Vin

Ibias

Common-Drain: Amplifier Measurements

V1

GND

Ibias

Vdd

GND

Vout

Mb

M6

M7

Common Drain

What about above-threshold operation:

Amplifies the input signal at the output

Ibias = (K/2) ( Vin - VT )2 (1 + (Vout/VA) )

100A

Vdd

GND

Vout

Vin

Ibias

Operating region decreases (Vout > Vin - VT)

Derive using quadratic functions:

Vout = VA( - 1)

Common E / S: Resistive Load

High-Gain Amplifier ExperimentsLoad-line Analysis

Common Base

Amplifies the input signal at the output (non-inverting gain)

Ibias 100A

Vdd

Vout

Vin

Common Base / Common Gate

Assuming an ideal current source:

Ibias = Ico e (Vb -Vin )/UT eVout /VA

Vout = -VA ln(Ibias/Ico) + (VA / UT) Vin (VA / UT) Vb

Vb

Gain = VA / UT = Av

Common Gate

Using a subthreshold MOSFET :

100pA

Vdd

Vout

Ibias

Vin

Vb

Ibias = Io e (Vb -Vin )/UT eVout /VA

Vout = -VA ln(Ibias/Io) + (VA / UT) Vin ( VA / UT) Vb

Gain = VA / UT = Av

Problem: Large input current

Common G: Resistive Load

Cascode CircuitsUse a common-gate/base transistor to: 1. Improve the output resistance of another transistor.2. Reduce the Gate-to-Drain capacitance effect of another transistor.

Input resistance of common-gate is low Source is nearly fixed if connected to the drain of a transistor

Vdrain

Vin

GND

V1

Vgate

Cascode CircuitsVdrain

Vbias

GND

V1

Vgate

Fixes the voltage at V1 or isolates V1 from the output

GND

Vgate

Vdrain

Idrain = Io e (Vbias -V1 )/UT eVdrain /VA

= Io e Vgate/UT eV1 /VA V1 ~ Vbias - Vgate + (UT/VA) Vdrain

Drain is fixed

Idrain = Io e Vgate/UT e Vbias /VA eVdrain / (Av VA )

Cascode Common-Drain Amp

GND

Ibias

VbMb

GND

V1

Vdd

Vout

biasp

biasn

One Pole

HighOutput

Resistance / DC Gain

BJT Cascode Configuration

MOS Cascode Circuit

BJT - CMOS Cascode Circuits

Preserve High-gm/I

Cascade Configurations

Cascade Connection: Rout

BJT-MOS Cascades

A good way to get zero base current….

Cascades: More stuff