EE 4345 – Semiconductor Electronics Design Project · EE 4345 – Semiconductor Electronics Design Project Transistor Current Sources and Active Loads Anuj Shah Himanshu Doshi Jayaprakash

EE 4345 – Semiconductor ElectronicsDesign Project

Transistor Current Sources and Active Loads

Anuj Shah

Himanshu Doshi

Jayaprakash Chintamaneni

Nareen Katta

Nikhil Patel

Preeti Yadav

�Current sources made by using active devices have come to be widely used in

analog integrated circuits as Biasing elements and as load devices for amplifier

stages

�The use of current sources in biasing can result in superior insensitivity of circuit

performance to power supply variations and to temperature

Current Sources

A simple 2-transistor current source

Simple current sourceSince Q1 and Q2 have the same base-emitter voltage.

Their collector currents are equal, Ic1=Ic2

Summing currents at the collector of Q1 gives,

Thus, For identical devices Q1 and Q2, the output and the referenceCurrents are equal

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Collector characteristics for an npn transistor

Norton equivalent representationOf a transistor current source

Thevenin equivalent Representation of aTransistor current source

Simple current source with current gain

The collector current Ic2 is different form the reference Current by afactor [ 1 + ( 2/ßf )]

The emitter current of the transistor Q3 is given by,

Summing currents at the collector of Q1,

Since Ic1 and Ic2 are equal,

Thus, the reference and the output current differ only by a factor of 1/ßf

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Widlar current source

Widlar current source

Assuming that the transistors are identical and that Vce2 is high enough

to allow Q2 to operate in the active region,The collector current of Q2 is

given by,

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Voltage equation around the base-emitter loop gives,

Solving for R2 gives

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Design a widlar current source , given Vcc=15volts. Assume identicaltransistors.Also, design a 10uA current mirror and compare total resistancerequired in the two circuits.

Let Iref =1mA

Design of current mirror,

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Cascode current source with bipolar transistors

Performing a full small-signal analysis on this circuit, including the finite

small-signal resistance of Q1 and Q4

We obtain, Ro =ß0 r0/2

Bipolar Wilson current source

Conventional Differential Amplifier

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� Resistors are used as load elements

� Large Voltage Gain – requires large power-supply voltage and

large values of resistors

Example:

� A voltage gain of 500 would require

and , Rc would have to be

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Current Sources as Active Load

�Common-Emitter Amplifier with Active Load

�Emitter-Coupled Pair with Active Load

�Input Offset Voltage of the Emitter-Coupled Pair with

Active Load

�Common-Mode Rejection Ratio of the Emitter-Coupled

Pair with Active Load

�Common-Source Amplifier with Active Load

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Fig.Common-emitter amplifier with active load

Transistors NPN PNPInitially at Vi=0 at point 1 Off SaturatesAs Vi Increases at point 2 On On at point 3 On On at point 4 Saturated Saturated

Fig.npn collector characteristicwith pnp Load line superimposed

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Fig.dc transfer characteristics Ofcommon-emitter Amplifier with activeload

�Typical values for voltage gain are 1000 to 2000 range.

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Fig.Small-signal equivalent Circuit forcommon-emitter Amplifier with activeload

�Drawback:

The quiescent value of the common-mode output voltage is very

sensitive to the value of the emitter-biasing current source and

the active-load current sources.

Fig.Emitter-coupled pair withActive load

Fig.Common-mode half-circuit for emitter-coupled pair with active load

�Output voltage Voc is very sensitive to the voltage at the base of Q6,

which is influence by Iref2.

�Example:

If Iref1 and Iref2 are different by 4% , the output voltage Voc will change by

about 2V.

The same change result from a 1mV mismatch between Q6 And Q7.

�This circuit eliminates the common-mode problems but provides

a single output with much better rejection of common-mode

Input signals.

Large signal behavior model

�Load resistance is zero.

�Vbias is adjusted in order to keep Q2 and Q4 in forward Active region.

Fig.Active-load stage with output connected to a voltage source

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Device Mismatch Effects:

� Presence of Component mismatches within the Amplifier itself and drifts

of component values with Temperature produce differential voltages at the

output that are indistinguishable from the signal being amplified.

� For Transistor Differential Amplifiers the effect of mismatches is represented

by two Quantities,the input offset voltage and the input offset current.

Circuit Containing Mismatches Equivalent Circuit with identically matched devices

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Input Offset Voltage is given by the expression

where mVqkTVT 26�� at 300�K

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Wb(VCB) is the base width of the function VCB

NA is the acceptor density in the base and A is the emitter Area

Input Offset Voltage of the Emitter Coupled Pair with Active Load

� For a Resistive Load the Input Offset Voltage arises mainly due to the

mismatches in Is in the Input Transistors and from mismatches in the

collector load Resistors.

� For an Active Load the input offset voltage results from mismatches in the input Transistors and load devices and from the base current of the load devices.

Emitter Coupled Pair with Active Load

Input Offset Voltage of the Emitter Coupled Pair with Active Load

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The Collector Current Ic4 is related to Ic3 by

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The Current Ic1 equals (-Ic3),plus the base currents in the pnp transistors

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In a Worst Case, the Offset Voltage is 0.2VT

�These Circuits have significantly higher offset Voltage than the

Resistively loaded case

�The offset Voltage can be reduced by inserting resistors in the Emitters of

Q3 and Q4.

Actively Loaded Emitter Coupled Pair for improved Offset

Common-Mode Rejection Ratio of the Emitter Coupled Pair withActive Load

� CMRR is defined as the Magnitude of the ratio of differential to Common Mode Gain.

� Provides conversion from a differential signal to a signal that is referenced to ground.This type of Conversion is required in all differential input,single-ended output amplifiers.

�Simplest differential to single-ended converter is the resistively Loaded emitter coupled pair.

Differential to Single ended ConversionUsing resistively loaded Emitter coupled Pair

Differential to Single ended ConversionUsing actively loaded Emitter coupled Pair

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Transistor Q3 operates at twice the current of Q1 and Q2

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CMRR of the resistively loaded stage is the inverse of the � of the current source Transistor when a simple current source is used as a biasing element.

For an active-loaded case

�In case of Resistively loaded case ,changes in the common Mode input will cause

changes in the bias current IEE because of the finite output resistance of the biasing

current source.This will cause a change in Ic2 and an identical change in Ic1.

�Because of the active load ,the change in Ic1 will produce a change in the currents

flowing in the pnp load transistors ,which produces a change in the collecter current

of Q2.So the output does not change at all in response to common Mode Inputs.

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Common-Source Amplifier with Active Load

Common Source Amplifier using an NMOS driver and PMOS activeload.

When M1 and M2 are forward active,the small signal gain is

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Gain for NMOS depletion-load transistor

Common-Source Amplifier with p channel transistorCurrent source load

Transfer Characteristic

I-V Characteristic of n-channel depletion mode load transistor

Common Source Amplifier with depletion mode transistor load and dc transfer characteristic

Common Source Amplifier with Enhancement-Mode load

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Common-Source Amplifier with enhancement-mode load and transfer characteristic

Source Coupled Pair with Active load

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� Widely used in CMOS Circuit Design

References:

� Analysis and Design of Analog Integrated Circuits, 3rd Edition by Paul R.Gray and Robert G.Meyer.

�Analog integrated circuit design,1st edition by David Johns and Ken Martin

� Electronics,2nd Edition, by Allan R.Hambley