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Non Ideal Characteristics of

Differential Amplifier

Integrated Circuits

9/29/2010 HITECH University 1


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Introduction

Input offset voltage

Input Bias and offset currents

Input common mode range



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Input offset Voltage



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Conti

Both inputs are grounded.

If two sides are perfectly matched then

current I would split equally b/w the Q1 and

Q2, and Vo would be zero.

Practical circuits exhibit mismatches that

results in a dc output voltage Vo even with the

inputs grounded.

Vo is the output dc offset voltage.



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Input offset voltage Vos can be obtained

simply by dividing Vo by differential gain of

the amplifier.

Vos=Vo/Ad



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Second Case

If we apply a voltageVos Between the input

terminal of differential amplifier then output

voltage reduced to zero.

The offset voltage results from the

mismatches in the load resistance Rc1 and Rc2

and from mismatches in Q1 and Q2.



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Conti..

1 = (

)

2 =

Then

1 = (

)( (

)2 = (

)( (

)

Vo=Vc2-Vc1

As

Vos=

Vos=( (

)())/



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Conti

Now consider the effects of mismatches in

transistors Q1 and Q2, means in particular let

transistors have a mismatch in their emitter

base junction.

There are other possible sources for input

offset voltage such as mismatches in the

values of and r0.



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Solution

For compensating offset voltages it involves a

deliberate mismatch in the values of two

collector resistances such that the differential

output voltage is reduced to zero when both

input terminals are grounded.

Offset-nulling scheme



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Input Bias and Offset Currents

In a perfectly symmetric differential pair thetwo input terminals carry equal DC currents.

1 = 2 = (/2)/ 1

This is the input Bias current of the differential

amplifier.

Mismatches in the amplifier circuit and mostly

a mismatch in makes the two input currentsunequal. The resulting difference is the inputoffset current, Ios



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1 = 2 =

+

= 1 2

1=

2=

After calculation

Ios=

+(

)



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Input common mode range

The input common mode range of adifferential amplifier is the range of inputvoltage Vcm over which the differential pair

behaves as a linear amplifier for differentialinput signals.

The upper limit of the common mode range isdetermined by Q1 and Q2 leaving the activemode and entering the saturation mode ofoperation.



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Thus the upper limit is approximately equal to

the dc collector voltage of Q1 and Q2.

The lower limit is determined by the

transistors that supplies the biasing current I

leaving its active region of the operation and

no longer functioning as a constant current

source.



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Biasing in BJT IC

With the present IC technology it is almost

impossible to fabricate large capacitors and it is

uneconomical to manufacture large resistances.

Basically biasing in the integrated circuit design isbased on the use of constant current sources.

On a IC chip with a number of amplifier stages a

constant dc current is generated at one locationand then reproduced at various other locations

for biasing the various amplifier stages.



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Diode Connected Transistors

Shorting the base and the collector of the BJT

together results in a two terminal device

having an i-v characteristics identical to the iE-

VbE.

Fig on white board



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Fig shows the two diode connectedtransistors, one npn and the other is pnp.

BJT is still operating in active mode, so the

current I divides b/w the base and collectoraccording to the value of the BJT .

Thus internally BJT still operates as a transistor

in the active mode this is the reason the i-vcharacteristic of the resulting diode is identicalto the IE-V be.



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Current Mirror

Current mirror is a circuit designed to copy acurrent through one active device by controllingthe current in another active device of a circuit,

keeping the output current constant regardless ofloading. The current being 'copied' can be, andsometimes is, a varying signal current.Conceptually, an ideal current mirror is simply an

ideal current amplifier. The current mirror is usedto provide bias currents and active loads tocircuits



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Mirror characteristics

There are three main specifications that characterize a currentmirror.

1. The first is the current level it produces.

2. The second is its AC output resistance, which determines howmuch the output current varies with the voltage applied to the

mirror.3. The third specification is the minimum voltage drop across the

mirror necessary to make it work properly. This minimum voltageis dictated by the need to keep the output transistor of the mirrorin active mode.

The range of voltages where the mirror works is called thecompliance range and the voltage marking the boundary betweengood and bad behavior is called the compliance voltage. There arealso a number of secondary performance issues with mirrors, forexample, temperature stability.



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Simple Current Source

Current source is an electrical or electronic

device that delivers or absorbs electric

current.

Fig On White Board



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This circuit will be used in BJT current steering

circuitry.

It utilizes a pair of matched transistors with

the input reference current, determined by

the resistor R connected to the positive power

supply V cc then the current I ref will be

=



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V be is the base emitter voltage corresponding

to an emitter current. Neglecting the effects of

and Dependence of Io on Vo the output

currents Io will be equal to I ref.



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Current Steering Circuits



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Dc reference current is generated in one location andthen its is reproduced at other locations for thepurpose of biasing the various amplifier stages in IC

From Fig I ref is generated in the branch that consists ofthe diode connected transistors Q1 and resistor R

Now for the sake of the simplicity assume that alltransistors have high and thus base currents arenegligibly small.

Diode-connected transistors Q1 forms a current mirrorwith the Q3.

Thus Q3 will supply a constant I equal to I ref



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Conti

To generate the DC current twice the value of

the I ref then two transistors should be

connected in parallel.

Current mirrors are indeed used to provide

multiples of the reference current by simply

designing the transistors to have an area ratio

equal to the desired multiple.



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Comparison with MOS Circuits

The MOS mirror doesnt suffer from finite effect.

Current transfer ratio in a bipolar mirror is

determined by the relative areas of thetransistors , where as in MOS mirror its isdetermined by relative (W/L) ratios

Both basic bipolar and MOS mirrors have anoutput resistance r0=VA/I

However VA is usually lower for MOS devices.


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