Linear Integrated Circuits

Introduction

We are now going through a period of micro electronic circuits.Micro electronics refers to the circuit technology that is capable of producing circuits that contain millions of components in a small piece of silicon(known as silicon chip)whose area is in the order of 100sq.mm.The integrated circuits or IC is a miniature, low cost electronic circuit consisting of active and passive components that are inseparably joined together on a single crystal chip of silicon.These circuits offer a number of distinct advantages over those made by the interconnecting discrete components.

Advantages

1. Miniaturization and hence increased equipment density
2. Cost reduction due to batch production
3. Increased system reliability due to elimination of soldered joints
4. Improved functional performance(as it is possible to fabricate even complex circuits for better characteristics)
5. Matched devices
6. Increased operating speeds
7. Reduction in power consumption

Integrated circuits offers a wide range of application and are broadly classified as

1)Digital IC’s 2) Linear IC’s

Digital IC’s are complete functioning logic networks that are equivalents of basic transistor logic circuits.They are used to form such circuits as gates,counters,multiplexers,demultiplexers,shift registers etc

Digital circuits are primary concerned with only two levels of voltage ‘high’ and ‘low’ therefore,accurate control of operating region characteristics is not required in digital circuits,unlike in linear circuits.For this reason,digital circuits are easy to design and are produced in large quantities as low cost devices.

Linear ICs are equivalent to discrete transistor networks ,such as amplifiers,filters,frequency multipliers and modulators that often require additional external components of satisfactory operation.

Of all presently available linear ICs ,the majority are operational amplifiers(op-amp).Op-amps are further classified into two groups:general purpose and special purpose.General purpose op-amps may be used for a variety of applications such as integrator,differentiator,summing amplifier etc.A special purpose op-amp is used for the specific operations they are designed for eg:LM380 op-amp can be used only for audio power applications.

Operational Amplifier(op-amp)

An operational amplifier is a high gain amplifier usually consisting of one or more differential amplifier and usually followed by a level translator and an output stage.

The op-amp is a versatile device that can be used to amplify dc as well as ac input signal and was originally designed for performing mathematical operations such as addition ,subtraction,multiplication and integration.Thus the name operational amplifier stems from its original use for these mathematical operations and is abbreviated to op-amp.The first op-amp was introduced by Fairchild semiconductor in 1963,its μA 702 which set the stage for development of other IC op-amps

Internal Block Schematic of op-amp

The input stage is the dual input balanced output differential amplifier.This stage generally provides most of the voltage gain of the amplifier and also establishes the input resistance of the op-amp.The intermediate stage is usually another differential amplifier,which is driven by the output of the first stage.On most amplifiers,the intermediate stage is dual input,unbalanced output. Because of direct coupling,the dc voltage at the output of the intermediate stage is well above ground potential.Therefore,the level translator(shifting)circuits is used after the intermediate stage downwards to zero volts with respect to ground.The final stage is usually a push pull complementary symmetry amplifier output stage.The output stage increases the voltage swing and raises the ground supplying capabilities of the op-amp.a well designed output stage also provides low output resistance.

Different Stages of an op-amp(input & intermediate)

Input Stage of an op-amp

The input stage is a dual input,balanced output differential amplifier.It has 2 inputs V_in1 and V_in2 which are applied at the bases B₁ and B₂ of transistors Q₁ and Q_2.The output V₀ ois measured between the two collectors C₁ and C₂ which are at the same dc potential.Because of the equal dc potential at the two collectors with respect to ground,the output is referred to as balanced output.

Intermediate Stage

The next stage is dual input,unbalanced output difference amplifier.Here two input signals are used however the output is measured at only one of the two collectors with reference to ground.The output is referred to as an unbalanced output because the collector at which the output voltage is measured is at some finite dc potential with reference to ground.In other words,there is some dc voltage at the output terminal without any input signal applied.

evel Translator

There are two good reasons for using a level shifter in an IC(op-amp).As we want an op-amp to operate down to dc,no coupling capacitor is used.Because of direct coupling,the dc level rises from stage to stage.The increase in dc level tend to shift the operating point of next stage.This,in turn,limits the output voltage swing and even distort the output signal.It,therefore becomes essential to shift the quiescent voltage before it is applied to the next stage.Another requirement to be satisfied is that the output should have voltage level of 0V for input signal.

he simplest type of a level shifter is shown in fig.1 given below.This is basically an emitter follower.Hence the level shifter also acts as buffer to isolate the high gain stages from output stage.The amount of shift obtained,

V_O - V_i = -V_BE ≈-0.7V

If this shift is insufficient,the output can be taken at a junction of two resistors R₁ and R₂ (fig.2).the voltage in shift is now increased by the drop across R₁ .How ever this arrangement has the disadvantage that signal voltage also get attenuated by R₂ /(R₁ +R₂) .This can be avoided if R₂ is replaced by ‘I’(fig.3).The shift is now

V_O - V_i = -(V_BE +IR₁)

There is no ac attenuation due to high resistance of the current source.

Another voltage source commonly used in μA741 op-amp is shown below.It can be seen that if base current is negligible compare to the current in R₃ and R₄ ,then the circuit behaves as a multiplier as.

V=V_BE (R₃ + R₄)/ R₄ = V_BE[1+( R₃/ R₄)]

The voltage source can also be used to replace R₁

Output Stage of an op-amp

The function of the last stage in an op-amp is to supply the load current and provide a low impedance output.a simple output stage is an emitter follower with complementary transistors.When Vi is positive,transistor Q1 is on and supplies current to load RL .If Vi is negative,Q1 is cutoff and Q2 acts as a sink to remove current from the load RL .There is however,a limitation in this circuit.The output voltage VO remains zero volt until the input Viexceeds VBE(cut in)=0.5V.This is called cross over distortion.It can be eliminated by applying a bias voltage V greater than QVBE(cut in)=1V between the two bases so that a small current flows in the transistor even in the quiescent state.

Differential Amplifier

The figure shows the open loop configuration of differential amplifier in which the input signals Vin1 and Vin2 are applied to the positive and negative input terminals since the op-amp amplifies the difference between the two input signals,this configuration is called differential amplifier.Vin1 and Vin2 could be either ac or dc voltages.The source resistances Rin1 and Rin2 are negligible compared to the input resistance Ri.The voltage drop across the resistors can be assumed to be zero,which then implies that V1=Vin and V2=Vin2.The the output voltage Vo=A(Vin1-Vin2).

The output voltage is equal to the voltage gain A times the difference between the two input voltage.The polarity of the output voltage is dependent on the polarity of the input difference voltage(Vin1-Vin2).In open loop configuration gain A is commonly referred to an open loop gain.

Inverting and Non-inverting Amplifier

In the inverting amplifier,only one input is applied and that is to the inverting input terminal.The non-inverting input terminal is grounded since V1=0V and V2=Vin.

Therefore Vo = -A(Vin)

The negative sign indicates the output voltage is out of phase with respect to input by 180 degree or is of opposite polarity.Thus in the inverting amplifier,The input signal is amplified by gain A and is also inverted at the output.

Non-Inverting Amplifier

In this configuration ,the input is given to the non inverting input terminal and the inverting terminal is connected to ground.

In the circuit,V1=Vin and V2=0V therefore the output voltage Vo=AVin.This means that the output voltage is larger than the input voltage by gain A and is in phase with the input signal

Frequency response of an op-amp

We have treated the gain of the op-amp as a constant.However it is a complex number that is a function of frequency.Therefore at a given frequency the gain will have a specific magnitude as well as phase angle.The manner in which the gain of the op-amp response to different frequencies is called the frequency response.A graph of the magnitude of the gain and frequency is called a frequency plot.

Op-amp Parameters

1)Input offset Voltage
Input offset voltage is the the voltage that must be applied between the two input terminals of an op-amp to null the output.Typical value of 741 IC is 6mV dc.

2) Input offset current

The algebraic difference between the current in the inverting and non inverting terminal is known as the input offset current Iio.As the matching between two terminals increases,the difference between IB1 and IB2 become smaller.Typical value for 741 IC is 200mA(max).

3)Input Bias Current

IB is the average current flows in the inverting and non-inverting terminal of an op-amp.

IB = (IB1 + IB2 )/2

Typical value for 741 is 500mA

4) Large Signal Voltage gain

It is the ratio of the output voltage and the differential input voltage

A = Output voltage/Differential input

= Vo/Vid

Typical value for 741 IC is 200,000.

5) Output Voltage Swing

This parameter indicates the values of positive and negative saturation voltage of the op-amp.For 741IC,it is +13 and -13V.

6) Differential input resistance Ri

Differential input resistance Ri is the equivalent resistance that can be measured at either the inverting or non-inverting input terminals with the other terminal connected to ground.Typical value for 741 IC is 2 mega ohm.

7) Input Capacitance Ci

Input capacitance is the equivalent capacitance that can be measured at either the inverting or non-inverting input terminal with the other terminal connected to ground.Typical value for a 741 IC is 1.4 pF.

8) Common Mode Rejection Ratio(CMRR)

When the same voltage is applied to both the input terminals the voltage is called a common mode voltage Vcm and the op-amp is said to be operating in the common mode configuration,CMRR is defined as the ratio of the differential voltage gain to common mode gain.

CMRR = Ad/Acm

9) Supply voltage Rejection Ratio

The change in an op-amps input offset voltage Vio caused by variations in the supply voltage is called the SVRR.It is expressed in microvolts per volt or in decibels.

SR= ΔVio/ΔV

Typical value for a 741IC is 150μV/V

10) Slew Rate

Slew rate is defined as the maximum rate of change of output voltage per unit of time and is expressed as volt per micro second.

SR=(|dVo|/|ds|)max ie V\μs

11) Gain Bandwidth Product

The gain bandwidth product(GB) is the bandwidth of the op-amp when the voltage gain is 1.Typical value for 741 IC is 1MHz.

The Ideal Op-amp

An ideal op-amp would exhibit the following characteristics

1. Infinite Voltage gain,A
2. Infinite input resistance,Ri so that almost any signal source can drive it and there is no loading of the preceding stage
3. Zero output resistance,Ro so that output can drive infinite number of other devices
4. Zero output voltage when input voltage is zero
5. Infinite bandwidth so that any frequency signal from zero to infinite Hertz can be amplified without attenuation
6. Infinite common mode rejection ratio(CMRR)so that the output common mode noise voltage is zero
7. Infinite slew rate so that the output voltage changes occur simultaneously with input voltage changes

Equivalent Circuit of an Op-Amp

Output volatge,Vo=AVid=A(V1-V2)
Vid=difference input voltage
V1= Voltage at the non-inverting input terminal with respect to ground
V2= voltage at the inverting terminal with respect to ground
Polarity of the output voltage depends on the polarity of the difference voltage.

Ideal Voltage Transfer Curve

Vo=A(V1-V2)

This is the basic op-amp equation in which the output offset voltage is assumed to be zero.The graphic representation of this equation is shown;where the output voltage ,Vo is plotted against input difference voltage Vid,keeping gain A constant.The output voltage cannot exceed the positive and negative saturation voltage.These saturation voltages are specified by an output voltage swing ratings of an op-amp for given values of supply voltages.The output voltage is directly proportional to the input difference voltage until it reaches the saturation voltages and thereafter the output voltage remains constant.

This curve is called ideal voltage transfer curve

Open Loop Op-Amp Configurations

When connected in open-loop configuration,the op-amp functions as a high gain amplifier.These are three open loop op-amp configurations.

1. Differential Amplifier
2. Inverting Amplifier
3. Non-Inverting Amplifier

These configurations are classed according to the number of inputs used and the terminal to which the input is applied when a single input is used.

Disadvantages of open loop configurations

1. The open loop of the op-amp is very high.Therefore only the smaller signals having low frequency may be amplified accurately without distortion.
2. Open loop Voltage gain of the op-amp is not a constant voltage gain varies with changes in temperature and power supply as well as mass production techniques.This makes op-amp unsuitable for many linear applications
3. Bandwidth of most open loop op-amps is negligibly small or almost zero therefore op-amp is impractical in ac applications.

For these reasons,the open loop op-amp is generally not used in linear applications.In certain applications,the open loop op-amp is purposely used as a non -linear device;that is a square wave output is obtained by applying a relatively large input signal.

We can select as well as control the gain of the op-amp if we introduce a modification in the basic circuit.This modification involves the use of feedback ie an output signal is fed back to the input either directly or via another network.If the signal fed back is of opposite polarity or out of phase of 180 degree.with respect to the input signal,the feedback is called negative feedback.Negative feedback is also known as degenerative feedback.

If the signal fed back is of the same polarity or in the phase with the input signal,the feed back is called positive feed back.In positive feedback,the feedback signal is aid the input signal.It is also known as regenerative feedback.

When used in amplifiers,negative feedback stabilizes the gain,increases the bandwidth and changes the input and output resistance.Other benefits include a decrease in harmonic or non-linear distortion and reduction in the effect of input offset voltage at the output.It is also reduces the effect of variations in the temperature and supply voltages on the output of the op-amp.

Op-amp in closed loop configuration

Op-amp is in closed loop configuration when there is a connection either direct or via another network exist between the input and output terminals.We can control the gain of op-amp if we introduce a modification in the basic circuit.This modification involves a feedback ie the input signal is fed back to the input either direct or via another network.
Different feedback configurations

An op-amp that uses feedback is called a feedback amplifier.A feedback amplifier is sometimes referred to as a closed loop amplifier because the feedback forms a closed loop between the input and the output.A close loop amplifier can be represented by using two blocks one for op-amp and the other for a feedback circuit.These connections are classified according to whether the voltage or current is fed back to the input in series or parallel.

1. Voltage series feedback
2. Voltage shunt feedback
3. Current series feedback
4. Current shunt feedback.