notes opamp sem 4

Applied and Digital Electronics - OPAMP

DEE/Sem-4/ Applied and Digital Electronics 1.3 Operational Amplifier:1.3.1 Basic differential amplifier circuit using BJT.1.3.2 Pin diagram of OPAMP IC741& functions of each pin. Definition of offset voltage, input bias current, input offset current, differential mode gain, CMRR, slew rate1.3.3 OPAMP as Non-inverting and Inverting amplifier, Adder, Subtractor, Integrator, Differentiator, Unity Gain Buffer, Schmitt Trigger, Zero Crossing Detector.1.3.4 Instrumentation amplifier – Operating principle using OPAMP, Applications.

1.3.1 Basic differential amplifier circuit using BJT.Differential amplifiers are in general very useful. They consist of two inputs and one output, as indicated

by the generic symbol in Fig. 1 The output is proportional to the difference between the two inputs, where the proportionality constant is the gain.

Among the two inputs, one input is labeled “ - “ being inverted and then added to the other non-inverting input, labeled “+”. This “differential pair” is then transmitted and then received by a differential amplifier. Any noise pickup will be approximately equal for the two inputs, and hence will not appear in the output of the differential amplifer. This “common mode” noise is rejected. This is often quantified by the common-mode rejection ratio (CMRR) which is the ratio of differential gain to common-mode gain. Clearly, a large CMRR is good.

Differential amplifier (Linear active device) with two input signals & and one output signal is shown in Fig. 1. In an ideal differential amplifier, output signal is given by

Where, = gain of the differential amplifier.

The circuit shown in Fig. 2 represents a differential amplifier design. It looks like two common-emitter amplifiers whose emitters are tied together at point A. It is simplest to analyze its output if one writes each input as the sum of two terms, a sum and a difference.

Consider two signals . In general, we can rewrite these as

Therefore, we can break down the response of the circuit to be due to the response to a common-mode inputand a difference input .

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Let’s analyze the difference signal first. Therefore, consider two inputs .

The signals at the emitters then follow the inputs, as usual, so that at point A we have

Again from the common-emitter amplifier principle, we have

Defining differential gain as the ratio of the output to the input difference, we get

and similarly for output 2

Generally, only one of the two outputs is used. Referring back to Fig. 1, we see that if we were to choose our one output to be the one labeled “out2”, then “In1” would correspond to “+” (non-inverting input) and “In2” would correspond to ” (inverting input). Keeping in mind these results for the relative signs, it is usual to write the differential gain as a positive quantity:

Now consider the common mode part of the inputs: From Fig. 2 we have

Again from the common-emitter amplifier principle, we have

So each output has the same common-mode gain:

Common mode rejection ratio (CMRR) is defined as the ratio of the differential voltage gain to the

common-mode voltage gain .

Generally, is very small and is very large and therefore, CMRR is very large.

CMRR is a quantity which serves as a figure of merit for differential amplifier. This being a large value, often expressed in decibel (dB). The higher the value of CMRR, the better is the matching between two input terminals and smaller is the output common mode voltage.

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The operation amplifier:

An operational amplifier is a direct coupled high gain amplifier consisting of one or more differential (OPAMP) amplifiers and followed by a level translator and an output stage. An operational amplifier is available as a single integrated circuit package.

The block diagram of OPAMP is shown in fig. 1.

Fig. 1

The input stage is a dual input balanced output differential amplifier. This stage provides most of the voltage gain of the amplifier and also establishes the input resistance of the OPAMP. The intermediate stage of OPAMP is another differential amplifier which is driven by the output of the first stage. This is usually dual input unbalanced output.

Because direct coupling is used, the dc voltage level at the output of intermediate stage is well above ground potential. Therefore level shifting circuit is used to shift the dc level at the output downward to zero with respect to ground. The output stage is generally a push pull complementary amplifier. The output stage increases the output voltage swing and raises the current supplying capability of the OPAMP. It also provides low output resistance.

The symbolic diagram of an OPAMP is shown in fig.2.

Fig. 2

Ideal OPAMPAn ideal op-amp is characterized by seven properties

Infinite open-loop voltage gain Infinite input impedance, means that input current i = 0. It means that an ideal OP-AMP

is a voltage-controlled device. Zero output impedance, means that V0 is not dependent on the load resistance

connected across the output, i.e. the op-amp can drive any load impedance to any voltage.

Zero noise contribution Zero DC output offset i.e. VO = 0 when V1 = V2= 0 Infinite bandwidth Differential inputs that stick together

Though these characteristics cannot be achieved in practice, yet an ideal OP-AMP serves as a convenient reference against which real OP-AMPs may be evaluated.

These ideals can be summarized by the two "golden rules":I. The output attempts to do whatever is necessary to make the voltage difference between the inputs zero. (Negative feedback will ensure that this is the case.)II. The inputs draw no current. (This is true in the approximation that the Zin of the op-amp is much larger than any)

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http://nptel.ac.in/courses/117107094/lecturers/lecture_5/lecture5_page1.htm


LM 741c is most commonly used OPAMP available in IC packages. It is an 8-pin chip. Package and Pin configurations are as follows:

Operational Amplifier : LM741 Pin Diagram:

The LM741 series are general purpose operational amplifiers which feature improved performance over industry standards like the LM709. They are direct, plug-in replacements for the 709C, LM201, MC1439 and 748 in most applications.

The amplifiers offer many features which make their application nearly foolproof: overload protection on the input and output, no latch-up when the common mode range is exceeded, as well as freedom from oscillations.

The LM741C is identical to the LM741/LM741A except that the LM741C has their performanceensured over a 0°C to +70°C temperature range, instead of -55°C to +125°C.

Absolute Maximum Ratings (indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is functional, but do not ensure specific performance limits.)

LM741A LM741 LM741CSupply Voltage ±22V ±22V ±18VPower Dissipation (4) 500 mW 500 mW 500 mWDifferential Input Voltage ±30V ±30V ±30VInput Voltage (5) ±15V ±15V ±15VOutput Short Circuit Duration Continuous Continuous ContinuousOperating Temperature Range -55 oC to+125 oC -55 oC to +125 oC 0°C to +70°CStorage Temperature Range -65 oC to +150 oC -65 oC to +150 oC -65°C to +150 oCJunction Temperature 150 oC 150 oC 100 oC

Definition of offset voltage, input bias current, input offset current, differential mode gain, CMRR, slew rate

1. Input Offset Voltage: Input offset voltage is defined as the voltage that must be applied between the two input terminals of an OPAMP to null or zero the output fig. 1, shows that two dc voltages are applied to input terminals to make the output zero.

Vio = Vdc1 – Vdc2

Vdc1 and Vdc2 are dc voltages and RS represents the source resistance. Vio is the difference of

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Vdc1 and Vdc2. It may be positive or negative. For a 741C OPAMP the maximum value of V io is 6mV. It means a voltage ± 6 mV is required to one of the input to reduce the output offset voltage to zero. The smaller the input offset voltage the better the differential amplifier, because its transistors are more closely matched.

Fig. 1

2. Input offset Current: The input offset current Iio is the difference between the currents into inverting and non-inverting terminals of a balanced amplifier.

Iio = | IB1 – IB2 |The Iio for the 741C is 200nA maximum. As the matching between two input terminals is improved, the difference between IB1 and IB2 becomes smaller, i.e. the Iio value decreases further.For a precision OPAMP 741C, Iio is 6 nA

3.Input Bias Current: The input bias current IB is the average of the current entering the input terminals of a balanced amplifier i.e.

IB = (IB1 + IB2 ) / 2For 741C IB(max) = 700 nA and for precision 741C IB = ± 7 nA

4. Differential Input Resistance: (Ri) Ri is the equivalent resistance that can be measured at either the inverting or non-inverting input terminal with the other terminal grounded. For the 741C the input resistance is relatively high 2 M . For some OPAMP it may Ωbe up to 1000 G ohm.

5. Input Capacitance: (Ci) Ci is the equivalent capacitance that can be measured at either the inverting and noninverting terminal with the other terminal connected to ground. A typical value of Ci is 1.4 pf for the 741C.

6. Offset Voltage Adjustment Range: 741 OPAMP have offset voltage null capability. Pins 1 and 5 are marked offset null for this purpose. It can be done by connecting 10 K ohm pot between 1 and 5 as shown in fig. 3.

Fig. 3

By varying the potentiometer, output offset voltage (with inputs grounded) can be reduced to zero volts. Thus the offset voltage adjustment range is the range through which the input offset voltage can be adjusted by varying 10 K pot. For the 741C the offset voltage adjustment range is ± 15 mV.

7. Input Voltage Range : Input voltage range is the range of a common mode input signal for which a differential amplifier remains linear. It is used to determine the degree of matching between the inverting and noninverting input terminals. For the 741C, the range of the input common mode voltage is ± 13V maximum. This means that the common mode voltage applied at both input terminals can be as high as +13V or as low as –13V.

8. Common Mode Rejection Ratio (CMRR). CMRR is defined as the ratio of the differential voltage gain Ad to the common mode voltage gain ACM

CMRR = Ad / ACM.

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For the 741C, CMRR is 90 dB typically. The higher the value of CMRR the better is the matching between two input terminals and the smaller is the output common mode voltage.

9. Supply voltage Rejection Ratio: (SVRR) SVRR is the ratio of the change in the input offset voltage to the corresponding change in power supply voltages. This is expressed in m V / V or in decibels, SVRR can be defined as

SVRR = D Vio / D VWhere D V is the change in the input supply voltage and D Vio is the corresponding change in the offset voltage. For the 741C, SVRR = 150 µ V / V. For 741C, SVRR is measured for both supply magnitudes increasing or decreasing simultaneously, with R3 =10K. For same OPAMPS, SVRR is separately specified as positive SVRR and negative SVRR.

10. Large Signal Voltage Gain: Since the OPAMP amplifies difference voltage between two input terminals, the voltage gain of the amplifier is defined as

Because output signal amplitude is much large than the input signal the voltage gain is commonly called large signal voltage gain. For 741C is voltage gain is 200,000 typically.

11. Output voltage Swing: The ac output compliance PP is the maximum unclipped peak to peak output voltage that an OPAMP can produce. Since the quiescent output is ideally zero, the ac output voltage can swing positive or negative. This also indicates the values of positive and negative saturation voltages of the OPAMP. The output voltage never exceeds these limits for a given supply voltages +VCC and –VEE. For a 741C it is ± 13 V.

12. Output Resistance: (RO) RO is the equivalent resistance that can be measured between the output terminal of the OPAMP and the ground. It is 75 ohm for the 741C OPAMP.

Example - 1 Determine the output voltage in each of the following cases for the open loop differential amplifier of fig. 4:

a. vin 1 = 5 m V dc, vin 2 = -7 µVdc b. vin 1 = 10 mV rms, vin 2= 20 mV rms

Fig. 4

Specifications of the OPAMP are given below: A = 200,000, Ri = 2 M , RΩ O = 75 , + VΩ CC = + 15 V, - VEE = - 15 V, and output voltage swing = ± 14V. Solution: (a). The output voltage of an OPAMP is given by

Remember that vo = 2.4 V dc with the assumption that the dc output voltage is zero when the input signals are zero. (b). The output voltage equation is valid for both ac and dc input signals. The output voltage is given by

Thus the theoretical value of output voltage vo = -2000 V rms. However, the OPAMP saturates at ± 14 V.

Therefore, the actual output waveform will be clipped as shown fig. 5. This non-sinusoidal waveform is unacceptable in amplifier applications.

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Fig. 5

13. Output Short circuit Current : In some applications, an OPAMP may drive a load resistance that is approximately zero. Even its output impedance is 75 ohm but cannot supply large currents. Since OPAMP is low power device and so its output current is limited. The 741C can supply a maximum short circuit output current of only 25mA.

14. Supply Current : IS is the current drawn by the OPAMP from the supply. For the 741C OPAMP the supply current is 2.8 m A.

15. Power Consumption: Power consumption (PC) is the amount of quiescent power (v in= 0V) that must be consumed by the OPAMP in order to operate properly. The amount of power consumed by the 741C is 85 m W.

16. Gain Bandwidth Product: The gain bandwidth product is the bandwidth of the OPAMP when the open loop voltage gain is reduced to 1. From open loop gain vs frequency graph At 1 MHz shown in. fig. 6, It can be found 1 MHz for the 741C OPAMP frequency the gain reduces to 1. The mid band voltage gain is 100, 000 and cut off frequency is 10Hz.

Fig. 6

17. Slew Rate: Slew rate is defined as the maximum rate of change of output voltage per unit of time under large signal conditions and is expressed in volts / m secs.

To understand this, consider a charging current of a capacitor shown in fig. 7.

Fig. 7

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If 'i' is more, capacitor charges quickly. If 'i' is limited to Imax, then rate of change is also limited. Slew rate indicates how rapidly the output of an OPAMP can change in response to changes in the input

frequency with input amplitude constant. The slew rate changes with change in voltage gain and is normally specified at unity gain.

If the slope requirement is greater than the slew rate, then distortion occurs. For the 741C the slew rate is low 0.5 V / m S. which limits its use in higher frequency applications.

18. Input Offset Voltage and Current Drift:It is also called average temperature coefficient of input offset voltage or input offset current. The input offset voltage drift is the ratio of the change in input offset voltage to change in temperature and expressed in m V /° C. Input offset voltage drift = ( D Vio / D T). Similarly, input offset current drift is the ratio of the change in input offset current to the change in temperature. Input offset current drift = ( D Iio / D T). For 741C,

D Vio / D T = 0.5 m V / C. D Iio/ D T = 12 pA / C.

1.3.3 OPAMP as Non-inverting and Inverting amplifier, Adder, Subtractor, Integrator, Differentiator, Unity Gain Buffer, Schmitt Trigger, Zero Crossing Detector.

Open loop OPAMP Configuration: In the case of amplifiers the term open loop indicates that no connection, exists between input

and output terminals of any type. That is, the output signal is not feedback in any form as part of the input signal.

In open loop configuration, The OPAMP functions as a high gain amplifier. There are three open loop OPAMP configurations.

Open-loop gain The open-loop gain of an operational amplifier is the gain obtained when no feedback is used

in the circuit. Open loop gain is usually exceedingly high; in fact, an ideal operational amplifier has infinite open-loop gain. Typically an op-amp may have a maximal open-loop gain of around . Normally, feedback is applied around the op-amp so that the gain of the overall circuit is defined and kept to a figure which is more usable. The very high open-loop gain of the op-amp allows a wide range of feedback levels to be applied to achieve the desired performance.

The open-loop gain of an operational amplifier falls very rapidly with increasing frequency. Along with slew rate, this is one of the reasons why operational amplifiers have limited bandwidth.

The definition of open-loop gain (at a fixed frequency) is

where is the input voltage difference that is being amplified. The dependence on frequency is not displayed here.

The Differential Amplifier: Fig. 3, shows the open loop differential amplifier in which input signals vin1 and vin2 are applied to the positive and negative input terminals.

Fig. 8

Since the OPAMP amplifies the difference the between the two input signals, this configuration is called the differential amplifier. The OPAMP amplifies both ac and dc input signals. The source resistance Rin1

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http://en.wikipedia.org/wiki/Bandwidth_(signal_processing)

http://en.wikipedia.org/wiki/Slew_rate

http://en.wikipedia.org/wiki/Frequency

http://en.wikipedia.org/wiki/Electrical_network

http://en.wikipedia.org/wiki/Electrical_network

http://en.wikipedia.org/wiki/Feedback

http://en.wikipedia.org/wiki/Gain

http://en.wikipedia.org/wiki/Operational_amplifier


and Rin2 are normally negligible compared to the input resistance Ri. Therefore voltage drop across these resistances can be assumed to be zero.

Therefore v1 = vin1 and v2 = vin2. vo = Ad (vin1 – vin2 ) where, Ad is the open loop gain.

The Inverting Amplifier: If the input is applied to only inverting terminal and non-inverting terminal is grounded then it is called inverting amplifier. This configuration is shown in fig. 4.

v1= 0, v2 = vin. vo = -Ad vin

Fig. 9

The negative sign indicates that the output voltage is out of phase with respect to input 180 ° or is of opposite polarity. Thus the input signal is amplified and inverted also.

The non-inverting amplifier: In this configuration, the input voltage is applied to non-inverting terminals and inverting terminal is ground as shown in fig. 5.

v1 = +vin v2 = 0 vo = +Ad vin

This means that the input voltage is amplified by Ad and there is no phase reversal at the output.

Fig. 10

In all there configurations any input signal slightly greater than zero drive the output to saturation level. This is because of very high gain. Thus when operated in open-loop, the output of the OPAMP is either negative or positive saturation or switches between positive and negative saturation levels. Therefore open loop op-amp is not used in linear applications.

Closed Loop Amplifier: The gain of the OPAMP can be controlled if feedback is introduced in the circuit. That is, an output signal

is feedback to the input either directly or via another network. If the signal feedback is of opposite or out phase by 180° with respect to the input signal, the feedback is called negative feedback.

An amplifier with negative feedback has a self-correcting ability of change in output voltage caused by changes in environmental conditions. It is also known as degenerative feedback because it reduces the output voltage and, in turn, reduces the voltage gain.

If the signal is feedback in phase with the input signal, the feedback is called positive feedback. In positive feedback the feedback signal aids the input signal. It is also known as regenerative feedback. Positive feedback is necessary in oscillator circuits.

The negative feedback stabilizes the gain, increases the bandwidth and changes, the input and output resistances. Other benefits are reduced distortion and reduced offset output voltage. It also reduces the effect of temperature and supply voltage variation on the output of an op-amp.

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Inverting AmplifierThe inverting amplifier configuration is shown in Fig. It is “inverting” because our signal input comes to the “-“ input, and therefore has the opposite sign to the output. The negative feedback is provided by the resistor Ri connecting output to input.

We can use our rules to analyze this circuit. Since “+” input is connected to ground, then by rule 1, “-“ input is also at ground. For this reason, the “-“ input is said to be at virtual ground. Therefore, the voltage drop across R1 is vin - v- = vin, and the voltage drop across R2 is vout — v_ = vout. So, applying Kirchoff’s first law to the node at input ‘-‘ , we have, using golden rule 2:

Or

Inverting amplifier configuration.

The input impedance, as always, is the impedance to ground for an input signal. Since the - input is at (virtual) ground, then the input impedance is simply R1:

Zin = R1

Non-inverting AmplifierThis configuration is given in Fig. Again, its basic properties are easy to analyze in terms of the golden rules.

where the last expression is from our voltage divider result. Therefore, rearranging gives

The input impedance in this case is given by the intrinsic op-amp input impedance. As mentioned above, this is very large, and is typically in the following range:

Non-inverting amplifier configuration

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OPAMP APPLICATIONS

Analog Inverter and Scale Changer: The circuit of analog inverter is shown in fig. 1. It is same as inverting voltage amplifier.

Assuming OPAMP to be an ideal one, the differential input voltage is zero.

i.e. vd = 0 Therefore, v1 = v2 = 0

Since input impedance is very high, therefore, input current is zero. OPAMP do not sink any current.

iin= if

vin / R = - vO / Rf

vo = - (Rf / R) vin

If R = Rf then vO = -vin, the circuit behaves like an inverter. If Rf / R = K (a constant) then the circuit is called inverting amplifier or scale changer voltages.

Fig. 1

Inverting summer: The configuration is shown in fig. 2. With three input voltages va, vb & vc. Depending upon the value of Rf and the input resistors Ra, Rb, Rc the circuit can be used as a summing amplifier, scaling amplifier, or averaging amplifier. Again, for an ideal OPAMP, v1 = v2. The current drawn by OPAMP is zero. Thus, applying KCL at v2 node

This means that the output voltage is equal to the negative sum of all the inputs times the gain of the circuit Rf/ R; hence the circuit is called a summing amplifier. When Rf= R then the output voltage is equal to the negative sum of all inputs.

vo= -(va+ vb+ vc)

Fig. 2

If each input voltage is amplified by a different factor in other words weighted differently at the output, the circuit is called then scaling amplifier.

The circuit can be used as an averaging circuit, in which the output voltage is equal to the average of all the input voltages. In this case, Ra= Rb= Rc = R and Rf / R = 1 / n where n is the number of inputs. Here Rf / R = 1 / 3.

vo = -(va+ vb + vc) / 3 In all these applications input could be either ac or dc.

Non-inverting configuration: If the input voltages are connected to noninverting input through resistors, then the circuit can be used as a summing or averaging amplifier through proper selection of R1, R2, R3 and Rf. as shown in fig. 3.

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To find the output voltage expression, v1 is required. Applying superposition theorem, the voltage v1 at the noninverting terminal is given by

Hence the output voltage is

Fig. 3

This shows that the output is equal to the average of all input voltages times the gain of the circuit (1+ Rf / R1), hence the name averaging amplifier. If (1+Rf/ R1) is made equal to 3 then the output voltage becomes sum of all three input voltages.

vo = v a + vb+ vc

Hence, the circuit is called summing amplifier.

Differential Amplifier: The basic differential amplifier is shown in fig. 4.

Fig. 4Since there are two inputs superposition theorem can be used to find the output voltage. When Vb= 0, then the circuit becomes inverting amplifier, hence the output due to Va only is Vo(a) = -(Rf / R1) Va

Similarly when, Va = 0, the configuration is a inverting amplifier having a voltage divided network at the noninverting input

Integrator: A circuit in which the output voltage waveform is the integral of the input voltage waveform is called integrator. Fig. 5, shows an integrator circuit using OPAMP.

Fig. 5 Here, the feedback element is a capacitor. The current drawn by OPAMP is zero and also the V2 is virtually grounded.

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Therefore, i1 = if and v2 = v1 = 0

Integrating both sides with respect to time from 0 to t, we get

The output voltage is directly proportional to the negative integral of the input voltage and inversely proportional to the time constant RC.If the input is a sine wave the output will be cosine wave. If the input is a square wave, the output will be a triangular wave. For accurate integration, the time period of the input signal T must be longer than or equal to RC.Fig. 6, shows the output of integrator for square and sinusoidal inputs.

Fig. 6 Example - 3

Prove that the network shown in fig. 7 is a non-inverting integrator with .Solution: The voltage at point A is vO / 2 and it is also the voltage at point B because different input voltage is negligible.

vB = VO / 2 Therefore, applying Node current equation at point B,

Fig. 7

Differentator: A circuit in which the output voltage waveform is the differentiation of input voltage is called differentiator.as shown in fig. 8.

Fig. 8

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The expression for the output voltage can be obtained from the Kirchoff's current equation written at node v2.

Thus the output vo is equal to the RC times the negative instantaneous rate of change of the input voltage vin with time. A cosine wave input produces sine output. fig. 8 also shows the output waveform for different input voltages. The input signal will be differentiated properly if the time period T of the input signal is larger than or equal to Rf C.

T > Rf C As the frequency changes, the gain changes. Also at higher frequencies the circuit is highly susceptible at high frequency noise and noise gets amplified. Both the high frequency noise and problem can be corrected by additing, few components. as shown in fig. 9.

Fig. 9

Comparators: An analog comparator has two inputs one is usually a constant reference voltage VR and other is a time varying signal vi and one output vO. The basic circuit of a comparator is shown in fig. 10. When the noninverting voltage is larger than the inverting voltage the comparator produces a high output voltage (+Vsat). When the non-inverting output is less than the inverting input the output is low (-V sat). Fig.10, also shows the output of a comparator for a sinusoidal.

Fig. 10 vO = -Vsat if vi > VR = + Vsat if v i < VR

If VR = 0, then slightest input voltage (in mV) is enough to saturate the OPAMP and the circuit acts as zero crossing detector as shown in fig. 11. If the supply voltages are ±15V, then the output compliance is from approximate – 13V to +13V. The more the open loop gain of OPAMP, the smaller the voltage required to saturate the output. If vdrequired is very small then the characteristic is a vertical line as shown in fig. 11.

Fig. 11 If we want to limit the output voltage of the comparator two voltages (one positive and other negative) then a resistor R and two zener diodes are added to clamp the output of the comparator. The circuit of such comparator is shown in fig. 12, The transfer characteristics of the circuit is also shown in fig. 12.

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Fig. 7 The resistance is chosen so that the zener operates in zener region. When VR= 0 then the output changes rapidly from one state to other very rapidly every time that the input passes through zero as shown in fig. 13.

Fig. 13 Such a configuration is called zero crossing detector. If we want pulses at zero crossing then a differentiator and a series diode is connected at the output. It produces single pulses at the zero crossing point in every cycle.

Schmitt Trigger: If the input to a comparator contains noise, the output may be erractive when vin is near a trip point. For instance, with a zero crossing, the output is low when vin is positive and high when vin is negative. If the input contains a noise voltage with a peak of 1mV or more, then the comparator will detect the zero crossing produced by the noise. Fig. 14, shows the output of zero crossing detection if the input contains noise.

Fig. 14 Fig. 15 This can be avoided by using a Schmitt trigger, circuit which is basically a comparator with positive feedback. Fig. 15, shows an inverting Schmitt trigger circuit using OPAMP. Because of the voltage divider circuit, there is a positive feedback voltage. When OPAMP is positively saturated, a positive voltage is feedback to the non-inverting input, this positive voltage holds the output in high stage. (vin< vf). When the output voltage is negatively saturated, a negative voltage feedback to the inverting input, holding the output in low state. When the output is +Vsat then reference voltage Vref is given by

If Vin is less than Vref output will remain +Vsat. When input vin exceeds Vref = +Vsat the output switches from +Vsat to –Vsat. Then the reference voltage is given by

The output will remain –Vsat as long as vin > Vref.

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Fig. 16 Fig. 17 If vin < Vref i.e. vin becomes more negative than –Vsat then again output switches to +Vsat and so on. The transfer characteristic of Schmitt trigger circuit is shown in fig. 16. The output is also shown in fig. 17 for a sinusoidal wave. If the input is different than sine even then the output will be determined in a same way.Positive feedback has an unusual effect on the circuit. It forces the reference voltage to have the same polarity as the output voltage, The reference. voltage is positive when the output voltage is high (+vsat) and negative when the output is low (–vsat). In a Schmitt trigger, the voltages at which the output switches from +vsat to –vsat or vice versa are called upper trigger point (UTP) and lower trigger point (LTP). the difference between the two trip points is called hysteresis.

Fig. 18

The hysteresis loop can be shifted to either side of zero point by connecting a voltage source as shown in fig. 18.When VO= +Vsat , the reference. Voltage (UTP) is given by

When VO= -Vsat , the reference. Voltage (UTP) is given by

If VR is positive the loop is shifted to right side; if VR is negative, the loop is shifted to left side. The hysteresis voltage Vhys remains the same.

Non-inverting Schmitt trigger: In this case, again the feedback is given at non-inverting terminal. The inverting terminal is grounded and the input voltage is connected to non-inverting input. Fig. 19, shows an non-inverting schmitt trigger circuit.

Fig. 19 To analyze the circuit behaviour, let us assume the output is negatively saturated. Then the feedback voltage is also negative (-Vsat). Then the feedback voltage is also negative. This feedback voltage will hold the output in negative saturation until the input voltage becomes positive enough to make voltage positive.

When vin becomes positive and its magnitude is greater than (R2 / R1) Vsat, then the output switches to +Vsat. Therefore, the UTP at which the output switches to +Vsat, is given by

Simillarly, when the output is in positive saturation, feedback voltage is positive. To switch output states, the input voltage has to become negative enough to make. When it happens, the output changes to the negative state from positive saturation to negative saturation voltage negative.

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When vin becomes negative and its magnitude is greater than R2 / R1 vsat, then the output switches to -vsat. Therefore,

The difference of UTP and LTP gives the hysteresis of the Schmitt trigger.

In non inverting Schmitt trigger circuit, the β is defined as

Voltage Follower: The lowest gain that can be obtained from a non-inverting amplifier with feedback is 1. When the non-inverting amplifier gives unity gain, it is called voltage follower because the output voltage is equal to the input voltage and in phase with the input voltage. In other words the output voltage follows the input voltage.

vout = Avd= A (v1 – v2) v1 = vin v2 =vout v1 = v2 if A >> 1 vout = vin.

The gain of the feedback circuit (B) is 1. ThereforeAf = 1 / B = 1

Fig. 20

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