Analog circuits unit 4

Analog Circuits
Unit 4
10 hrs, Semester 4, ECE
By : Swamy T N
Assistant Professor
ECE Department, Dr. Ambedkar Institute of Technology, Bengaluru
Mob: 9620216633
Email: swamyohm@gmail.com
Op-amp with Negative Feedback

Syllabus contents
• Op-Amp with Negative Feedback and general
applications Inverting and Non inverting Amplifiers –
Closed Loop voltage gain, Input impedance, Output
impedance, Bandwidth, Total output offset voltage
with feedback. DC and AC Amplifiers, Summing, Scaling
and Averaging Amplifiers, Instrumentation amplifier,
Comparators, Zero Crossing Detector, Schmitt trigger.
• Text Book: Ramakant A Gayakwad, “Op-Amps and
Linear Integrated Circuits”, 4th edition, Pearson
Education, 2000
2
Swamy TN, Assistant Professor, ECE, Dr.AIT,
Bengaluru

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Voltage series feedback amplifier
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Voltage series feedback amplifier-
Negative Feedback
5
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Voltage series feedback closed loop
voltage gain
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voltage gain
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voltage gain
8
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voltage gain
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difference input voltage ideally zero
10
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input resistance with feedback
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output resistance with feedback
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(bandwidth with feedback)
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22
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Total output offset voltage with
feedback
23
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Voltage follower
24
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Voltage shunt feedback amplifier
26
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Voltage shunt feedback amplifier-
closed loop gain
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closed loop gain
28
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closed loop gain
29
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Inverting input terminal at virtual
ground
30
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Input resistance with feedback
31
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Output resistance with feedback
32
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Bandwidth with feedback
33
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Total output offset voltage with
feedback
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DC Amplifier
• In DC amplifier, the output signal changes in response
to changes in its DC input levels.
• The DC amplifier can be inverting, non-inverting or
differential as shown in fig 6.1
• To reduce the output offset voltage to zero, that is, to
improve the accuracy of the DC amplifer, the offset null
circuitry of the op-amp should be used. For op-amp
without offset null capability, the external offset
voltage compensating network should be used as
shown in Fig 6.2.
• Otherwise, a high-precision op-amp such as the uA714,
which has smaller offset and drifts must be used.
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AC Amplifier
• Circuits shown in figure 6.1 and 6.2 respond
to ac input signal as well.
• To prevent the amplification of dc levels
coupling capacitors must be used between the
stages.
• Figure 6.3 shows the AC inverting and
noninverting amplifiers with coupling
capacitors.
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AC amplifier
• The coupling capacitor not only block the DC voltage
but also sets the low-frequency cutoff limit, which is
given by
• fL = 1/[2πCi(RiF + Ro)]
• Where
• fL = low frequency cutoff or low end of the bandwidth
• Ci = capacitance between two stages being coupled or
dc blocking capacitors
• RiF = AC input resistance of the second stage
• Ro = AC output resistance of the first stage or the
source resistance, Rin
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AC amplifier
• The bandwidth of the amplifier
• BW = fH – fL Depends on the desired value of fL and the
closed loop gain of the amplifier
• The coupling capacitor Ci, besides providing the low-
frequency cutoff limit, also helps to eliminate dc level
amplification from stage to stage.
• The closed loop gain of inverting amplifier is AF = - RF/R1
• The closed loop gain of noninverting amplifier is
AF = 1 + RF/R1
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Summing Scaling and averaging
amplifiers
• Inverting configurations
– Summing amplifier
– Scaling or weighted amplifer
– Average circuit
• Noninverting configuration
– Averaging amplifier
• Differential configuration
– Subtractor
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Inverting configuration
• Figure 6.6 shows
inverting configuration
with three inputs Va, Vb,
Vc.
• Depending upon the
relationship between
feedback resistor RF and
the input resistors Ra, Rb,
Rc, the circuit can be used
as a summing amplifier,
scaling amplifier, or an
averaging amplifier.
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Inverting configuration
• KCL at node V2
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Inverting configuration-summing
amplifier
• Here Ra = Rb = Rc = R, then output voltage
expression is
• Vo = -RF(Va + Vb + Vc)/R
• This means that the output voltage is
equal to the negative sum of all the input
times the gain of the circuit RF/R: hence
the circuit is called as summing amplifier.
• When the gain of the circuit is 1, that is
Ra=Rb=Rc=RF, the output voltage is equal
to the negative sum of all input voltages.
• Thus
• Vo = -(Va + Vb + Vc)
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Inverting configuration-scaling or
weighted amplifier
• If each input voltage is amplifier by a
different factor, in other words,
weighted differently at the output,
the circuit is then called a scaling or
weighted amplifier.
• Ra, Rb, and Rc are different
• Thus the output voltage of the scaling
amplifier is
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Inverting configuration-Average Circuit
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Non inverting configuration
If input voltage sources and
resistors are connected to
the noninverting terminal as
shown.
The circuit can be used
either as a summing or
averaging amplifier through
selection of appropriate
values of resistors that is R1
and RF
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Non inverting configuration
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Non inverting configuration- averaging
amplifier
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• Equation shows that the
output voltage is equal to
the average of all input
voltages times the gain of
the circuit (1+RF/R1),
hence the name
averaging amplifier.
• If the gain is 1, the output
voltage will be equal to
the average of all input
voltages.

Non inverting configuration- summing
amplifier
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Differential configuration- A subtractor
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Differential Configuration-Summing
amplifier
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Fig: Deriving the output voltage equation for
the summing amplifier shown above
Fig: Summing amplifier using differential
configuration

Instrumentation amplifier
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Refer text book for theory

Instrumentation amplifier using
transducer bridge
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Refer text book for theory

• Comparator
• Zero crossing detector
• Schmitt trigger
Given as self study assignment
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Note: slides are only for reference. It is mandatory to refer prescribed Text book
Text Book: Op-Amps and Linear Integrated Circuits by Ramakant A. Gayakwad

Analog circuits unit 4

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