Download Now that we know how to make a summing amplifier, ponder over

Electronics Club
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Analog Electronics
Introduction:
Most of you have had your share of experiences with digital circuits in the previous
workshops. Digital programming, though extremely robust and comparatively easy, is
not all that is to circuit designing. The analog electronics is also as vast as and much
more challenging than the digital electronics. In the upcoming lecture and the
workshops, we will take a glimpse of a device that is among the most useful for
analog circuitry: An Operational Amplifier, or for simplicity, an Op-Amp.

Analog Electronics: Well, the digital electronics dealt simply with two states –
high or low; high corresponding to 5 V & low to 0 V in our case. But, analog
comes in with whole range of all possible voltages between the two states; i.e.
there is a continuous voltage range between the maximum & the minimum
voltages, making it the in thing for very interesting & high level circuit designs,
bringing in much more complex & fun to work to work with.

Op-Amp (Operational Amplifier):An op-amp is, in
technical words, a differential amplifier. It has two input
pins and one output pin One input pin is marked –ve
(Inverting Input) & the other is marked +ve (Noninverting Input). There are also two pins for +VSS & -VSS
whose importance you will learn in the following pages.
It can be operated in a variety of ways (as will be the case
with most of the devices we learn about, e.g. 555). Here in the workshop, we
would deal with the three most important applications- Comparator, Amplifier &
Switch.

Digital Multimeter (DMM): A digital multimeter is a very
interesting & accurate device which can be used to measure
resistances, DC & AC voltages etc. It has a wheel in middle
which can be used to set whether resistance is to be measured or
the voltage. (Units are written over it , so you would be able to
identify it easily where it has to be set.) Then the red & black
pins can be put across the device whose resistance or the voltage
difference is to be measured, & whoa…measurement done!

Amplifier: An amplifier is a device
which increases the amplitude of the
input signal as shown in the figure:
This is very important in analog circuits
as u may have small signals & u might
have to use them in following circuit as
an input where the existing voltage
difference might not be enough. So ,use
an amplifier there & move ahead.

Comparator: In digital, it is very easy to compare the stateseither it is 1 or it is 0. But in analog, u have to use some other
device to do the comparison. Here comes in the comparator. It
has two inputs- one is the reference voltage with respect to
which the other signal is compared. If the input is greater than
the VREF, ith gives one particular range of voltage as output ; &
if less, other range.

Single Input Mode :
It refers to the mode of operation when, understandably, out of the two pins
intended for input, one will almost always be set to a reference voltage, which is
generally GND. Our output depends only on one of the inputs.
As we can now see why input pin marked –ve is called an inverting input & +ve
pin is called a non-inverting input.
 Impedance: It is the total resistance, capacitance and the inductance of a given
circuit part. Refresh what you learned about the AC currents, jω, etc.
 We will now define a term that we will often use: gain. Gain is merely the ratio
between the output voltage to the input voltage of any device.
AV =Vo/Vi
Op-Amps
A small peek into the black box:
Now, a small word about the Rin and the internals of the Op-amp. While talking
about ANY device, there will be a few parameters you need to keep in mind, more so
when you are working with Analog and not digital signals. Whenever you attach a
new device to you existing circuit, you are actually making changes in the RLC
parameters of the circuit, which might have disastrous results, eg. you attach a device
which has a very low R in parallel to your circuit which was functioning just about
right with the meticulously adjusted R-thevnin, but there you go… the whole circuit
fails, because now, suddenly, the net resistance falls too low!!!
To cope up these changes, there are some internal properties of the chips & devices
hat we use.
X
Z
Y
It has infinite input impedence RIN.
 It has 0 output impedence R0.
 It draws in no current from the input pins.
 There is a virtual short connection between the input terminals.
Ideally, the input impedance of the black-box device named the op-amp is infinite.
What it means is that the internal resistance in the op-amp between the two input pins
is of a very high magnitude such that it can be taken as infinity. This being the reason
for it not drawing any current & so all the current coming to X goes through RF.
The output impedance is so small that it can be taken as 0 and so the output voltage is
Av* Vi taken from Z.
Now, very important concept- VIRTUAL SHORT : In the figure above, There is a
virtual short between the X & Y points. This means that X & Y have the same
potentials i.e. in the figure above, as Y is connected to ground, so the X point also
gets to ground voltage. This may seem to be a little strange, but just accept the fact
for now. This concept would be very important for deriving the amplification
equations, switching etc.
 Saturation:
Just think of it, can we amplify a signal to any extent as we like? If yes, whr does that
voltage difference come from?
Actually, we can amplify the signal only upto a particular Voltage which has to be fed
into the op-amp. It can’t amplify the signals beyond that (obvious reason –Energy
consideration) Thes voltage is called VSATURATION which is the same VSS as shown in
the diagram on the first page.
Note: The +Vss in our case will be Vcc and the –Vss is the GND.
Now, if the parameters as set such that the amplified signal is less than the VSAT, then
the signal is represented as it should be. But if the amplification makes the output
signal greater than the +VSS or less than the –VSS , then:
1) If (inverting input) > (non-inverting input) output moves towards -Vss
2) If (inverting input) < (non-inverting input) output moves towards +Vss
Owing of these extreme responses, we can use the op-amp as an electronic switch, or
as a comparator, the technically correct name. Say you have fixed
the inverting voltage at some reference value Vref. While the
voltage at the non inverting input (Vin) is more positive than the
voltage at the inverting input (Vref), the output voltage will be in
positive saturation (+Vsat). As the voltage of the non inverting
input (Vin) becomes less positive than the voltage at the inverting
input (Vref), the output voltage will be in negative saturation
(-Vsat).
--Remember the internal diagram of the 555 timer, this might be a
nice time for a little revision.
 Feed Back:
Although the immediate jumping to saturation voltages helps us in making a
comparator, but when you come to think of it, it is not a very useful thing to have. We
would be much better off with a device which provided gains as per our needs. The
good news is that our ‘uncanny’ device can do that too, though in a somewhat twisted
manner. Imagine what will happen if we take the output and plug it into one of the
input of the op-amp. The op-amp will be providing itself the voltage it is outputting,
i.e. feeding back. These connections made from the output pin of the op-amp to the
input pins are called feedback connections.
This negative feedback ensures that the voltage at both the pins will remain the same
(sorted) while no current will be drawn at all from the circuit.
Ignore the
capacitanc
e attached
to the first
op-amp for
now.
These configurations provide gains which can be easily manipulated by changing the
values of the resistors. DO NOT confuse this new gain with the old gain of the
differential op-amp. The open loop gain of the op-amp is still infinity. The closed loop
gain, however, is the G shown here. Now whenever we talk about gains, they will be
the closed loop gains of the op-amps.
Chip Used :
LM324
It is a quad Op-Amp i.e. it is a chip which has four independent op-amps in it. The pin
connections are as follows:
Applications
1) Amplifier
Op-Amp is the most commonly used amplifier in analog circuitry. All it does is when
you have a slight voltage difference in the terminals, then the output is the voltage
difference between the two pins multiplied by a very large factor.
For op-amps, to avoid confusion, this gain AV is often called the open loop gain.
Also, the input given to the pin marked positive is called the non-inverting input(V+)
and to the negative pin the inverting input (V-). The input voltage for the op-amp is, in
our mode of working,
Vi= V+- VThere are two basic kinds of amplifiers that can be made : namely – Inverting
Amplifier & Non- Inverting Amplifier (does it ring any bell????)
a) Invering Amplifier : In this kind of
amplifier, the amplified signal has
opposite polarity as that of the input
signal; reason being that the input signal
is in the inverting input (-ve).
Working the expression
Vout = -(R2/ R1) * Vin
Is pretty trivial using the concepts of
virtual ground & that the op-amp
doesn’t draw any current in from the
input. So, current through R1 is Vin/ R1. As the whole current passes through R2 & due
to virtual short, V at X is 0, So, Vout – VX = Vout = - (R2 / R1)* Vin. using the simple
thevenin’s laws.
b) Non- Inverting Amplifier: Input goes
to the non-inverting input & the resulting
expression for Vout using the same funda is
:
Vout = (1 + R2 /R1 )* Vin
(work out this expression yourself)
2) Comparator
Now let us probe into another very common &
important application: Comparator
In this, in the –ve input, the Vref is fed in . & so
we get the output waveform as shown in the
figure.
3) The Schmitt trigger:
Remember we delayed the discussion of the
noise-proofing of the comparator as a switch?
Now we see a simple way of doing it. See the
adjoined figure.
The eagle eyed reader would have noticed a
peculiar thing about the circuit: Its positive
feedback.
This has interesting implications, this circuit
being one of them.
Now, we had been working with the negative
feedback till now. This positive feedback applications
gives a very interesting hysteresis as follows:
4) Summing Amplifier:
Consider the following circuit:
Now, using the simple Thevenin’s Theorems & op-amp characteristic, it can be
derived that:
Vo = -Rf (V1/R1 + V2/R2 + V3/R3)
Now that we know how to make a summing amplifier, ponder over what this
particular circuit will do: