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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 (Non- inverting 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 states- either 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-AmpsA 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.

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.

X

Y

Z

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 -Vss2) 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

capacitance 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+- V-

There 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) ComparatorNow let us probe into another very common & important application: ComparatorIn 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: