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Basic Operational Amplifier (Op-Amp)

Operational amplifiers were first developed by George Philbrick when he was an employee for the National Defense Research Council in the early 1940s. The op-amp later went through many transitions; Fairchild Semiconductor corp. developed a standard in 1962 - the uA741.The operational amplifier, also called op-amp, is the workhorse of the electronics field. It evolved as a vacuum tube circuit, from the World War II era, to its present form as an Integrated Circuit (IC). The op-amp is used in applications such as amplification, integration, summing, logarithmic scaling, wave shaping, oscillators, etc. An operational amplifier is a direct coupled high gain differential amplifier to which feedback is added to control its overall response characteristics. It has very high input impedance and low output impedance. It is used to perform a wide variety of linear functions and some nonlinear functions and is called as basic linear (analog) integrated circuit. Op-amp is provided with two input terminals called as Inverting and Non inverting input terminals and one output terminal. The signal applied at the inverting input terminal produces a phase shift 180 0 at the output terminal and no phase shift (00 phase shift) at the output terminal when signal is applied at the non-inverting input terminal. Some of the linear systems constructed with Op-amp as the basic building blocks are: Voltage to current and current to voltage converters, Amplifiers, Voltage followers, Active filters, etc. Some of the nonlinear systems constructed with Op-amp are: Amplitude modulators, Log and Antilog amplifiers, Precision rectifiers, Sample and hold circuits, Comparators, Square wave and triangular waveform generators, etc.

PIN DIAGRAM OF OPERATIONAL AMPLIFIER IC µA741C (µA stands for Fairchild)

Fig1: Pin-out of the op-amp

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V+ Vp Vo Vn

Fig 2: The circuit symbol for an op-amp

V-

Op-amps can also be used to build circuits that perform Analog-to-Digital (A/D) conversion, square-wave generators (oscillators) and a whole bunch of other stuff. The robustness of the op-amp is because the functionality of the op-amp can be extended by the use of feedback. The op-amp has 5 terminals. Two input terminals (+ and -, called the non-inverting and inverting terminals respectively), one output terminal (Vo) and two power supplies (V+ and V-). An opamp is a differential amplifier, which amplifies the difference between the two input terminals. If we have a source with voltage Vp connected to the + terminal and Vn to the terminal, the output is: ---1 where A is a dimensionless constant (called open-loop voltage gain) specific to the op-amp. Vo is the output voltage, V+ and V- are the power supplies.

Please note: the actual circuit inside an op-amp is complex and non-linear. The model below is an excellent linear approximation to the actual circuit inside an op-amp, and that is all you need to know for LIC subject.

Vp

Vo Vn

Fig 3: An approximate linear circuit model for an op-amp

------2

(G/A is a gain, Output is open, no current flows through Ro/Rout)

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If we graph Vo as a function of (Vp-Vn), we get the Voltage Transfer Characteristic (VTC) of an op-amp, as shown below: Vo

Keeps increasing

Vp-Vn

(micro volt) Keeps Decreasing

Fig 4. Ideal VTC of an op-amp

The graph shows that Vo varies linearly as a function of Vp-Vn. This should be obvious from equation 1 since A is a constant. In reality, we all know that you cannot get infinite gain - the amplifier cannot amplify if we input a trillion volts. This is where the functionality of the power supplies comes into play. A real op-amp will have a VTC shown below:

Vo V+

V-

Vp-Vn

(micro volt)

Fig 5. Non-ideal VTC of an op-amp

As you can see, the op-amp cannot output a voltage greater than its positive power supply (V+) or lower than its negative power supply (V-), a phenomenon called railing. Remember this when you use an op-amp.

Power supply is one of the most confusing aspects new students encounter with an op-amp. Please be very careful about your power supply values and connections.

Great, how do you use an actual op-amp? A real op-amp does not look like the triangle above. Rather, it comes in the form of Dual-Inline-Package Integrated Circuit (DIP IC) as shown below:

Fig 6. Pins of Op-amps

As you can see, the actual op-amp is very tiny indeed! In order to use the op-amp above, you refer to the product's datasheet. A datasheet (as the name suggests) has important data describing the many features of your product. In order to correctly read the pin numbering on the op-amp IC: hold the op-amp with its top-side (the LM741 lettering side) facing you. You will see

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a notch corresponding to the notch AND/OR you will see the circle shown in figure. The pin to the left of the notch or circle is pin number 1, and it increases going down. On the right side, the bottom pin is 5 and it increases going up. Don't worry about the NC and the Offset Null pins. You can leave them disconnected when you breadboard/spring board your op-amp. We are almost done with the theory part. The final concepts we have to learn are a few simple rules for analyzing an op-amp circuit and the concept of negative feedback. Both these concepts arise out of the constant A in equation 2.

Basic Ideal Op Amp Assumptions

Op-amp golden rules As stated earlier, the scale of Vp-Vn in the VTC of an op-amp is in microvolts. This is because the open-loop gain (the constant A in equation 2) is usually in the order of 106. For all practical purposes, you approximate the open-loop gain of an ideal op-amp as infinity. ---3 You also make the following approximations for an ideal op-amp: ---4 ---5

Resistor color codes

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Important Instructions: Check the AFO (Audio Frequency Oscillator) and CRO (Cathode Ray Oscilloscope). Keep the OFFSET knob at its minimum (off); otherwise it may cause clamping of the output waveform. Check the patch cords/ single stand wires for continuity before connecting. Avoid insulation while using single strand wires on the spring boards. Check the connecting board for proper connections. Adjust the voltages Ve=-15V and Vc=15V for the op-amp. Scale should be mentioned in the graph without fail before drawing the waveforms. Keep the CRO in proper mode (AC or DC) depending on the experiment. Check the connections before switching on the power supply. All grounds of the circuits should be shorted together without fail. If supply goes while conducting the experiment, if staff member calls you or whenever you are leaving the table switch off the main of that table. When Lab experiment is over then take the sign in your observation book from the staff member then remove the connections with staff permission. Do not move instruments from one lab to another lab.

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