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## Electrical engineering

### Course: Electrical engineering>Unit 3

Lesson 1: Operational amplifier

# What is an operational amplifier?

The "operational amplifier" has two differential inputs and very high gain. Willy describes the symbol and properties of an op-amp. Op-amps are the backbone of analog circuit design. Created by Willy McAllister.

## Want to join the conversation?

• At , are BOTH of the power supplies, POSITIVE power supplies? • Hello Wagner,

Yes, the power supplies are identical. For example, you could use two 9 VDC batteries. Notice how they are connected at . The ground makes one power supply appear positive and the other appear negative. The op amp takes advantage of this by alternatively "pulling" the load to the positive supply and then to the negative supply.

Regards,

APD
• What are the internal workings of the operational amplifier? Is it a transistor or something? • Hi. How do i differentiate an inverting op amp from a non inverting op amp? • At , why might the the orientation of the symbols be changed? • The opamp triangle symbol is symmetric, and the "pointy" output end aways (usually) points to the right. But there is nothing preventing the person creating the schematic from putting the + input on top or the - input on top. So you always take a quick glance at the signs to avoid making a bad assumption. My habit when I draw opamp schematics is to make the top opamp terminal the one that takes in the main input signal coming from the left. This can be either the + or - input depending on the opamp configuration I'm using. Each designer tends to have some sort of habit, but we are not all the same.
• what will happen to the op amp if the differential inputs are given with interchanged polarities • Hello Venkata,

The short answer is that the direction of the output will change.

In practice nearly all op-amps operate using a negative feedback loop. If you connect the terminals incorrectly the feedback loop will be broken. Instead of having helpful negative feedback you end up with positive feedback. The circuit will either oscillator or more than likely, lock up on one of the power rails.

Sorry, if you are new to op-amps I have included many new terms. May I suggest that you print this note and set it aside. Continue your study of op-amps and take a look back from time to time to see if you understand the concept.

Regards,

APD
• What is the relation between the open loop gain and closed loop gain of an op-amp? • Open Loop gain refers to the gain of the opamp itself (just the triangle symbol). When you include the opamp in a circuit with resistors, there is always a resistor that connects from the opamp output back to one of the inputs. That forms the 'closed loop'.

If you connect everything up right the Closed Loop gain of the circuit will be determined only by ratios of resistors. The open-loop gain of the opamp drops out of the computation of closed-loop gain.
• At he mentions about 20 to 50 transistors and resistors being used so if vacuum tubes were used instead of transistors what would be the number of vacuum tubes? • Hello Amresh,

This is a complex question. In modern op-amps you will find both PNP and NPN transistors. Know that there is only one type of vacuum tube - think of it as an NPN transistor. Consequently, the circuits must be designed differently.

You may be interested in the Philbrick K2-W - link below. This is one of the first recognizable op amps on the market. It uses two each dual triodes (12AX7 vacuum tubes) as the active elements. By modern standards the circuit is very inconvenient with its + and - 300 VDC power rails and the 6.3 V (likely AC) for the tube heaters.

Fun fact - the vacuum tube identifier tells you something at a glance. The first number (12) indicates the heater voltage. For this particular tube the heaters may be run in parallel or series. Philbrik chose to use a parallel 6.3 V connection.

The last number in the tube identifier (7) tells you the number of active components. Here we have a dual triode (2 x 3) plus the heater.

Enjoy,

APD

Ref:
http://www.philbrickarchive.org/k2-w_refurbished.pdf
http://www.philbrickarchive.org/
• Is it possible to operate a UA741 op amp with a single power supply?. And if so, Should the voltage be twice of the dual supply voltage?
(1 vote) • At , what does he mean by "v+ goes this way"?? does that mean raising the value of +v? same question with -v also..
(1 vote) • Hello Karthikeyan,

V+ physically describes the non-inverting input terminal to the op-amp. It is also a description of the input voltage. We need to look at the context to determine which thing we are talking about.

Similar statement for V-.

Vo physically describes the output terminal. The same term is also used to describe the output voltage as measured at the output terminal.

The op amp will respond to the difference between V+ and V-. If V+ is greater than V- then Vo will swing positive. Likewise if V- is greater than V+ the output will swing negative. The amount of swing is (V+ - V-) * Gain. Where gain is a very large number.

BTW I read your profile where you described your search. May I recommend: taking a look at

Please let me know if you have any other question.

Regards,

APD
• what does it mean to have a negative voltage?
I mean, if i use a battery of 1.5v, then the positive terminal is at +1.5v and the -ve at 0v right?
Or is the other terminal at -1.5v?
But that would mean a 3V potential difference right?
(1 vote) • Voltage is a relative measurement. When you say a voltage value, it is always with respect to some other point. Even if you don't mention the other point explicitly, there is always another point. If it is not mentioned, that means it has to be assumed, and you have to know that everyone talking about the voltage shares the same assumption about the other point.

If you have a 1.5v battery, the positive terminal is at +1.5v relative to the negative terminal. It is also equally correct to say the negative terminal of the battery is at -1.5v relative to the positive terminal. If you say "the voltage of the negative terminal is 0v", that means is you are assuming the "other point" needed to specify voltage is also the negative terminal.

Voltage is kind of like height. The top of my head is 5' 9" above my feet (+5'9"), and I can also say my feet are 5' 9" below the top of my head (-5'9"). Usually when we talk about the height of a person the assumed reference for 0 height is the bottom of your feet.