Input Differential Amplifier - Basics Tutorial 741 IC - Op-Amp Structure And Testing

An amplifier is a circuit which can produce an output voltage, which is the product of input voltage with a value called voltage gain. An op-amp (operational amplifier) is a kind of amplifier circuit which can perform an operation (addition, subtraction etc.) on the input voltages, apart from simply amplifying the input.
An op-amp (operational amplifier) is an electronic circuit made of several active devices (transistors) and passive devices (resistor, capacitors) etc. which is capable of realizing following the common features:The term op-amp first appeared in a patent produced by Karl D. Swartzel of Bell Labs in 1941. This amplifier was capable of doing a summing operation on the input voltages.

Suggested Reading:-
Operational Amplifiers Tutorials Series 
Linear Op-amp Operation-Inverting And Non-Inverting Configuration
Voltage Follower and Difference Amplifier Configuration
Instrumentation and Summation Configuration
Op-amp with T-bridge Feedback Network
Op-amp Integrator Network

Functional blocks of An Op-Amplifier

Op-Amplifier is  made of several other basic electronic components like transistors, resistors, capacitors etc. Almost all op-amp ICs internally has the same basic functional blocks, built by the basic electronic components. These functional blocks are namely,                      

A. Input Differential Amplifier

A differential amplifier is the most important module inside an op-amp. Input voltages are applied to the pins of differential amplifier block. Let's discuss about the differential amplifier in detail.

A normal amplifier amplifies the entire signal voltage with reference to the ground and is fed to the output. And those amplifiers normally have a single input and obviously a single output. For example if we give a 5V as input with reference to the ground to a normal amplifier and the voltage gain of the amplifier is say 2, then the output will be 10V, provided that the circuit is given a supply voltage of more than 10V.

In the circuit shown above, you can see an amplifier which is connected to a power supply of 20V, and having a voltage gain of 2. When a voltage of 5V is fed to the only input pin, the output will be 10V. The GND is considered as a common reference point for both input voltage and the output voltage.

Differential amplifier on the other hand amplifies only the difference between the two input voltages. For example if the gain of the differential amplifier is say 2, and if we give a voltage 3V to one of its input pin and on the other pin we give a voltage say 5V. Now the difference between those two voltages i.e. (3~5=2) is amplified and will be available at the output. Hence the output voltage is 2V*2=4V.

Note: Common mode voltage -

Typical differential amplifier hence rejects or masks the effect of common mode voltage in its output.Common mode voltage means the voltage which is common to both the input pins. For example if we apply a voltage of 5V to one input and 3V to another input pin, then the common mode voltage is 3V.
Hence if the inputs voltages are,
Input voltage1, V1 = 5V,
Input voltage2, V2 = 3V, then
Common mode voltage = 3V, and
Difference voltage = 5 ~ 3 = 2V

The differential amplifier simply rejects the common mode voltage and amplifies the difference voltage only.

Differential amplifiers basically have two power-supply, two inputs and two outputs. The two outputs are then combined to a single output using circuit called current mirrors. The voltages across the input pins are called differential input voltage and the voltage across the two output pins are called differential outputs.

In the above circuit, there are two input terminals, marked as Vin+ and Vin-. The output voltage is obtained differentially across the collectors of two transistors. 

The two output pins can be combined to a single output pin using a differential to single ended conversion. We call such a converting circuit as current mirror.

The inverting input produces a negative amplified voltage in its output and the non-inverting input produces a positive amplified voltage in its output. The differential to single ended convertor converts that differential voltage to single ended voltage.


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