Operational Amplifier: Understanding Operational Amplifiers

Operational Amplifier
Operational Amplifier

Introduction to operational amplifier:

Operational amplifier is the most useful device in analog electronics. It is also quite affordable and simple. It is usually abbreviated as Op-amp. It can be defined as a direct coupled high gain amplifier which amplifies difference between two input signals.

The block diagram of an Op-amp is a multistage amplifier which consists of input stage, intermediate stage, level shifting stage and output stage which is as shown in figure-

Op-amp has two inputs inverting and non-inverting. The input stage is dual input balanced output differential amplifier which provides voltage gain of the amplifier and rejects noise.

The intermediate stage is usually dual input unbalanced output differential amplifier which gives single ended output i.e. output with respect to ground.

After this, level shifter circuit is used to shift the DC level to zero volt because the output of intermediate stage is well above ground level.

The last block is nothing but an output stage which is complementary push pull amplifier. It increases the output voltage and current carrying capability.

Symbol of Operational Amplifier:

 

Operational Amplifier
Operational Amplifier

 

V1 is Non-inverting input. Input signal applied at this terminal produces in phase signal at the output.

V2 is Inverting input. Input signal applied at this terminal produces out of phase signal at the output.

Vo is Output signal which is the amplification of difference between two inputs signals.

Vo is A*(V1-V2)

Where A= open loop voltage gain (gain without feedback and gain )

More about 741 Operational Amplifier:

Now days, the most popularly used op-amp is IC LM741. The 741 IC was designed by David Fullagar of Fairchild Semiconductor in 1968. This is very much useful in integrator, summing amplifier, voltage follower and other basic applications. The IC 741 is available in the market as 8 pin DIP (Dual Inline Package). The pin configuration is shown below:

 

Ideal characteristics of Operational Amplifier:

 

  • Infinite voltage gain
  • Infinite input resistance
  • Zero output resistance
  • Zero offset voltage
  • Infinite bandwidth
  • Infinite Common Mode Rejection Ratio (CMRR)
  • Infinite slew rate

We know that, no component can be ideal so practical characteristics of operational amplifiers are as discussed below.

Input offset voltage (Vio) of Operational amplifier:

It is the voltage that is applied between the input terminals of op-amp to nullify the output (i.e. to make the output is zero). An ideal op-amp amplifies the differential input i.e. if both inputs are at the same voltage with respect to ground; the output is zero. But due to manufacturing process, the differential input transistors of real op-amps might not exactly match. This causes output to zero at non-zero value of differential input which is called input offset voltage. Its maximum value is 6mV for 741 op-amp.

Input offset current (Iio) of Operational amplifier:

The algebraic difference between the currents entering into non-inverting and inverting terminals is called as input offset current.

Iio =|IB1- IB2|

IB1= non-inverting input current

IB2= inverting input current

For IC 741, its value is 200nA.

Input bias current (IB):

The average of currents entering into inverting and non-inverting terminals of op-amp is called input bias current.
Its value is 500nA for IC 741.

Differential input resistance (Ri):

It is the equivalent resistance that is measured from any one of the input terminals by keeping the other terminal connected to ground. Its value is high as high 2MὨ.

Input capacitance:

It is the equivalent capacitance that is measured from any one of the input terminals by keeping other terminal connected to ground. It value is 1.4pF for IC 741.

Common Mode Rejection Ratio(CMRR):

It is the ratio of differential mode voltage gain to common mode voltage gain. It is measured in decibel (dB).

CMRR= 20log (Ad/Ac)

If the value of CMRR is high, there is better matching between the 2 input terminals.

For IC 741, CMRR is 90dB.

Large signal voltage gain (A):

It is the ratio of output voltage to the differential input voltage.

A= (Output Voltage) / (Input Voltage)

Its value is 2, 00,000 for IC 741.

Supply voltage rejection ratio (SVRR):

It is defined as changes in the op-amp’s input offset voltage due to changes in the supply voltage.

SVRR= Change in input offset voltage (dVio)/Changes in supply voltage (dV)

The lower the value of SVRR, the better will be the performance.For 741 IC, SVRR= 150μV/V.

Output voltage swing:

This is nothing but the range of values of positive and negative saturation voltages of op-amp. The op-amp voltage swing values cannot be greater than the supply voltages +Vcc and –Vee. For op-amp 741, its value is between +13V and -13V.

 Slew rate (SR):

Slew rate is one of the essential parameters of op-amp when op-amp is used at higher frequencies. It is defined as the maximum rate of change of output voltage per unit time. It is measured in volts/μSeconds. By calculating slew rate, we can easily find out the rate in which the output of op-amp changes in response to changes in the input frequency. The SR of particular op-amp is fixed.

For IC 741, its value is 0.5V/μs.

 

 Gain bandwidth product:

As the name suggests, it is the product of gain of op-amp and bandwidth. It is the bandwidth of op-amp when voltage gain is unity. Its value is 1MHz for IC741.

Applications of Operational Amplifier:

  • In signal processing.
  • Control system.
  • Instrumentation.
  • Communications.

Digital Clock using 8051

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