Question

Discuss the properties of an ideal op-amp and compare these properties to those of a 741C op-amp.

Discuss how the frequency of the input signal to an op-amp affects the voltage gain.

Please NO hand Writting

Answer 1

Properties of Ideal Opamp and comparison with IC741 opamp

1. It has INFINITE open loop voltage gain (the gain when there is no feedbackin the circuit of opamp)

  

Whereas for IC741 opamp it is :

  

2. It has INFINITE input resistance i.e. ideal opamp draws zero current from input source.

   Whereas for IC741 opamp it is :

   to   

3. It has ZERO output resistance. Hence output voltage of ideal opamp is independent of Load resistor.

Note: Ideal opamp is ideal voltage amplifier. It is also called Voltage controlled Voltage source.

     

Whereas for IC741 opamp it is :

   to

4. It has INFINITE bandwidth i.e Ideal opamp can amplify signal of any frequency.

Whereas for IC741 opamp the bandwidth is 5 Hz.

5. It gives ZERO offset (output voltage of opamp when both input terminal voltages are zero) i.e.

If and then   

where   and are the voltages at the two input terminals of Opamp

is the output voltage of opamp.

  Whereas for IC741 opamp it is NON ZERO i.e.

  If and then   

6. It has INFINITE Common mode rejection ratio (CMRR) (It is the measure of the ability of amplifier to reject comon mode input). A high CMRR implies better rejection of Common Mode input.

If    then

Ideal opamp is true Difference Amplifier.

  Whereas for IC741 opamp it is or 120 dB.

7. It has INFINITE Slew Rate (It is the maximum rate at which output voltage of opamp can change)

Unit: V/µsec

Whereas for IC741 opamp it is 0.5 V/µsec to 1 V/µsec

Frequency response of IC741 opamp:

A plot of open loop gain () vs frequency:

Opamp is a Direct coupled Amplifier. Therefore Voltage gain remains constant at low frequencies     and decreases at higher frequencies due to a Compensation Capacitor present in the internal structure of opamp.

Open Loop gain of opamp at any frequency can be calculated by using:

where

   in the plot is called cuttoff  / 3dB / open loop bandwidth

   in the plot is the maximum gain or DC gain ( gain at frequency=0).

   in the plot is frequency

in the plot is unity gain bandwidth i.e. the frequency at which open loop gain becomes unity

At = UGB, Gain of opamp