For small signal sinusoidal applications the AC characteristics of op-amp are discussed as follows:

  1. Frequency response
  2. Stability of an op-amp
  3. Frequency Compensation
  4. Slew Rate

Frequency Response :

An ideal op-amp should have infinite band width. If open loop gain is 90 dB with DC signal then it should remain the same for audio and radio frequencies also but practically the op-amp gain deceases / roll’s off at higher frequencies hence a capacitor component is placed in a circuit of op-amp 
     For an op-amp with one corner frequency all the capacitor effects can be represented by single capacitor.
     The following is a high frequency model of an op-amp:


The open loop voltage gain is obtained as:

        V0  = [-jXc/(R0 – jXc)] AOLVd

(or) A = V0/Vd = AOL/ 1+j2πfRC

A = AOL/ 1+j(f/f1)

f1 = 2πRC

The magnitude and phase angle of open loop gain is

│A│ = AOL/ 1+(f/f1)2   (or) Ф = tan-1 (f/f1)

Leave a Reply