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For small signal sinusoidal (AC) application one has to know the ac characteristics such as frequency response and slew-rate.

**AC Characteristics:**

For small
signal sinusoidal (AC) application one has to know the ac characteristics such
as frequency response and slew-rate.

**Frequency Response:**

The
variation in operating frequency will cause variations in gain magnitude and
its phase angle. The manner in which the gain of the op-amp responds to
different frequencies is called the frequency response. Op-amp should have an
infinite bandwidth Bw =∞ (i.e) if its open loop gain in 90dB with dc signal its
gain should remain the same 90 dB through audio and onto high radio frequency.
The op-amp gain decreases (roll-off) at higher frequency what reasons to
decrease gain after a certain frequency reached. There must be a capacitive
component in the equivalent circuit of the op-amp. For an op-amp with only one
break (corner) frequency all the capacitors effects can be represented by a
single capacitor C. Below fig is a modified variation of the low frequency
model with capacitor C at the o/p.

There is
one pole due to R0 C and one -20dB/decade. The open loop voltage gain of an
op-amp with only one corner frequency is obtained from above fig.

f1 is the
corner frequency or the upper 3 dB frequency of the op-amp. The magnitude and
phase angle of the open loop volt gain are fu of frequency can be written as,

The
magnitude and phase angle characteristics from eqn (29) and (30)

1. For
frequency f<< f_{1} the magnitude of the gain is 20 log AOL in
dB.

2. At
frequency f = f_{1} the gain in 3 dB down from the dc value of AOL in
dB. This frequency f1 is called corner frequency.

3. For
f>> f_{1} the fain roll-off at the rate off -20dB/decade or
-6dB/decade.

From the
phase characteristics that the phase angle is zero at frequency f =0.

At the
corner frequency f_{1} the phase angle is -45^{0} (lagging and
a infinite frequency the phase angle is -90^{0} . It shows that a
maximum of 90^{0} phase change can occur in an op-amp with a single
capacitor C. Zero frequency is taken as te decade below the corner frequency and
infinite frequency is one decade above the corner frequency.

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