What is Slew Rate? | Op Amp

Slew rate is defined as the maximum rate of change of the op amp output voltage per unit of time in the closed-loop configuration under large-signal condition. Slew rate indicates how rapidly the op amp output can change in response to change in the input frequency. It is given as

    \[ \boxed{Slew\; rate=\frac{\mathrm{d} V_o}{\mathrm{d} t}\mid_{max}} \]

The Slew rate of an op-amp is fixed. If the slew rate is 10\frac{V}{\mu sec}

then it means that the op-amp output voltage can change maximum by 10 V in 1 \mu sec.

The ideal value of the slew rate is infinite. Practically it is a finite value and it is due to the following reasons

  1. saturation of internal stages and
  2. the finite time constant of internal capacitors
Slew rate
Fig. 1 Operational amplifier

Consider figure 1. Vi and Vo are respectively the input and output of the op amp. Av is the closed-loop voltage gain.

V_{i}=V_{m}\sin \omega _ot

Therefore, the output voltage V_o=A_vV_i

    \[\Rightarrow V_{o}=(A_vV_{m})\sin \omega _ot\]

Now,

    \[\frac{\mathrm{d} V_o}{\mathrm{d} t}=A_v\frac{\mathrm{d} V_i}{\mathrm{d} t}\]

\Rightarrow \frac{\mathrm{d} V_o}{\mathrm{d} t}=A_vV_{m}\omega _o\cos \omega _ot

Therefore,

\Rightarrow \frac{\mathrm{d} V_o}{\mathrm{d} t}\mid_{max}=A_vV_{m}\omega _o\times 1

    \[\frac{\mathrm{d} V_o}{\mathrm{d} t}\mid_{max}=A_vV_{m}\omega _o\]

For producing undistorted op amp output, \frac{\mathrm{d} V_o}{\mathrm{d} t}\mid_{max}

must be less than the slew rate i.e.

    \[ \boxed{\frac{\mathrm{d} V_o}{\mathrm{d} t}\mid_{max}\;<Slew\; rate} \]

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