Mumbai University > Computer Engineering > Sem 3 > Electronic Circuits and Communication Fundamentals

Marks: 10 Marks

Year: May 2016

CMRR

It is the ratio of differential voltage gain $A_d$ to the common mode voltage gain $A_C$.

$$CMRR=\dfrac{A_d}{A_C}$$

Now $A_d$ is nothing but open loop voltage gain $A_{OL}$. And $A_c$ is measured by using the circuit as shown in figure

The common mode input $V_C$ is applied to both the input terminals of OP-amp. Then the output $V_{OC}$ is measured. Then common mode gain $A_C$ can be obtained as,

$$A_c=\dfrac{V_{OC}}{V_c}$$

It is generally very small and not specified in the data sheet. The CMRR is generally specified for the op-amp and is expressed in dB. For op-amp 741C it is 90dB.

Slew Rate

The slew rate is defined as the maximum rate of change of output voltage with time.

The slew rate is specified in V/μsec. Thus,

$$Slew \ \ Rate=S=\dfrac{dV_0}{dt}\bigg| \max$$

The slew rate is caused due to limited charging rate of the compensating capacitor and current limiting and saturation of the internal stages of an op-amp, when a high frequency, large amplitude signal is applied.

The internal capacitor voltage cannot change instantaneously. It is given by $(dV_c)/dt=I/C$. For large charging rate, the capacitor should be small or charging current should be large. Hence the slew rate for the op-amp whose maximum internal capacitor charging current is known, can be obtained as

$$S=\dfrac{I_{|max}}{C}$$

For example, for IC 741 the charging current is 15 μA and the internal capacitor is 30 pF, hence its slew rate is

$$S=\dfrac{15 \times 10^{-6}}{30 \times 10^{-12}}=\dfrac{0.5}{10^{-6}}V/\sec \\ =0.5 V/\mu \sec$$