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Op-amp characteristics

It is important to recognize the limitations of op-amps so that measurement errors may be avoided in instrumental applications. We will explore two important characteristics of several different common integrated circuit op-amps.
To measure the input offset voltage $V_{\rm offset}$, connect the op-amp as a voltage follower and connect the non-inverting input to common. The output voltage equals $V_{\rm offset}$. Perform this measurement for the three different types of op-amps and include the data in the table. Repeat for both of the LF353 dual op-amps. Note that the pinout of the 741 op-amp is identical to that of the LF351.

To measure the input bias current, $I_{\rm bias}$, a 10 M$\Omega$ resistor should be connected between the non-inverting input of the voltage follower and common. The IR drop across the resistor results from the bias current. The output voltage $V_{\rm out}$ is the sum of the offset voltage, $V_{\rm offset}$, and the IR drop across the resistor. $I_{\rm bias} = (V_{\rm out}-V_{\rm offset})/R$. Carry out this determination for each of the op-amps under investigation, tabulate, and comment on the results.
   Op-Amp    $V_{\rm offset}=V_{\rm out}$, for $R_{\rm in}=0$ $V_{\rm out}$, for $R_{\rm in}=10$M$\Omega$   $I_{\rm bias}$, nA  
LF353-1      
LF353-2      
LF351      
$\mu$A741      


next up previous contents
Next: Advanced op-amp designs Up: Building circuits with op-amps Previous: Summing amplifier

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Last revised: 2007-01-05