Op_amp_troubleshooting

Published:2009/7/24 20:50:00 Author:Jessie | From:SeekIC

The troubleshooting approach described for the amplifiers of chapter 1 can be applied to op-amp circuits of this chapter. That is, first test the circuit to see if it performs the desired function, If not, try correcting the problem with adjustments. Then, trace signals using a meter or scope from input to output. If any portion of the circuit has a normal input, and an abnormal output, you have located the area in which the trouble occurs. From that point on, it is a matter of voltage measurements and/or point-to-point resistance measurements. The following are a few examples. In the circuit of Fig. 10-19, there should be a TTL-level output for a small differential input voltage. For example, if you ground one input, and apply a signal to the opposite input, there should be a TTL-level output at pin 6. If you ground both inputs, there should be no signal output. However, the output might shift in voltage level because of the input-offset voltage. If there is no output with a differential-signal input, suspect the LM101A. If there is an output from the LM101A, but not at the desired TTL level, suspect the LM103. In the circuit of Fig. 10-20, there should be an output when the photodiode is exposed to light. Because it is difficult to monitor the input current, measure the voltage across R2 (from pin to ground). If there is no change in output voltage when the photodiode is exposed to varying light conditions, suspect the photo-diode, the LM101A, or D2. If the output does change with light current, check the values of R1, R2, and D2. It is also possible that the photodiode is defective, and producing a nonlinear output.



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