This applies to the problem of proper biasing in transistor amplifiers. Take a look at the diagram shown below:

The biasing on the left is the correct biasing while the one on the left is the incorrect one. The wire from Ci should be should be shorted with the resistor network as shown in the circuit and should not overlap it like the second example. In the case of the wrong biasing, since the base-emitter junction had got no biasing at all, the BJT never gets on and you will not get any output at all from the amplifier. Many people tend to make this mistake and thus the biasing of the BJT is not proper.

Opamp input voltages

When connecting a circuit with an opamp remember that the input voltages given to the opamp should not exceed the Vmax and Vmin values set in the opamp properties. Alternatively while using a five-pin opamp, the input voltages should not exceed the voltages set for the supply voltages for the opamps. If the opamp uses a supply of +15 to -15 V then the input should not cross this threshold. If it does happen  to then the circuit simulation may happen to fail. While designing the circuit, ensure that you give input voltages that do not exceed the voltage limit set by the opamp.

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