Electronic – What exactly is stopping OpAmps from reaching VCC/GND


So I have been reading about OpAmps and also simulated a bit with Ltspice. I made a simple Integrator with an LM324 OpAmp and its getting close to the positive rail but Its not really exact.

What is causing OpAmps not reaching exact positive rail values ? Is it the circuit inside the OpAmp limiting it ?

Best Answer

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Figure 1. The LM324 op-amp internal circuit.

Note that when the output is driven high that the Darlington transistor pair, Q5 and Q6 must turn on. Since the base-emitter junctions of Q5 and Q6 each drop about 0.7 V when on it stands to reason that the most we could expect out of this circuit is V+ - 1.4 V. Since Q5's base current comes from the 100 µA current source we must allow for the voltage drop of that too.

When swinging negative Q13 provides a path to ground. Since its base must be pulled low by Q12 to turn on we have a similar, although slightly improved, version of the problem.

You should be able to simulate each of the cases for the top and bottom drive circuits and see what the minimum and maximum output voltages are in each case. Then compare these with the datasheet.

Note that the voltage range will degrade as the load Iout increases (mA) .