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Steven Ramsey's Essay on the LM158N chip


Entry ID #: 2158
Created: Mon, Jan 11, 2016 5:29 PM

I chose a LM158N ozone collecting chip. I was always fascinated that the special chip has two circuits that are independent, high gain, internally frequency compensated which were designed specifically to operate from a single power supply over a wide range of voltages. The low power supply drain is independent to the magnitude of the power supply voltage. Application areas include transducer amplifiers, DC gain blocks and all the conventional op-amp circuits which now can be more easily implemented in single power supply systems. For example, these circuits can be directly supplied with the standard +5V which is used in logic systems and will easily provide the required interface electronics without requiring any additional power supply. In the linear mode the input common-mode voltage range includes ground and the output voltage can also swing to ground, even though operated from only a single power supply voltage. I discovered that the chip has two inputs, an inverting input and a non-inverting input.  I didn’t think the chip would have two inputs.  The supply voltage is +/-16 or 32 volts. The input voltage is -0.3 to +32 volts. However the differential input voltage is +32. This input current only exists when the voltage at any of the input leads is driven negative. It is due to the collector-base junction of the input PNP transistor becoming forward biased and thereby acting as input diodes clamps. In addition to this diode action, there is also NPN parasitic action on the IC chip. This transistor action can cause the output voltages of the Op-amps to go to the VCC voltage level (or to ground for a large overdrive) for the time duration than an input is driven negative. This is not destructive and normal output will set up again for input voltage higher than -0.3V.  


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