# Op-Amp Question

1. Jun 24, 2008

### hemant03

Hi,
I am new to circuit designing and I have a question on one of the parameters in the spec sheet of op - amp.
I donot quiet understand (see the graph) what does the output saturation voltage v/s (overdrive voltage or load current) is and why is it almost the same for outout being low or high.
I have attached the pdf of the specsheet for easy viewing.

#### Attached Files:

• ###### LT1638_LT1639.pdf
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2. Jun 24, 2008

### NoTime

Basically an OP-amp just multiplies the input by the gain.
You can't get a result greater than the supply voltage.

What exactly happens when your input exceeds the maximum allowed by your supply voltage depends on the internal construction of the OP-amp.
The graphs are there to show the result of doing this.

3. Jun 25, 2008

### hemant03

Hi,
Thanks for the response but my question is specific to the graph showing output saturation voltage v/s load current for output being high or low. Basically I am unable to understand the relation between output sat. voltage, load current and output being a high or low.

Thanks,
- Hemant

4. Jun 29, 2008

### GTrax

Opamps have transistors in their output stages to deliver these currents. A saturated output stage is one that cannot deliver any more current, even if you drive it harder. It is saturated !

Look at page 12 of the PDF data to see the output stage is two bipolar transistors, the PNP one for pull-up, and the NPN one for pull-down. A saturated transistor is not a zero resistance device. There will be some volts across it that will increase as the current increases.

Notice also that both axes of the data graph are logarithmic.
Thus the very nearly straight sloping part of line still indicates proportionality. ie. the apparent resistance of the fully turned on transistor is nearly constant! The increasing current simply delivers increasing saturation voltage. You should appreciate that to get this measurement, the current is being controlled by some external circuit. The transistor, being saturated, no longer has any say in limiting the current.

Yet the logarithmic scale use does exaggerate the display of voltages and currents that are very small. Thus you get to see that there is a certain minimum voltage drop across the transistor that is independent of the current, and is there even when the current drops to zero. It is, however, somewhat dependent on temperature. There are three separate curves given for different temperatures. When the current is reduced to the point the voltage drop due to bulk resistance is not significant, you get to see the remaining voltage left on the collector because of the saturated state of the base-emitter junction.

Finally - be aware that the saturation voltage is not the output voltage of the op-amp at the time. Notice there are two data graphs. One for 'output high' and the other for 'output low'. The saturation voltage is the difference between the (low or high) supply rail and the output terminal, when the opamp is driven so hard the output saturates.

I hope that does it, because I am better at using the things instead of thinking about their insides.

Last edited: Jun 29, 2008