Saturation Determinant

[Vizier87]

A 5V supply to an LM358 results in 3.7V max saturation output for my non-inverting amplifier. I'd like it to be 5V max output to maximize my output range to my uC, so I suppose adjusting the supply voltage helps that?

Cheers.

 

[audioguru]

Simply increase the supply to +6.7V then the max output will be +5.0V.
Or use a "rail-to-rail" opamp with a 5V supply.

 

[Vizier87]

Wait a mo... rail-to-rail is only inherent to specific op-amps, eh?

All devices are "commoned" in my circuit, and some is restricted to a supply of max 5.25V. I suppose since I'll be using a 6.7V supply, dropping the voltage to the limited-voltage device using diodes will aid this?

Lastly, AG, you said in a another post that a 0.2V (200mV) schottky diode is needed to limit the voltage spike between the two inputs so that the op-amp doesn't get damaged. I found out that LM35's output is 300mV as an input to the non-inv input to the LM358 (in room temp), and the op-amps responded normally. Can you please explain this?

 

[audioguru]

A Schottky diode at an input of an LM358 to ground will limit its negative voltage to 0.2V so that the input is not destroyed by a high current that flows when it is -0.3V or more.
The max allowed differential voltage (between inputs) is 32V regardless of the amount of supply voltage.
The inputs work fine all the way down to 0V.

 

[Vizier87]

Okay thanks AG.

 

[Vizier87]

AG, (or anyone else) since the Vce (sat) of a transistor is 0.2V, does a diode connected transistor mimics a schottky diode?

The stores nearby don't sell schottky diodes, so I'm thinking of improvising.

 

[audioguru]

Your transistor will not saturate with its base connected to its collector. It is just a 0.65V silicon diode. The diode will have avalanche breakdown at about 6V to 7V when the polarity is reversed.

Why don't you post your entire schematic for us to see if input protection is required?

 

[Vizier87]

It's from a previous post you answered, a piezo disdrometer connected to the opamp.

On the other hand, I tested my TIP41C, diode-connected and my DMM read 0.193 V in the diode reader mode. I suppose I've just re-invented the schottky diode?

 

[RCinFLA]

Most bipolar based op amps have darlington output stage that results in a least two diode drops below Vcc at output saturation.

If you look for a CMOS based op amp that specifically specs rail to rail output range you can get near Vcc to ground output swing.



The vocabulary word of the week is "disdrometer"

 

[Vizier87]

A disdrometer is........ googled.
Are we in the same tune here? I'm referring to protection diodes across input for the op-amps. the details you mentioned is impertinent to the... jump three posts backwards, I'm afraid.