# [SOLVED] Op-amp output voltage has a drop, why is that?

#### Vikram50517

Joined Jan 4, 2020
81
PFA the circuit( A Non inverting amplifier) , my Vcc is 9.5v , iam also giving the same to non-inverting input. Iam using LM-358 as my amplifier.
My gain as i have calculated is 11 times the input voltage (1 + Rf/R1) which is (1 + 100k/10k). So if i give 9.5 v as input , shouldn't my output saturate and spit out 9.5 v? but there was a drop of around 1.2 v consistently ( I even tried the circuit in different boards, but the result is the same), i wonder why? Let me post a schematic of my circuit

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#### Ian0

Joined Aug 7, 2020
5,082
https://www.onsemi.com/pdf/datasheet/lm358-d.pdf
See page 4, and the section "Output voltage - high limit". It is always about 2V lower than the supply voltage.
If you look at the circuit diagram on page 2, you will see that the output stage has a darlington at the top (Q13/Q14) and a 25Ω resistor in series. That would explain the voltage drop.

#### Vikram50517

Joined Jan 4, 2020
81
Oh i see , i get it know, Thanks !

#### Ian0

Joined Aug 7, 2020
5,082
If you need the output to get to 9.5V, search for "rail-to-rail output", but check the maximum supply voltages as many rail-to-rail output op-amps have a maximum supply of 5V.
I'll also mention that the INPUT of many op-amps have similar restrictions - it only works over a limited range known as the "input common mode voltage range" - on a LM358, it goes all the way to the negative supply, but not to the positive. The LM358 will not work properly if the input is higher than 2V below the positive supply. This would also apply to your circuit.