LDO output when Voltage reference is not grounded Properly

Thread Starter


Joined Mar 21, 2020
Dear Team,

I was buiding a simple linear regulator using a pass transistor ,opamp and zenar diode.
When I am connecting zenar ground properly output is coming as expected.When the Zenar ground is removed output is coming higher than expected.
I know when the ground is not proper I will not get the desired output.

Last edited:


Joined May 3, 2013
The Zener may not be having a direct connection to the other "ground" points.
All the "grounds" MUST be connected electrically in your circuit.


Joined Jan 11, 2015
1N750 is a 4.7 volt 500mW zener. So when working at a reasonable current the voltage across the zener will be about 4.7 volts.
The opamp will try to drive Q1 until both of the opamp inputs are equal so the junction of R1 and R2 will try to reach 4.7 volts
and thus the Vout will try for 2*4.7 volts -> 9.4 volts. With only 10 volts input to Q1 that only leaves .6 volts across Q1 which
isn't enough as your "normal" output is only 8.9 volts. The 10 volt input is too low for an output of 9.4 volts.