I was experimenting with point-to-point wiring with LM3886. Notice that almost full DC power supply voltage at output terminal! I thought I was losing one side of power supply but after carefully check, the power supply working correctly. Later on, I found that part of the input is not connected, after reconnect it works. I also try shorting the input to ground, there is no DC at output as well.
Why is that? Is it because incorrect input bias when disconnected?? I tried simulate on TINA-TI, and it is kinda show what I experienced.
Closed SW1 = Normal/ correct wiring. Only 1.18mV at output.
Open SW1 = Disconnected input bias circuit. Simulation show 22 V DC at output.
Please share your thoughts.
AP
Why is that? Is it because incorrect input bias when disconnected?? I tried simulate on TINA-TI, and it is kinda show what I experienced.
Closed SW1 = Normal/ correct wiring. Only 1.18mV at output.
Open SW1 = Disconnected input bias circuit. Simulation show 22 V DC at output.
Please share your thoughts.
AP
I don't think I want to share my thoughts with strangers ;-)
But this is what happens. If you open SW1, there's no DC connection to the +input.
The input bias current flowing in/out of the input pins will flow into an infinite resistance (the open switch) and will drive that input to infinite voltage.
Fortunately it stops at the supply voltage! ;-)
That input voltage is amplified, again limited to the supply voltage.
It is what you should normally expect with an open input.
Edit: the LM3886 datasheet specifies the input bias current at each input pin at typical 0.2uA, max 1uA.
BTW What is the purpose of SW1?
Jan
But this is what happens. If you open SW1, there's no DC connection to the +input.
The input bias current flowing in/out of the input pins will flow into an infinite resistance (the open switch) and will drive that input to infinite voltage.
Fortunately it stops at the supply voltage! ;-)
That input voltage is amplified, again limited to the supply voltage.
It is what you should normally expect with an open input.
Edit: the LM3886 datasheet specifies the input bias current at each input pin at typical 0.2uA, max 1uA.
BTW What is the purpose of SW1?
Jan
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Thanks Jan for your explanation. SW1 is just a simulation for "broken wire" or "bad joint" that I experienced.