LTP transistor Matching

[phoenix87]

It looks like the winner is input resistor mismatch.

Left input transistor has a 26.3K resistance to ground.
Right input transistor has a 13K resistance to ground.
Input pair current looks like about 2.25mA each. 2N5401 can be low beta, but let's say it's 100. 22.5uA*13.3K = 300mV.

Quickest fix is to double the size of the feeback resistors, both Rseries and Rshunt. Harder fix is a bias servo.

Hi Russel ,
I changed the feed back resistors to 27k and 680 Ohms , DC offset dropped from 109mV to 70mV. This was a quick check , i will try to get the feedback resistors to 26.3K.

Thanks a ton

 

[nigelwright7557]

An LTP out of alignment can cause DC offset on the output too.
The LTP base resistors should be same value to keep voltage across them the same too.
I use a trimmer resistor in the 2 emitters and tail of LTP to correct output DC offset.