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Old 17th April 2016, 06:33 PM   #1
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Default Important parameters LTP transistor matching

Hi guys.
When matching Long Tail pairs for an amplifier (BJT), which parameters do you match for? Obviously Vbe is important, but what about gain?
When designers use current mirrors to match the current through each transistor isn't Vbe the only parameter to consider?
I'm asking this as a noob. I usually match at Ic of 2,5mA.
Thanks in advance.
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Old 17th April 2016, 06:52 PM   #2
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As long as the Hfe is more than you need, all should be OK. If you are using a balanced pair, then Vbe will be best matched.
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Old 18th April 2016, 09:05 AM   #3
AndrewT is offline AndrewT  Scotland
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I explained in a post yesterday how to check the currents and voltages around the input LTP and why these voltages result in output offset.
D.Self shows a similar calculation in his power amplifier book. He also shows how close the device currents need to be to minimise distortion in the LTP.

When you do this, you begin to see why the output offset and current balance becomes dependant on the LTP device parameter matching.
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Last edited by AndrewT; 18th April 2016 at 09:09 AM.
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