SSM2220/SSM2212 for LTP/Mirror

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Neutrality

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Joined 2004
#1
Been looking at using the SSM2220(dual PNP) and SSM2220(dual NPN) for LTP input transistors as well as current mirror.

Now the question is, where to use what? SSM2220 dual PNP has max Hfe of around 165 while the SSM2212 dual NPN has max Hfe of around 600.

SSM2220 for LTP input transistor and SSM2212 for current mirror? or the other way around?
 
#3
lineup said:
High gain, hfe, is very good for an input pair.
Click to expand...

That was my thinking as well.

However, I hoped it would be almost as good using it the other way around since Im desiging something with PNP LTP input pairs, so I hoped the dual PNP SSM2220 would be ok to use as LTP input transistor and then using the dual NPN SSM2212 as a current mirror.

However, redesigning the circuit the other way around and using NPN as inputs should be just about the same.

But thanks. 🙂
 
#4
Hi,

There are several threads here which mention the effect of low hfe transistors in the input pair. The first I think about is output dc offset.

Have you considered the 2sa1349/2sc3381 dual transistors? These can still be obtained, even on this forum. They where not intended as input transistors, however they sometimes are used as input pairs. I used them myself in a symmetrical design.

Greetz
 
#5
Bensen said:
Hi,

There are several threads here which mention the effect of low hfe transistors in the input pair. The first I think about is output dc offset.

Have you considered the 2sa1349/2sc3381 dual transistors? These can still be obtained, even on this forum. They where not intended as input transistors, however they sometimes are used as input pairs. I used them myself in a symmetrical design.

Greetz
Click to expand...

This is an all(almost) SMD design so dual matched SMD BJTs is a requirement.

I am not too worried about a few extra mV's of output dc offset.

I always use a DC servo so the only issue with a slighty higer output DC offset is that it might lead to somewhat higher distortion, but still more or less a neglible increase.

However, I have revised my circuit and it is now done "the other way around" so I can use high gain dual NPN as input pairs and lower gain dual PNP as current mirror.

Problem solved. 🙂
 
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