High gain diff amp

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automagp68

Joined Nov 13, 2011
81
So i need to build a high gain diff amp. Only BJT,s available.

need +- 7.5 Volt swing

Typically i can't get a but diff amp to give me a gain of more then a 100 or so. I need 800 differential gain

So the first thought is to cascade a few together. However, as the output voltage goes up and goes into the base of the next stage all i get is clipping.

Say i built a single stage diff amp and then wanted to just get a bunch of gain out of it. Take the output and maybe run it into a Common emitter maybe
How do i get the common emitter to not clip with a large input signal?

Any other thoughts or proposed ideas on how to do this?
 

Jony130

Joined Feb 17, 2009
5,157
Simple use diff amp as a input stage, next stage common emitter and finally stage (output stage) common collector.
And CE stage will provide the most of a needed gain.
 

Thread Starter

automagp68

Joined Nov 13, 2011
81
Whats the point of the last stage?

High output impedance? I only need to drive a 1k load?

How do i keep the CE from clipping with an input voltage of a few hundred milivolts?

Right now my diff amp is outputting around 700 mv with a 10mv input which I'm happy with i guess.s Now i need to crank up the gain.
Will a CE take a 700mv input and no clip with +- 7.5 volt swing on a 15v supply?
thanks for your input
 

Thread Starter

automagp68

Joined Nov 13, 2011
81
Let me also say because this is a deign for a discrete chip i can't use a bypass cap for a CE

Any ideas on how to do this?

Without a bypass cap i am seeing low single digit gains
I need a 7.5 volt swing. Frequency response is unimportant as long as it works for around 50k-100k
 

Thread Starter

automagp68

Joined Nov 13, 2011
81
Essentially what I'm doing is building the first few stages of an low grade op amp

However i do not have the understanding to use feedback like an op amp does

Needs to be done without that
 
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