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Old 24th March 2003, 01:27 PM   #1
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Join Date: Oct 2002
Location: USA
Default help with BJT preamp design

I was going through my junkbox the other day, and thought I would throw something together to function as a preamp. I have a bunch of MPSA06 transistors, and various resistors, etc. So, I decided to make a simple current-sourced differential pair. I put the circuit together, but the gain I am getting is not symetrical on both outputs. So, I decided to add followers on the output (haven't done that yet, but thought it may help by taking the load off the differential pair). I don't know much of anything about designing with BJT's and I would appreciate it if some of you with more experience would look at my design and give me a shove in the right direction.
Some specific questions:
1. What is the best way to go about getting a symmetrical balanced output from a single-ended source?
2. How critical is matching between gain transistors in the differential pair, and is there a way to make this less critical?
3. I have taken into account the dissipation of the transistors, but is there an optimal current for this circuit? What is a typical Q current for a circuit like this?
4. What is a good way to lower the gain of the circuit? It seems like I am always running into this problem. If I use emitter resistors, it degrades the matching on the outputs of the circuit (right?) but if I don't use them, the gain is out of control, and the circuit clips with the full output of my cd player (2V max RMS).
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