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previous: Jay Doyle Re: A Couple of Simple Ideas -- 10/31/2000 5:03 PM View Thread

BJT Stuff

10/31/2000 7:15 PM
Doug H
BJT Stuff
Forgot to add this to the last one:  
"BJT based opamp exhibit lower input Z, but they also need more current on their inputs to run properly, which is why you would need to lower the value of the bias resistor to accomodate them. "
The first version of the Rat used a 1M for the bias resistor, with an LM308 bjt opamp. Not sure if that value is really a problem or not. In later versions they lowered it to 220k, I don't know if that was to supply more current, or to lower input Z to reduce noise. I know with the TL072 it has the latter effect, without noticeably rolling off highs. That came in pretty handy for a gainy circuit like this.  
I'll be curious how it behaves when my LM308's come in. Like I mentioned before, I'm not too interested in BJT opamps, except this particular model. But it does make a good testbed, like you said.