Hi Since it's the input stage, it's likely the point most impacted by a higher flicker noise part. That might make one want to look at alternatives.
Of course, it's not real clear that a super low noise amp is needed in this case. Bob On Feb 12, 2010, at 8:46 PM, Bruce Griffiths wrote: > The series RC to ground keeps the high frequency impedance seen by Q1 and Q7 > low so that the base current noise which increases significantly as the > frequency approaches the ft of these transistors. > However such a series RC network does little to suppress the the rise due to > gain peaking. > A shunt capacitor from the output stage collectors to the output stage bases > is much more effective for the 2x gain stage. > > Such a capacitor increases the noise for the 1x gain White emitter follower. > Using an input transistor with higher bandwidth is more effective in this > case. > > Bruce > > Bob Camp wrote: >> Hi >> >> I suspect your noise spike can be cured by a series R-C to ground from the >> junction of Q1 base, Q7 base and all the other stuff. Something is going to >> have to set a high frequency roll off. With no coils some combo of R and C >> is going to have to do it. >> >> You might also try returning all of the upper emitter resistor bypasses to >> ground rather than B+. Another alternative would be emitter to emitter >> bypass as shown on the JPL schematic. I'm guessing both would improve >> isolation in a real world circuit. >> >> Bob >> >> >> On Feb 11, 2010, at 8:34 PM, Bruce Griffiths wrote: >> >> >>> life speed wrote: >>> >>>> Message: 2 >>>> Date: Fri, 12 Feb 2010 12:12:29 +1300 >>>> From: Bruce Griffiths<[email protected]> >>>> The output (collectors of Q5, Q6 emitter of Q4) of the input amplifier >>>> sets the dc voltage at the inputs ( Q1 base, Q7 base respectively) of >>>> the output amplifiers. >>>> >>>> The circuit consists of a unity gain input amplifier (Q4, Q5, Q6) that >>>> drives a pair of output amplifiers (Q1, Q2, Q3 and Q7, Q8, Q9 >>>> respectively) each with a gain of 2x (6dB). >>>> The input amplifier is essentially a white emitter follower with a >>>> complementary symmetry output stage (shown in transistor electronics >>>> books from the 1960's) where an input CE transistor drives a >>>> complementary pair of CE transistors with feedback from the common >>>> collectors of the 2 output transistors to the input transistor emitter. >>>> In effect its merely a very simple unity gain opamp. Its usually best to >>>> ensure that the CE output stage pair provide the dominant open loop >>>> pole. Using a higher ft (2 to 3x) input transistor than the output pair >>>> is the usual way of ensuring this. >>>> >>>> Well, it is so obvious now that you explained it. I had forgot about the >>>> need for one of the stages to set the dominant pole. >>>> >>>> Thanks Bruce and Bob for sharing your obsession with frequency controls. >>>> I'll simulate this further, and have a prototype PCB built within the next >>>> few weeks. I did notice the resistor at the base of Q2,5,8 is responsible >>>> for significant noise. I'll have to be careful with the bias circuit. >>>> >>>> Have to get busy for now, but I will report back with results. >>>> >>>> Best regards, >>>> >>>> Clay >>>> >>>> >>>> >>> Clay >>> >>> One can always use a smaller resistor in series with an RF choke that has >>> no resonances in the region of interest. >>> >>> The attached circuit schematic illustrates one method of biasing for which >>> the emitter current of the input transistor can be largely sourced via a >>> resistor rather than from the collector current of the npn output >>> transistor. >>> >>> My simulations indicate if that one uses 2N3904's as the input device >>> rather than the 2N5179's shown that there is an enormous peak in the output >>> noise spectrum at around 150-200MHz or so. >>> When the 2N5179 is used this noise peak is much smaller and broader. >>> >>> Use the same bias divider bypassing techniques that NIST used including the >>> use of electrolytic caps (they used tantalum caps) to reduce the low >>> frequency noise from the power supply. The ceramic bypass caps ensure >>> sufficient isolation between stages. >>> Simulating the reverse isolation with realistic component parasitics is >>> always informative/useful. >>> >>> Bruce >>> <Transformerless_10MHz_disA.gif>_______________________________________________ >>> time-nuts mailing list -- [email protected] >>> To unsubscribe, go to >>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >>> and follow the instructions there. >>> >> >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. >> >> > > > > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
