Common Base Transistor Ampilfier Question - Page 1
Forum section: electronics / beginners

author: Bryan
author_rank: Frequent Contributor
timestamp: October 25, 2016, 09:37:06 am
content: Hello:Watching another great (older) Utube video from Allan W2AEW on Transistor Amplifiers. Bread boarded the common base amplifier (better sued for RF amplifiers) and used a input signal of 10Mhz. Of course it did not work and as a matter of fact the output was quite distorted and attenuated. So increased the capacitance of the 10uf capacitor to 100uf and it worked much better, signal was amplified as expected.Question I have and I know there is other variables such as limits to the transistor I used 2N2222 and a BC547 for a 10Mhz frequency. But what would the proper capacitance values that should be used for a 10Mhz signal. Would appreciate the mathematical calculations one would use. There is two capacitors in the circuit. I hope Allen is ok if I post his schematic from his video.https://youtu.be/zXh5gMc6kyU

author: StillTrying
author_rank: Super Contributor
timestamp: October 25, 2016, 03:44:58 pm
content: Electrolytics aren't very good at high frequencies, in theory at least, a single 0.1uF disc decoupling cap on the base should give a better result at 10Mhz than any electrolytic.1 / (2*pi*F*C) is a good starting point.

author: danadak
author_rank: Super Contributor
timestamp: October 25, 2016, 03:50:23 pm
content: CB Zin quite low, approximated by 1 / gm.http://www.prenhall.com/howe3/microelectronics/pdf_folder/lectures/tth/lecture18.fm5.pdfRegards, Dana.

author: TimFox
author_rank: Super Contributor
timestamp: October 25, 2016, 08:47:19 pm
content: Quote from: danadak on October 25, 2016, 03:50:23 pmTo first order, at low-to-medium frequencies, that becomes Zin approximately (26 mV)/Ie. At emitter current 5 mA, that is only 5 ohms.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 25, 2016, 10:31:53 pm
content: And at high frequencies, inductance on the base terminal is reflected as negative resistance at the emitter. I've measured a CB stage as around -5 ohms, once...Tim

author: Bryan
author_rank: Frequent Contributor
timestamp: October 27, 2016, 08:19:49 am
content: Sorry, folks gotten me lost here, I did breadboard the circuit again using a common emitter amplifier and it surprisingly did quite well with 10Mhz, from the video the "Miller" effect should have really compromised the signal in in this arrangement, thus the recommendation to use a common base for RF.

author: Bryan
author_rank: Frequent Contributor
timestamp: October 27, 2016, 08:21:27 am
content: Quote from: StillTrying on October 25, 2016, 03:44:58 pmIsn't that to calculate the reactance. But yes using a 0.1uf improved the output considerably, still not as well when I used a common emitter amplifier, which I did not expect to work as well at 10Mhz.

author: danadak
author_rank: Super Contributor
timestamp: October 27, 2016, 11:16:38 am
content: A representative solution, attached.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: October 27, 2016, 11:41:22 am
content: And input Z sim, attached.Regards, Dana.

author: danadak
author_rank: Super Contributor
timestamp: October 27, 2016, 03:01:38 pm
content: Here is a couple of sims where decent G occurs, but Zinat 10 Mhz is ~ 50 ohms.You tradeoff G vs Zin vs C3.Zin : 1 / C3, G : C3.Of course you can run Q1 at higher Ic to get more Gm.......Regards, Dana.