Common Base Transistor Ampilfier Question

[Bryan]

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

[StillTrying]

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.

[danadak]

CB Zin quite low, approximated by 1 / gm.

http://www.prenhall.com/howe3/microelectronics/pdf_folder/lectures/tth/lecture18.fm5.pdf


Regards, Dana.

[TimFox]

CB Zin quite low, approximated by 1 / gm.

http://www.prenhall.com/howe3/microelectronics/pdf_folder/lectures/tth/lecture18.fm5.pdf


Regards, Dana.

To first order, at low-to-medium frequencies, that becomes Zin approximately (26 mV)/Ie.  At emitter current 5 mA, that is only 5 ohms.

[T3sl4co1l]

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

[Bryan]

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.

[Bryan]

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.

Isn'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.

[danadak]

A representative solution, attached.


Regards, Dana.

[danadak]

And input Z sim, attached.


Regards, Dana.

[danadak]

Here is a couple of sims where decent G occurs, but Zin
at 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.