Common Base Amplifier - Practical Build (2/2)
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- Опубліковано 17 бер 2023
- #185 In this video I proceed to build and then test the common base amplifier that was designed in episode 1. I go trough the various steps that where treated in the simulator, but also end up being somewhat limited by the real requirements that measuring such a circuit require - mainly the very small input signals.
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Common base amplifier were used in early 2000's late 90''s at input of FM receivers for public service broadcast 88 - 108MHz. (impedance adaptors or matching impedances main reason ) .Of course was use in many other aplications as you said . Thx.
Thanks for the interesting video. The few drawbacks you mentioned (input impedance depends in DC bias, output impedance is high, low bias gives higher noise and lower cut-off frequency) can be overcome by a particular type of common base topology - the Norton amplifier. The dc bias is increased to a couple of mA to get an emitter resistance of 5 ohm or less. Then a transformer is used with one turn in series with the emitter to the signal source. As the emitter is practically zero ohms to ground the input voltage is across that one turn. In the collector are more turns of the same transformer. The output is tapped a few turns down from the supply line, the winding continues with some extra turns to the collector, see further down why. The voltage gain of this amp is the ratio between these turns and the one input turn. Usually not very high, let’s say 4. Now the impedance matching trick - the output voltage is 4x the input voltage. In order to have the same input and output impedance the output current should also be 4x the input current. Which happens when the input has 4x as many turns in total as the output. And that is where the extra turns in the collector come into play. So to recap - from the supply down to the output 4 turns, then continue with 11 turns to the collector and in the emitter another 1 turn, making it a total of 16 turns. With a gain of 3 or 5 of course your total primary turns is either 9 or 25.
A picture says more than a 1000 words but YT doesn’t let me draw so I hope you get what I mean.
One turn primary can also be multiples of course, the mutual inductance should give an impedance at the operating frequency much higher than 50 ohms
Excellent video - really appreciate the effort you put into making these videos and your mix between simulation / experiment is perfect... I'm looking forward to see what's coming next!!
Thanks! I always appreciate those dives into the practical issues that affect real circuits :-)
interesting demonstration my friend. Now what we need is exploration of a new endeavor: A common base amplifier directly coupled to a common collector amplifier so you get voltage gain at a low output impedance.
The edit was me saying common emitter, I was supposed to say common collector.
So I edited it to change one word
The desirable trait here is thar the common collector or emitter follower has no voltage gain but it is a current buffer
Wow, a most wonderful example. It sure takes a lot of good circuit demos to fine tune the simulation libraries for future use. 😎 Thank you.
Thanks mate. Love your videos!
Absolutely excellent ! Thanks.
I like very much the real life test, that show all is not perfect, but "close enough" (I take this in my technical vocabulary, now 😁)
Will you do a summary video to show the advantages and use case of all 3 topologies ?
I'm happy you enjoyed it! I'm not sure about the summary video, but based on another viewers suggestion, I am considering extending the series to a 4th topology - the cascode - which is basically a combination of 2 basic topologies, but since it can be built with a single transistor (dual gate mosfet) I think it disserves the analysis.
@@FesZElectronics Thanks for your answer. Yes of course, the cascode topology will be even more interesting, bipolar or mosfet. Impatient to see it 😁
I wonder if you could still use VNA even though its signal is too high, if you add attenuator between VNA output port and amplifier input port? The amplitude response graph you get would be shifted down, but I expect you would still get some information about the circuit response.
I did try that, but the more attenuation you add, the more noisy the signal becomes; at some point, the measured signal becomes so small the device can't really make out any useful information. Wit this particular amplifier, you would need about 40dB or more of attenuation to not overdrive the amplifier. The LiteVNA I have is just not sensitive enough to work properly with that sort of attenuation.
Hi. You are one of the most elaborative electronics channel. Keep up the good work. I have a question related to my project. I have a parallel lc tank circuit which oscillates at 3 Mhz. I am trying to get this signal through a buffer opamp configuration. I use lmh6646 which has enough features for my application. But at the output of the buffer circuit i see a 4 Mhz signal. How is this happening? The buffer circuit outputs different signal from input. Can you think of any reason for that to happen and how can i solve it?
Thanks.
осторожно, орудует гугл переводчик
I don't understand. An amplifier with a COMMON base. So the base must be connected to a potential with low resistance. Is not it so?
In an amplifier with a common base, the output current is approximately equal to the input current. This amplifier only amplifies the voltage. The capacitor in the base does not help, because there is not an alternating current flowing there, but a pulsating one direction. I am sure that if you measure the base, it sags when there are distortions ...
An explanatory team is urgently needed - why is there such a high DC resistance in the base?