It is impressive at which frequencies a typical audio signal transistor can operate in common base configuration. When I still was a kid at the age of 11 I have build my first TV antenna amplifier according to a circuit design I have found in a book to improve the TV signal in our home. I had no idea about common base transistor configuration but it worked up to frequencies of the UHF band of 800 MHz. I have used the BF255 with a transit frequency of 200 MHz. Everybody was happy that I have saved the family's TV reception.
Lol, it was funny. I have drawn an extra wire for the positive power supply line, because I was very skeptical that it is possible to use the inner wire of the coax cable for this purpose.
Super interesting, I really learned somethingon this transistor amplifier series. Always puzzled by the input impedance of transistor amplifiers. It may be interesting to make a video on the complex R+jX input impedance of this kind of arrangement, also how to chose the right transistor, and finally how to couple correctly a transistor amplifier to another.
Oooh are you going to talk about cascode configurations? In the RF world, high frequency amplifiers are often built using a dual-gate MOSFET wired to be a common-gate amplifier on the drain side of a common-source input amplifier. It's funny to see transistors with four pins!
i noticed that as the input signal gets larger the transistor saturates and the input impedance starts to deviate from the setpoint. for my circuit this starts to happen above 110mV and is much lower than what i would expect... what parameters determine the maximum input signal amplitude? how can i calculate or design for a specific amplitude?
Usually an AC simulation will give you the "small signal" response. small signal being, well, small. As you corectly observed, the larger the signal the more the behavior will deviate. The problem that you usually get is that the amplifying element has a variation in gain - this can be caused by the Vce getting to small (in the case of bipolar transistors) - here a solution is to use larger supply voltages; but this can also be caused by a limitation is slew rate (usually a problem in op-amps) - here you need to look for an amplifier that doesn't just have a high GBW (gain bandwidth product) but also a high slew rate. Last thing to keep in mind is that the higher the amplitude the more current is needed to drive it - using larger static point current should help in achieving larger amplitudes.
One of the best explanations of the common base amp. I always had troubles with it. I thought the input signal goes "against the current" but If I understand it well the signal on the emitter causes a changing Vbe just like a signal on the base would do without going through the transistor. The output is the result of that current change but doing it this way gives better isolation because the signal itself does not go through the collector. It more like the amp copies the signal that is on the emitter instead of amplifying it, Is that correct ?
"the amp copies the signal that is on the emitter instead of amplifying it, Is that correct ?". Yes, that's correct. Applications for common base circuits are cascode amplifiers and emitter switched bipolar transistor which can be viewed as a level shifting circuit.
I think of it differently - in the Common Collector circuit, you "push" current into the base, and that base current gets multiplied by a large number, like 100. The collector and emitter currents are nearly the same - there is 100 times more collector current in the emitter than there is base current. In Common Base circuit, you "pull" current from the emitter, but the emitter and collector currents are still nearly the same - the base current is still about 100 times lower than the emitter current you're pulling. I haven't simulated this, but I believe the improved isolation essentially derives from the fact that you're shrinking the voltage drop on the collector - base capacitance. For the Common Collector case, you're intentionally driving a voltage on the base, and it is about 180 degrees out of phase with the output. Since i = Cdv/dt, the higher the frequency you go, the more current is driven through the CB capacitance. Conversely, with the Common Base case, the AC base voltage is close to 0V, so you only have the collector voltage swing pulling current through that capacitance - you no longer an AC base voltage pulling in the opposite direction increasing that current even more.
@@Cynthia_Cantrell I tried it in a sim. You are right, the emitter and collector current are almost equal (difference is the very small base current ) The amount of base current is indeed pulled because it does almost makes no difference if I change the voltage divider. two times 100k or 2 times 100 ohm is a small difference in base current (in the uA's ) So thanks, I'm never to old to learn :-) edit: it is possible to get big differenced in current if you play around with the collector and emitter resistors. If they are far apart you get big DC offsets f.i 50 ohm collector and 1k emitter. That also causes a pretty big base current if you use 100 ohm in the voltage divider. Using 10k makes the basecurrent smaller but still a big DC offset. Enough to kill the 2n2222 in my sim (almost 50mA using 2x100 ohm) For me it was to see if the base is "modulated" by the ac signal on the emitter resistor shifting the Vbe up and down and it worked as I expected. All 3 signals are in phase. Interesting, I now I have to take my trusty "the art of electronics" and look at the math.
@@pa4tim Glad you found it useful! In most applications we see current "pushed" into the base to make an inverter, for example. But that emitter diode doesn't know or care how the current got into it! Tie the base to 3.3V with a suitable resistor, and the emitter to a logic output, and now you have a non-inverting level shifter that will go up to the transistor's Vce max - just tie the input of the device you're controlling to the collector - with an appropriate resistor to a high voltage rail, say 24 or 48V, for example. In this case current is "pulled" through the base when the logic output goes low, rather than being "pushed" in when the base is driven high (as happens with an inverter). Sometimes it's helpful to imagine things "backwards." Enjoy some "Art!" 🙂
![Gain block RF Amplifiers - Theory and Design [1/2]](http://i.ytimg.com/vi/8myFI-tmowo/mqdefault.jpg)
You are far more better than most college Professors teaching Electronics Engineering.
This guy is a great source to knowledge.
He helped me out a lot using his videos as a base
It is impressive at which frequencies a typical audio signal transistor can operate in common base configuration. When I still was a kid at the age of 11 I have build my first TV antenna amplifier according to a circuit design I have found in a book to improve the TV signal in our home. I had no idea about common base transistor configuration but it worked up to frequencies of the UHF band of 800 MHz. I have used the BF255 with a transit frequency of 200 MHz. Everybody was happy that I have saved the family's TV reception.
Lol, it was funny. I have drawn an extra wire for the positive power supply line, because I was very skeptical that it is possible to use the inner wire of the coax cable for this purpose.
Super interesting, I really learned somethingon this transistor amplifier series.
Always puzzled by the input impedance of transistor amplifiers.
It may be interesting to make a video on the complex R+jX input impedance of this kind of arrangement, also how to chose the right transistor, and finally how to couple correctly a transistor amplifier to another.
It would be great.
Oooh are you going to talk about cascode configurations? In the RF world, high frequency amplifiers are often built using a dual-gate MOSFET wired to be a common-gate amplifier on the drain side of a common-source input amplifier. It's funny to see transistors with four pins!
I guess I'll have to look at that some time after the common emitter configuration. Thanks for the suggestion!
thank you for your time and as always excellent content
How you calculate that inductance?
pick a value of L that will result in a high impedance at the desired frequency of operation, so at that freq it will 'look' like an 'open circuit'
how to choose the capacitor values?
i noticed that as the input signal gets larger the transistor saturates and the input impedance starts to deviate from the setpoint. for my circuit this starts to happen above 110mV and is much lower than what i would expect... what parameters determine the maximum input signal amplitude? how can i calculate or design for a specific amplitude?
Usually an AC simulation will give you the "small signal" response. small signal being, well, small. As you corectly observed, the larger the signal the more the behavior will deviate. The problem that you usually get is that the amplifying element has a variation in gain - this can be caused by the Vce getting to small (in the case of bipolar transistors) - here a solution is to use larger supply voltages; but this can also be caused by a limitation is slew rate (usually a problem in op-amps) - here you need to look for an amplifier that doesn't just have a high GBW (gain bandwidth product) but also a high slew rate. Last thing to keep in mind is that the higher the amplitude the more current is needed to drive it - using larger static point current should help in achieving larger amplitudes.
One of the best explanations of the common base amp. I always had troubles with it. I thought the input signal goes "against the current" but If I understand it well the signal on the emitter causes a changing Vbe just like a signal on the base would do without going through the transistor. The output is the result of that current change but doing it this way gives better isolation because the signal itself does not go through the collector. It more like the amp copies the signal that is on the emitter instead of amplifying it, Is that correct ?
"the amp copies the signal that is on the emitter instead of amplifying it, Is that correct ?". Yes, that's correct. Applications for common base circuits are cascode amplifiers and emitter switched bipolar transistor which can be viewed as a level shifting circuit.
I think of it differently - in the Common Collector circuit, you "push" current into the base, and that base current gets multiplied by a large number, like 100. The collector and emitter currents are nearly the same - there is 100 times more collector current in the emitter than there is base current.
In Common Base circuit, you "pull" current from the emitter, but the emitter and collector currents are still nearly the same - the base current is still about 100 times lower than the emitter current you're pulling.
I haven't simulated this, but I believe the improved isolation essentially derives from the fact that you're shrinking the voltage drop on the collector - base capacitance. For the Common Collector case, you're intentionally driving a voltage on the base, and it is about 180 degrees out of phase with the output. Since i = Cdv/dt, the higher the frequency you go, the more current is driven through the CB capacitance.
Conversely, with the Common Base case, the AC base voltage is close to 0V, so you only have the collector voltage swing pulling current through that capacitance - you no longer an AC base voltage pulling in the opposite direction increasing that current even more.
@@Cynthia_Cantrell I tried it in a sim. You are right, the emitter and collector current are almost equal (difference is the very small base current ) The amount of base current is indeed pulled because it does almost makes no difference if I change the voltage divider. two times 100k or 2 times 100 ohm is a small difference in base current (in the uA's ) So thanks, I'm never to old to learn :-) edit: it is possible to get big differenced in current if you play around with the collector and emitter resistors. If they are far apart you get big DC offsets f.i 50 ohm collector and 1k emitter. That also causes a pretty big base current if you use 100 ohm in the voltage divider. Using 10k makes the basecurrent smaller but still a big DC offset. Enough to kill the 2n2222 in my sim (almost 50mA using 2x100 ohm)
For me it was to see if the base is "modulated" by the ac signal on the emitter resistor shifting the Vbe up and down and it worked as I expected. All 3 signals are in phase. Interesting, I now I have to take my trusty "the art of electronics" and look at the math.
@@pa4tim Glad you found it useful! In most applications we see current "pushed" into the base to make an inverter, for example. But that emitter diode doesn't know or care how the current got into it!
Tie the base to 3.3V with a suitable resistor, and the emitter to a logic output, and now you have a non-inverting level shifter that will go up to the transistor's Vce max - just tie the input of the device you're controlling to the collector - with an appropriate resistor to a high voltage rail, say 24 or 48V, for example. In this case current is "pulled" through the base when the logic output goes low, rather than being "pushed" in when the base is driven high (as happens with an inverter).
Sometimes it's helpful to imagine things "backwards."
Enjoy some "Art!" 🙂