As a rule, the BJT common-base output characteristics are presented as a function of the collector current IC of two variables - the collector-emitter voltage VCE and the emitter current IC; IC = f(VCE, IE). The emitter current serves as a parameter to obtain a family of characteristics.
But we know (and discussed many times) that the CB stage is usually controlled by voltage than current. Then why not use the input emitter voltage instead of the emitter current when measuring the CB output characteristics of the bipolar transistor - IC = f(VCE, VE)?
Maybe we should use one or the other characteristic, depending on how we drive the transistor from the side of the emitter - by current or voltage. Or maybe it does not matter?
This question is dual to the question about CE stage...
https://www.researchgate.net/post/Why_do_we_use_Ib_instead_Vbe_as_a_parameter_when_measuring_common-emitter_output_characteristics
... and related to the question about the CB output resistance:
https://www.researchgate.net/post/Why_does_the_common-base_stage_have_higher_output_impedance_than_the_common-emitter_stage
In the attached picture below, the input current is set by a simple "resistor-type" current source (a resistor in series to a voltage source) but I have seen more sophisticated setups where a true current source is used. Is it necessary?
Hi Cyril,
(This is just my opinion and it doesn't directly answers you question.)
Most of the BJT circuits are designed to meet small-signal specifications like voltage gain or transconductance gain (so you have to use h, or y, etc. parameters that are a function of the quiescent Q point).
In my experience I haven't ever had to deal with such things as CB output characteristics.
Normally I used the CE characteristics (if anytime) to be sure the transistor is working in the desired (large signal) Q point. Most circuits can be designed in such a way that are CE configured for large signal and CB configured for small signal by using decoupling capacitors.
Since the device is the same for any configuration, once you have a set of characteristics (i.e. CE) it doesn't really matter if you have another set (like CB) or not, unless you are working with a circuit that has to perform with large signals.
Right, Leonardo... and I think so... CE output characteristics are sufficient to characterize the humble element (the transistor). For the purposes of the Semiconductor devices, there is no need to measure other (CB or CC) output characteristics since they will not give more info about the transistor. They will characterize the circuit... the manner in which the transistor is connected... but this belongs to the area of the Semiconductor circuitry.
Circuit characteristics can be different than device characterstics; the output characteristic of the CC stage (emitter follower) is a good illustration of this difference - it is vertical while the original transistor curve is horizontal:
https://www.researchgate.net/post/What_is_the_output_characteristic_of_the_common-collector_transistor_stage
Thanks for the support! I need it to gain confidence in the rightness of goal towards which I move.
Hello gentlemen,
we are able to use Ve as a parameter, sure. But it will be somewhat impractical due to the strong temperature dependence of Ve and the exponential dependence of current Ie on voltage Ve.The emitter current is the better option
Josef,
Can we say this is the same solution as in the case of the CE stage in the question below?
https://www.researchgate.net/post/Why_do_we_use_Ib_instead_Vbe_as_a_parameter_when_measuring_common-emitter_output_characteristics
It turns out that the CB stage seen by the side of the emitter is similar to the CE stage seen by the side of the base - i.e., both they have low input resistance, and this is the reason to control them by current?
Cyril,
The temperature dependence of beta - hence alfa - is substantially less than the temperature dependence of base - emitter voltage. We use this fact in stabilizing operating point for all circuits with BJTs. Voltage Vbe as a parameter I saw only in datasheet for the power transistors. Unfortunately I no longer remember the type.
Maybe the temperature (self heating) is the reason the second CE characteristic below (at VBE = const) to be worse than the first one (at IB = const)?
It seems the self heating and Early effect superimpose and act in the same direction thus increasing the collector current when collector voltage increases...
Josef, once you committed to cooperate with me in this noble mission - to reveal the secrets of basic transistor circuits:), can you comment my final remark about using "more sophisticated setups where a true current source is used".
Is it really necessary? Or the simple "resistor-type" current source (a resistor in series to a voltage source) can do this "donkey" work (setting the emitter current)? Or even a bare variable resistor (rheostat) can do it?
If we investigate CB circuit, large enough voltage and resistance is sufficient as current source, I think. Temperature changes in base-emitter voltage are then relatively negligible. In teaching I use connections - see Annex.
Yes, Josef... I think so... Even there is no need the emitter resistance and voltage to be very large because the transistor, acting as an emitter follower, maintains its emitter voltage nearly constant (almost equal to the voltage of the base).
The emitter resistor give us very useful negative current feedback. It eliminates also large changes of beta. For ac signal we can eliminate this feedback by bridging of resistor by appropriate capacitor.
If I need to demonstrate principle of high gain circuit, I am used to show circuit - see Annex - with very simple current source in collector.
If I need to demonstrate principle of DC amplifier, I will explain to students basic outline of the operational amplifier - see attachment.
Oh, this is my favorite circuit idea - dynamic load. Two years ago, I tried to make an interesting discussion about this ingenious circuit trick... but as you can see, I could not. Maybe now we can revive this discussion?
https://www.researchgate.net/post/Can_we_apply_an_input_current_to_the_collector_of_a_BJT_whose_base_is_held_at_a_constant_voltage_and_to_take_the_collector_voltage_as_an_output
I see very nice illustrations in your attachments. As I can see in the first picture, you have shown an attractive LED application - "lighting zener diode":)
In the second picture, we can see a bunch of great circuit ideas including the present one (injecting a current into the emitters of the differential pair)... and, of course, the great current mirror: And the fate of this issue was not very different:
https://www.researchgate.net/post/Can_we_reverse_a_BJT_by_injecting_the_input_current_into_the_collector_and_taking_the_base-emitter_voltage_as_an_output
Interesting thoughts about the external reference point... It is also interesting to discuss the topic about the external biasing in op-amps. Here is a related question:
https://www.researchgate.net/post/Can_we_connect_a_floating_voltage_source_between_the_two_inputs_of_a_differential_amplifier_If_so_at_which_conditions
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