Emitter resistor with BJT, how does it provide feedback?

[FvM]

Don't know why you ignored my answer to post #16 after requesting it forcefully. There are different quiescent point stabilization methods, the emitter resistor in combination with base voltage divider is one of it.

I even agree that emitter resistor feedback is the best method for for quiescent point and AC gain stabilization of general purpose amplifiers, e.g. audio up to low MHz RF range. But I never had the idea to hold it against analyzing this technique as feedback circuit. You say it's not feedback, it's necessary for circuit operation. Why not both?

 

[Vbase]

Don't know why you ignored my answer to post #16 after requesting it forcefully. There are different quiescent point stabilization methods, the emitter resistor in combination with base voltage divider is one of it.

I even agree that emitter resistor feedback is the best method for for quiescent point and AC gain stabilization of general purpose amplifiers, e.g. audio up to low MHz RF range. But I never had the idea to hold it against analyzing this technique as feedback circuit. You say it's not feedback, it's necessary for circuit operation. Why not both?

The reason I ignored your answer was that my question was if you can design the 4 resistors circuit which I described before having Re=0. I wasn't asking about other circuits ideas for stability, I know that you can offer many. Using Re for stability was agreed by you to be the point of discussion.
I accept that it is both feedback and stability. In post #3 I said that we call it feedback but it is a stability resistor. My explanation all along is that without Re there is no viable circuit, this is why it should be called Re and not feedback resistor. When you call it feedback resistor people think that that is the purpose of it, and that is misleading.

 

[FvM]

my question was if you can design the 4 resistors circuit which I described before having Re=0

May be in your mind. You have been talking about common emitter with or without Re and I've been answering exactly this question. That's the problem with unsaid prerequisites.

I accept that it is both feedback and stability. In post #3 I said that we call it feedback but it is a stability resistor.

I'm half way satisfied. The next point would be to accept that feedback is the means to achieve stability. Good night.

 

[D.A.(Tony)Stewart]

Use of emitter resistor when using BJT in its various configurations seems to be important. Lets take example of BJT common emitter amplifier.

The emitter resistor is used supposedly to make the bias point more stable, this refers to the Q-point. But how? What would happen if this emitter resistor was not used?

Re is called emitter degeneration as the rBE becomes insignificant..

If Re is removed you now have power controlled temperature with the Shockley NTC effect on VBE which destabilizes the Q point.

As most of Emitter current is Collector current. Using Re stabilizes the AC gain from V=IR where the ratio of Rc/Re is voltage gain is stable rather than dependant on a lower thermally sensitive rBE. If Re is too large for required high gain at low current then it is separated and the larger one is bypassed for DC bias and higher AC gain with the smaller Re unbypassed controls the gain.

When ever Vce gets low (<2V) it enters a region of saturation that decays hFE and causes reduced gain and thus increased THD of linear large sinusoidal signals.

Another way of looking at Common Emitter is that it is just a gain controlled resistance and the DC bias current must always be bigger than the AC peak current to avoid saturation. Without an Re this is not possible. The gain lower both the output and input impedance at collector and base respectively when feedback is used. The AC gain is the un-bypassed impedance ratio of collector to emitter and the DC feedback just raises the DC current above the required AC max current to control the Q point.

I mean it is possible to get 60dB Voltage gain but not power gain from a single stage with hfe=100

 

[Vbase]

May be in your mind. You have been talking about common emitter with or without Re and I've been answering exactly this question. That's the problem with unsaid prerequisites.


I'm half way satisfied. The next point would be to accept that feedback is the means to achieve stability. Good night.

Read post #13 and find out that the circuit was not only in my head.
If you are blaming me for unsaid prerequisites then the next point is good night.

 

[D.A.(Tony)Stewart]

I trust you understand my explanations.

 

[crutschow]

Yeah, let's widen the definition a little more and call a potato feedback too!

So we go from the sublime to the ridiculous. Nice transition.

 

[FvM]

Read post #13 and find out that the circuit was not only in my head.

Yes, you are right. Post #16 nevertheless talks generally about a common emitter amplifier, so I felt compelled to state that CE circuits can be stabilized without Re.

The three resistor circuit (without Re) was used in an electronics lab that I redesigned and supervised during my institute years - as an example of bad biasing. The DUT was part of a transistor array implementing an on-chip thermostat, so you could change the junction temperature almost instantly. You don't need to refer to current gain variations or resistor tolerances to understand why the circuit is effectively useless. I don't remember the theory part of the lab excercise well, it was more focussed on visual perception, but I presume the stabilisation methods have been correctly analyzed as (DC) feedback.

Reviewing post #13, I believe the error in reasoning is to see "feedback" working only in the signal (AC) path. But it's present in quiescent point (DC) realisation as well.

 

[D.A.(Tony)Stewart]

Yes with Rb and no Re, one can calculate Ib then Ic from hFE and thus Vce then rBE and thus gain, but gain & collector voltage is very sensitive to hFE wide tolerances and Vcc.

H Bias and Negative feedback bias are both much more stable due to emitter resistor degeneration.

This design I put together pushes the envelope for voltage gain >60dB in a single stage.

It uses a 2 mH choke with some self capacitance shown to get 62dB gain at 1MHz for 0Vdc out using 50Ω source and 10k load.

 

[LvW]

In post #3 I said that we call it feedback but it is a stability resistor. My explanation all along is that without Re there is no viable circuit, this is why it should be called Re and not feedback resistor. When you call it feedback resistor people think that that is the purpose of it, and that is misleading.

1.) Vbase - the question is (and was) NOT if a gain stage without Re would be a "viable circuit". You should not concentrate on a subject that was not questioned at all.
Again, I remind you on operational amplifiers: They even do not work as amplifiers without negative feedback.

2.) Are you familiar with feedback theory - in particular with the results and consequences of signal feedback?
Do you know that ALL these consequences can be observed in an Re stabilized gain stage?
(Increase of signal input and output impedance, reduction in signal gain, THD improvement (linearization), less sensitivity to BJT parameter tolerances, wider bandwidth)

3.) How would YOU explain to others these parameter changes without using the feedback theory? Do you know about the role of the loop gain in this context?

4.) That means: The only purpose of Re is to provide negative signal feedback! It is misleading and wrong to deny this fact! Do you need a visualization of the feedback loop for calculating the loop gain?