# Negative feedback problem, help please

Discussion in 'General Electronics Chat' started by burich, Jan 2, 2016.

1. ### burich Thread Starter New Member

Jan 2, 2016
2
0
Hey everyone!

I saw what is recommended to do with common-emitter amplifier if one is to prevent thermal runaway (connecting a resistor between the transistor's emitter terminal and circuit ground, R_load in the Figure below):

So it is said that the way it works is this: if base current happens to increase due to transistor heating, emitter current will likewise increase, dropping more voltage across the load, which in turn substracts from V_input, to reduce the amount of voltage dropped between base and emitter. Thus the reduced voltage between base and emitter, base current will decrease, so the thermal runaway has been stopped.

And here is my theory, what happens next, and causes me trouble in fully understanding: after the decrease of the base current, the emitter current has to drop as well, causing lower voltage dropping on the load, which in turn let's the voltage dropped between base and emitter grow (because the V_input is constant, and V_input = V_base-emitter + V_load).
As it grows, base current will increase too, so in the end we have arrived to where we came from, the whole process is about to restart. It looks to me though, there is no real equilibrium, but a constant alternation between ups and downs of the base current. Is it right?

Or, if it is not, where is the mistake in my theory? How does this negative feedback stops itself? I mean, it looks like there is a negative feedback to the first negative feedback too, for example, base current starts to increase (due to transistor heating), then in starts to deacrease (due to negative feedback), but if it is decreasing, it has to start to grow as well (just as I wrote it down above), then it should decrease again, then increase, then decrease, etc...

So what is the truth, is there a never ending oscillation (around an equlibrium) with a relatively small amplitude and thus the base current can't increase too much (no thermal runaway) OR there is and equilibrium that the process reaches after a time, staying there WITHOUT any ups and downs of the base current? And if there is and equlibrium, what's the mistake in my theory?

Every help is greatly appreciated!

2. ### burich Thread Starter New Member

Jan 2, 2016
2
0
Doh, picture seems to be gone ill. Here, I upload it instead:

File size:
12.3 KB
Views:
10
3. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
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No oscillation, just temperature dependence...

4. ### MikeML AAC Fanatic!

Oct 2, 2009
5,450
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Which is what you were supposed to do in the first place. I hate having to go those third-party image hosting sites...

5. ### crutschow Expert

Mar 14, 2008
13,056
3,245
Negative feedback can lead to oscillations if, at some frequency, the feedback becomes positive.
That can't happen for your circuit as it just slowly reaches an equilibrium point with no overshoot of that point and no oscillations.
Your assumption that it somehow overshoots the equilibrium point is incorrect. Why would it?