Followers, loops, and feedback

[SY]

sorry, but common-base stage does not amplify the current .......

Now we're REALLY getting into the nitty-gritty of language, jargon, and semantics. When we say "voltage follower," we mean something that doesn't have voltage gain. but swings sufficient current so that the output voltage "follows" the input voltage. So, analogously, a common-base stage could easily be thought of as a current follower, in that the input terminal (the emitter) is at a nearly constant voltage, but the output (at the collector) can swing whatever voltage is necessary to "follow" the input current.

 

[Steve Eddy]

"emitter being fed back to the base via the load"

Sorry, but I don't understand it. How the emitter feed back the base, via the load?
The load is not connected to the base.

Well, the base goes to ground and the bottom of the load goes to ground. Seems to me that connects the load to tbe base.

"What exactly is it that you're measuring? "

Apply 1kHz square wave on the input, and measure the collector of the first stage. You will see overshot on the squarewave. That is the delay.

Hmmm. I've never associated overshoot with delay. I associate it with resonance. Sure what you're seeing isn't just due to Gibbs phenomenon and not necessarily overshoot?

se

 

[Steve Eddy]

The emitter terminal goes straight to ground, but the emitter resistance, it seems to me, should be degenerating things, otherwise you'd have infinite gain. If I think of a T model for a bipolar, there's an equivalent resistance re in series with the emitter lead.

Oh, well yeah, the intrisnic emitter resistance is always there. You originally said "Rbe" which isn't the same as r_e.

se

 

[SY]

Ah, jargon. In any case, the emitter resistance does provide degeneration, even in a true common emitter amp. So there's always some feedback no matter how much a designer might want to wish it away.

I'm going to have to learn how to do those cool sub_scripts.

 

[Steve Eddy]

You have right. With common emitter connection You can create feedback loop.

But as I know the emitter degeneration resistor is not feedback loop.

From Horowitz and Hill's The Art of Electronics:

Emitter resistor as feedback

Adding an external series resistor to the intrinsic emitter resistance r_e (emitter degeneration) improves many properties of the common-emitter amplifier, at the expense of gain. You will see the same thing happening in Chapters 4 and 5, when we discuss negative feedback, an important technique for improving amplifier characteristics by feeding back some of the output signal to reduce the effective input signal.
The similarity here is no coincidence; the emitter-degenerated amplifier itself uses a form of negative feedback. Think of the transistor as a transconductance device, determining collector current (and therefore output voltage) according to the voltage applied between the base and emitter; but the input to the amplifier is the voltage from base to ground. So the voltage from the base to emitter is the input voltage, minus a sample of the output (I_ER_E). That's negative feedback, and that's why emitter degeneration improves most properties of the amplifier (improved linearity and stability and increased input impedance; also the output impedance would be reduced if the feedback were taken directly from the collector).

se

 

[Steve Eddy]

Ah, jargon. In any case, the emitter resistance does provide degeneration, even in a true common emitter amp. So there's always some feedback no matter how much a designer might want to wish it away.

Yes, there's always going to be the intrinsic and internal r_e. But the literal emitter resistor R_E is external.

I'm going to have to learn how to do those cool sub_scripts.

Damn! Quick learner!

se

 

[SY]

And the effective emitter resistance is the sum of the two- but never zero. I can quote Horowitz and Hill for the unconvinced.

On further review, one could get rid of the degenerative effect of the emitter resistance by connecting a negative resistance of equal magnitude to ground from the emitter terminal. Of course, one gets this negative resistance in the first place by the use of feedback...

 

[Steve Eddy]

And the effective emitter resistance is the sum of the two- but never zero. I can quote Horowitz and Hill for the unconvinced.

Hehehe.

Yes, it's never going to be zero. I'd already said as much in my previous reply to Frank. What I was saying is that with the emitter going straight to ground through no literal resistor, base emitter resistance (which is what I took your Rbe to mean) doesn't add to r_e and therefore doesn't increase negative feedback above its intrinsic level.

On further review, one could get rid of the degenerative effect of the emitter resistance by connecting a negative resistance of equal magnitude to ground from the emitter terminal. Of course, one gets this negative resistance in the first place by the use of feedback...

Hehehe. True. Negative feedback just seems to pop its head up everywhere.

se

 

[PMA]

Now we're REALLY getting into the nitty-gritty of language, jargon, and semantics. When we say "voltage follower," we mean something that doesn't have voltage gain. but swings sufficient current so that the output voltage "follows" the input voltage. So, analogously, a common-base stage could easily be thought of as a current follower, in that the input terminal (the emitter) is at a nearly constant voltage, but the output (at the collector) can swing whatever voltage is necessary to "follow" the input current.

Yes, this is absolutely correct. Common -base stage has current gain equal to 1 and has non-unity voltage gain. My objection was addressed to statement that "common-base stage can be considered as a current buffer". That is not right, as a current buffer amplifies the current and has voltage gain of +1.

Pavel