My simulations show severe distortion when there is no negative feedback.audioguru,
What is the point and conclusions of your simulations?
Ratch
My simulations show severe distortion when there is no negative feedback.
They also show that the distortion is reduced very much when there is a high resistance ahead of the feedback resistor so that it allows much more AC negative feedback that you disbelieve.
Your emitter resistor reduces the maximum output level, my input resistor adds the same negative feedback as your emitter resistor but does not reduce the maximum output level.
I simulated your idea of separating the AC negative feedback from the DC negative feedback. It makes no difference.
That's not the way you originally presented your arguments as you stated that R2 "was not there for negative feedback", hence some of my replies were targeting that.MrAl,
[R2 being there for DC and AC feedback]
That is the point I am trying to make. I believe R2 should only be used for bias and not AC feedback.
Well, again, we already have negative AC feedback. True it could be done a different way, but that doesnt mean there is no AC negative feedback now. The resistor R2 does both with one resistor. I also wont argue that an emitter resistor might help, but that's a different design.If you want negative AC feedback, then use a emitter resistor.
Not sure why you are stating this. R4 affects the gain, so it works with the feedback. It doesnt have to be in the feedback path itself, who cares. Also, dividing R2 in half does not remove it completely from the AC gain considerations. With R2a left side and R2b right side to collector, sized cap in the middle to ground, R2b does not enter in the AC gain equation but R2a still does.Yes, R4 will reduce the gain and protect the emitter-base junction, but not enter into feedback considerations.
Ok that's great, but check out R2a above.I don't have any problems with my analysis.
My new simulation shows the output level is reduced when I remove the C2 filter capacitor so it proves that there is AC negative feedback through R4 and R2.
That's not the way you originally presented your arguments as you stated that R2 "was not there for negative feedback", hence some of my replies were targeting that.
Well, again, we already have negative AC feedback. True it could be done a different way, but that doesnt mean there is no AC negative feedback now. The resistor R2 does both with one resistor. I also wont argue that an emitter resistor might help, but that's a different design.
Not sure why you are stating this. R4 affects the gain, so it works with the feedback.
It doesnt have to be in the feedback path itself, who cares.
With R2a left side and R2b right side to collector, sized cap in the middle to ground, R2b does not enter in the AC gain equation but R2a still does.
I haveOk that's great, but check out R2a above.
MrAl,
[R2a and R2b, and how R2a may or may not affect the gain]
If there is so little AC after the capacitor, there won't be any AC feedback from R2a.
Ratch
I'm getting trouble on getting (using PSPICE-like software) the input and output impedance of a common-emitter BJT amplifier with collector-base resistor, to try to see how it compares with a more common emitter degeneration feedback (or both). I attached schematic pictures of my set-up. I hope someone can help me see where am I missing something.
Well again if you look back you'll see that i never said there would be feedback from R2a, i just said that "R2a is still in the gain equation". That's why i said that we cant completely remove R2 from the gain considerations, even after dividing it into two section with a cap to filter out AC variations from the collector. It may be better to make modifications overall but the AC gain due to R2a still has to be considered. That's why i pointed your analysis back to R2a.
MrAl,
I assume you mean AC gain. R2a can be in the gain equation all it wants. If there is no AC present, it might as well not be there.
Ratch
If we talk about AC gain and how R2a affects it, how exactly does the viewpoint of zero AC input help here?
Here's an equation for the AC voltage gain of the original circuit [B=Beta]:
Av=-(B*R2*R3)/(B*R3*R4+R3*R4+R2*R4)
Note that the gain is negative because the output is 180 degrees out of phase with the input.
Also, it looks like a good approximation for the output impedance is:
Rout=(R3*R2)/(R2+R3*B+R3)
MrAl,
Could you explain why R3 is in the output impedance equation.? I thought that R3 was the load resistor R[L].
Ratch
Have a look at this circuit:
http://www.ecircuitcenter.com/Circuits/trce/trce.htm
The resistor which provides DC bias to the collector, RC, is separate from the load resistor, RL, which only receives an AC output signal.
Apparently MrAL assumed that R3 is the bias resistor, and the load resistor is not shown in the OP's first post.
This current source testing procedure is very typical and shouldnt come as any surprise to anyone. Would be interesting though if Ratch assumed otherwise.
What I don't understand is why the B-E terminals are shorted out. The output impedance depends on what is connected to the input terminals, so why short them out?
Ratch
I'd be happy to follow your analysis through but it's a little hard to follow with all those brackets. Nonetheless, if you'd defined all your input variables (explaining what each one is and why they are there) i'd be happy to give in another shot. For example, if everything was in terms of the actual resistors used like R1, R2, R3, etc., it would be a lot easier to follow.
In many equations like these you'll find abbreviations like Rg, Rf, etc., rather than R[g], R[f], etc., and it makes it easier to follow. It's easy to note that the 'g' is a subscript in 'Rg' for example because it is lower case.
You might also note that the equation i gave for the output impedance is quite accurate given the assumption of a constant base voltage, or to put it another way, a tiny test current in the output.
We could go over these in more detail if you like. I'd like to see how you developed your equation too. We could use a circuit simulator to check both formulas and see how they compare side by side.
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