........ With a capacitor across Re we end up with a reactance from the emitter to ground Xe. As we increase the capacitor value, we lower Xe. And now the AC gain is very roughtly:
Av(AC)=Rc/Xe
The difference here is that the gain for DC is roughly Rc/Re, while for AC it is roughly Rc/Xe. So we can maintain the bias point with Rc/Re yet get higher AC gain because of Rc/Xe.
Ohh - I think, this is an approximation that is "too rough" because with rising frequencies Xe is approaching zero and the gain goes to infinity.
Hi W,
A physical capacitors Impedance value, never goes to zero.
Ohh - I think, this is an approximation that is "too rough" because with rising frequencies Xe is approaching zero and the gain goes to infinity.
Here comes the gain formula, which is to be used for varying Xe:
G=-Rc/(1/g + Xe)
Here you can see that the mentioned approximation is valid only as long as Xe>>1/g .
(Remember: the inverse of the transconductance g equals a resistor, which noemally is in the range 1/g=10...100 ohms, strictly depending on the selected bias point) .
Thus, for large frequencies we have
G=-g*Rc.
If that is the case, then If i provide a Capacitor to short out The emitter resistor, then the AC will not cause any drop across Emitter Resistor and consequently the Operating point just remains there. Then how will there be a Voltage swing at the output? I am definitely missing something
* Please quote correctly. Let me repeat that Xe is APPROACHING zero. Don`t you agree ? (I never have argued that it "goes to zero").
is approaching zero and the gain goes to infinity.
At frequencies high enough that Ce shorts out Re, the internal emitter resistance is NOT shorted and the ratio Rc/REE (or Rc/r'e) gives a good approximation to the AC gain of your circuit.
Hi there Winterstone,
What part of "very rough" dont you understand
.........
For reasonable values it's a good enough approximation to show what is happening in the circuit. If i wanted to get more particular i would have just analyzed the entire circuit which includes other various things. For a beginner i think it explains it pretty well.
.........
I guess all i was trying to show in as simple a manner as possible, was that the capacitive reactance makes the total resistance from emitter to ground look smaller and thus it raises the gain and the gain does go up when the Rc/Re gets bigger as Re gets smaller. So perhaps we can modify the gain equation a little to make it more appealing to your sense of snow.
Its the gain goes to infinity statement which infers the Xc value [impedance] must be zero, which I am disputing.
The resistance labeled r'e is the same internal emitter resistance.
At frequencies high enough that Ce shorts out Re, the internal emitter resistance is NOT shorted and the ratio Rc/REE (or Rc/r'e) gives a good approximation to the AC gain of your circuit.
Thanks I get this now! Although here is something else I'm not getting: How does having a high Collector resistor compared to emitter resistor give me better gain? I am measuring my output from what is left out of the drop across the collector resistor. So having a higher voltage drop at the collector resistor should only give lesser voltage at the output but it is not so according the ratio I take. I don't seem to get this?
Hi again MrAl,
Certainly I know that it is your intention to help the questioner as much as possible. And you can be sure that the same applies to me.
Nevertheless, I cannot agree with your approach - let me explain:
1.) At first, many books on electronics basics contain the formula for a simple gain stage without feedback G=-Rc*h21/h11=-g*Rc (g=transconductance)
Each beginner would be lost knowing only the rough approximation as given by you: G=-Rc/Xe.
<snip>
Regards
W.
Hello again Winterstone,
You're just getting absurd here.
..................
....................
Maybe you are the one that is "lost" here because you dont seem to understand how approximations work
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?