emitter resistance and emitter capacitance

[electronics_kumar]

plz explain abt emitter resitance and its capacitance.
wht will happen if there is emitter resitance alone ,or emitter capacitance alone
ie. wht we are achieving by doing so

 

[andy1]

I am not sure if this is related to common collector amplifier.

The emitter resistance and capacitance both affects the gain among other things.

There is also a feedback issue with emitter resistance in common collector configuration.

I think your question has to be asked in a specific context otherwise there is no way to answer your question.

 

[fletcher]

If you mean common emitter gain stage emitter resistance decreases the gain and increases the bandwidth also it makes your bias independent of β and it has thermal compensation role. Emitter capacitance with proper position between two emitter resistor adjusts the gain while DC opertaing point doesn't change.

 

[electronics_kumar]

answer to my question with CE as reference

 

[sriramb]

As the question is clear enough for me if the connection s are in cascaded CE then emitter capacitance which is also coupled capacitance can b connected to the base then we can over come the distruction of the signal and for cascading without any loss of generality. I n the same way we can also use emitter reistance

 

[abhigopal]

In CE amplifier, Emitter capacitance if connected across the emitter resistance will actually make the V at emitter zero under bias conditions. mainly used for biasing. Otherwise, the emitter resistance while looking from the base will appear as (current gain * emmiter resisitance).

 

[anyaddress]

hi this a small reference about semiconductor capacitance (simple capacitor,diode capacitance,transistor capacitance and more

 

[Kevin Weddle]

Yeah, re is a signal impedance. If you just apply a voltage to the PN junction you will get the current. I believe the formula is e to the VBE/VT where VT is the thermal voltage. I have never used this equation. But it looks useful.

 

[papyaki]

Hi,

Why not use pSpice or another spice simulator and make a parametric simulation with different values for emitter resistance and emitter condensator ? You will see the effects of both compondents on DC polarization, AC gain, AC bandwith etc.

pSpice student version is available for free :

h**p://www.orcad.com/downloads/demo/default.asp

* = t

 

[yogi]

emitter resistance in a CE amplifier is for feedback and bias

emitter capacitance is to avoid voltage drop across
emitter resistor

 

[VVV]

The resistor connected between the emitter and ground is used to stabilize the Q-point of the transistor. Imagine the collector (and hence the emitter) current changes as a result of temperature variation.
An increase of this current will cause an increase of the voltage across this emitter resistor. But an increase of this voltage will diminish the base-emitter voltage of the transistor (assuming for simplicity that the base is at a fixed voltage with respect to ground). And a decrease of the base-emitter voltage will tend to decrease the collector current, thus compensating the initial unwanted increase in the collector current. Conversely, a decrease in collector current will be followed by a decrease of the emitter voltage, follwed by an increase of the B-E voltage and an increase of the collector current to compensate the unwanted decrease.

As can be seen, this resistor introduces a negative feeback. But this negative feeback does not know what caused the change in the collector current and it will work equally on the AC signal, reducing the gain. Sometimes this is undesirable, so we shunt the resistor with a capacitor. This capacitor works like a short-circuit for the useful AC signal, but has no effect on the DC component. Since the temperature variations are extremely slow, they affect the DC component of the collector current, so the emitter resistor can compensate them, stabilizing the Q-point. However, with the capacitor in place we get a higher gain in AC. The penalty we have to pay is a lower input impedance. The input impedance is Zin=h11+(h21+1)*Ze, where Ze is the impedance connected to the emitter. With the capacitor acting as a short, all that is left in AC is Zin=h11, which can be much lower.

Without the capacitor, it is clear what happens: the circuit works, having the same gain both in AC and in DC. Without the resistor, the transistor cannot be properly biased, so it will not work as a transistor; the actual functionality depends heavily on the actual circuit. The B-E diode might work as a peak rectifier together with the emitter capacitor.

 

[electron_boy]

hi

emitter resistance is re and it can be related to rb as rb=re/(β+1)

roughly re is calculated by re=26mV/Ie (at normal temperature)

for more details refer "electronic devices and circuit theory" boyelstad