Common emitter amp and BJT base-emitter resistance question

In summary, the base-emitter resistance seen in a common emitter amplifier circuit is a small-signal AC parameter that is affected by the DC bias values of collector current and hFE. This can cause confusion, but understanding the difference between DC and AC values and how they relate to the hybrid pi model can clarify this concept. "
  • #1
linear_shift
16
0
Hi,

In a common emitter amplifier circuit, I am considering the base-emitter junction. When you apply a base current, you get the base-emitter resistance, according to thermal_voltage/base_current. But when you get that resistance and place it in it's position in series with the emitter resistor in the amplifier circuit, it changes the base current. What gives? What keeps it at the same value (for given thermal voltage, beta/Hfe, etc)? Certainly I'm missing something here (probably something elemental ;D ). I know base-emitter resistance isn't supposed to matter much in a typical common emitter amp, but this question has been bugging me for some time now. :P

Thanks,

LS
 
Engineering news on Phys.org
  • #2
I think you are referring to small-signal models, the hybrid pi or the T model. Those models relate to the AC behavior of the amplifier. The base-emitter resistance is what a small amplitude AC signal would see.
 
  • #3
The base-emitter resistance seen from the base side of the bjt is called "r_pi". Seen from the emitter side, it is called "re".

r_pi = hfe/gm = hfe*Vt/Ic. But we must be careful & remember that "Ic" is the dc or quiescent value of collector current. The small signal, or ac part of the collector current is denoted by "ic". We must not confuse the two.

By definition, r_pi = vbe/ib, being sure to observe the lower case "v" in "vbe", & lower case "i" in "ib". Referring to the above equation, r_pi = hfe*Vt/Ic, we must remember that r_pi is a small signal ac parameter. But, the "Ic" in the equation is the dc bias value. So the dc value of collector current determines the ac small signal resistance value.

The "hfe" value denotes small signal ac conditions. So "Ic/hfe" is not really the base current. Ic = hFE*Ib, since "Ic, Ib, & hFE" are all dc values. Likewise ic = hfe*ib (ac).

The re value is Vt/Ie. Again Ie is dc value, but re is ac. Also, re = vbe/ie = alpha/gm, all ac values. I think that is where the confusion can take place. Have I helped or made matters worse.

Claude
 
Last edited:
  • #4
cabraham,

That makes sense now, the two values Ic and ic (or Ib and ib) are separate. Ic (DC) determines the AC resistance, so the AC circuit is different (adding r_pi), than the DC circuit determining r_pi which does not see r_pi (so we don't have the "looping" r_pi calculation). Wikipedia does not explain things well. :P

Thanks for all of your help,

LS
 
  • #5

1. What is a common emitter amplifier?

A common emitter amplifier is a type of transistor amplifier that uses a bipolar junction transistor (BJT) in a common emitter configuration. It is commonly used for amplification of analog signals in electronic circuits.

2. How does a common emitter amplifier work?

In a common emitter amplifier, the BJT acts as a voltage-controlled current source. The input signal is applied to the base terminal of the transistor, which controls the amount of current flowing through the emitter-collector junction. This amplified current is then converted back to a voltage at the output, resulting in signal amplification.

3. What is the role of the base-emitter resistance in a BJT?

The base-emitter resistance in a BJT is responsible for controlling the amount of current flowing through the base-emitter junction, and therefore, the amplification of the input signal. A lower base-emitter resistance leads to higher amplification, while a higher resistance reduces the amplification.

4. How is the base-emitter resistance calculated in a BJT?

The base-emitter resistance can be calculated using the formula Rbe = (Vt / Ib) - (Re + (β+1) * Rload), where Vt is the thermal voltage, Ib is the base current, Re is the emitter resistance, β is the current gain of the transistor, and Rload is the load resistance.

5. How does the base-emitter resistance affect the performance of a common emitter amplifier?

The base-emitter resistance has a significant impact on the performance of a common emitter amplifier. A lower resistance leads to higher amplification and better performance, while a higher resistance can cause distortion and reduce the overall performance of the amplifier.

Similar threads

Replies
10
Views
939
Replies
16
Views
4K
Replies
5
Views
2K
  • Electrical Engineering
Replies
19
Views
1K
  • Electrical Engineering
Replies
5
Views
2K
Replies
6
Views
1K
  • Electrical Engineering
2
Replies
60
Views
12K
  • Electrical Engineering
2
Replies
43
Views
4K
  • Electrical Engineering
Replies
2
Views
2K
  • Electrical Engineering
Replies
3
Views
1K
Back
Top