Transistor as an amplifier (doubt)

In summary, the npn transistor functions as an amplifier in its active state by utilizing majority of electrons from the base layer to be dragged into the collector layer. The resistance offered by the collector base junction is not considered in output power, rather the external resistance in series with the collector as the load being driven is used. However, for large values of effective collector resistance, the internal output resistance of the transistor may need to be taken into account in determining the gain.
  • #1
latencymech
The npn transistor is said to function as an amplifier only in its active state (emitter base is forward biased and collector base is reverse biased). It is considered that majority of the electrons reaching the base layer is dragged into the collector layer. So the emitter current (ec) is approximately equal to the collector current (cc).

The reason for amplification is given by the equation

P(input) = I^2 (ec) * R(emitter base)
P(output) = I^2(cc) * R(collector base)

Since R(collector base) >>> R(emitter base) & I(ec) = I(cc), P(output) > P(input)... [because the collector base junction is reverse biased and it offers high resistance to the flow of majority charge carriers across the junction]

My question:-

The resistance offered by the collector base junction is only to the flow of electron holes from base (p type) to collector (n type) or the flow of electrons from collector (n type) to base (p type). So why are we considering the resistance in output power to be R(collector base)? This resistance does not oppose the motion of electrons from the base to the collector right??
 
Physics news on Phys.org
  • #2
latencymech said:
So why are we considering the resistance in output power to be R(collector base)?
Hi, welcome to the PF. :smile:

I don't think we are considering the device's Rcb to be the output. You should be using the external resistance in series with the collector as the load being driven. It is denoted RC.

You need to indicate the circuit under consideration, though I can surmize it to be the common emitter amplifier.

Pin is (base current)^2 x rbe,
there may be a factor beta in there, too, depending on the terms you use.
 
  • Like
Likes latencymech
  • #3
Thank you.
 
  • #4
NascentOxygen said:
I don't think we are considering the device's Rcb to be the output. You should be using the external resistance in series with the collector as the load being driven. It is denoted RC.
.

Yes - that is the normal procedure for calculating the signal output voltage.
However, if the effective collector resistance Rc,eff (including the input resistance of the next stage - if existent) is rather large (tens of kohms) it may be necessary to take into account also the BJT`s internal output resistance r,o because the transistor is not an ideal current source. The value of r,o is, typically, in the range (20...50)kohms and is identical to the inverse slope of the Ic=f(Vce) characteristics. In this case, the gain determining resistance is (R,eff||r,o).
 

FAQ: Transistor as an amplifier (doubt)

1. How does a transistor function as an amplifier?

A transistor amplifies a weak input signal by using a small amount of current to control the flow of a larger current. This is achieved through the use of a small base current to control the flow of a larger collector current.

2. What are the different types of transistor amplifiers?

There are three main types of transistor amplifiers: common emitter, common base, and common collector. Each type has its own unique characteristics and is used for different applications.

3. What is the role of biasing in transistor amplifiers?

Biasing is the process of applying a steady DC voltage to the base of a transistor amplifier to ensure it operates in its linear region. This is necessary to achieve a faithful amplification of the input signal.

4. How is gain defined in a transistor amplifier?

Gain is a measure of the amplification ability of a transistor amplifier and is defined as the ratio of output voltage to input voltage. It is typically represented in decibels (dB) and can vary depending on the type of amplifier and its configuration.

5. What are the limitations of using a transistor as an amplifier?

Some of the limitations of using a transistor as an amplifier include non-linear distortion at high frequencies, temperature dependence, and the need for proper biasing and heat dissipation to prevent damage. Additionally, transistors have a limited voltage and current handling capability, which can restrict their use in certain applications.

Similar threads

Replies
1
Views
3K
Replies
4
Views
3K
Replies
4
Views
2K
Replies
59
Views
4K
Replies
5
Views
2K
Replies
1
Views
2K
Replies
6
Views
3K
Replies
3
Views
2K
Back
Top