Common collector mode cofiguration

Common collector mode (CC-configuration ):

In this configuration we use collector terminal as common for both input and output signals. This configuration is also known as emitter follower configuration because the emitter voltage follows the base voltage. This configuration is mostly used as a buffer. These configuration are widely used in impedance matching application because if their high input impedance.

In this configuration the input signal is applied between the base-collector region and the output is taken from the emitter –collector region.

In this configuration collector current is common in between input and output section. The input voltage and current are \(V_{CB}\) and \(I_B\) respectively whereas output and current are \(V_{CC}\) and \(I_E\) respectively shown in figure . The current amplification factor in CC-mode is denoted by \(\Upsilon\) and is defined as the ratio of output current to the input current.Here,

Output current =\(I_E\)

Input curretnt=\(I_B\)

\(\therefore\) amplification factor\((\Upsilon)=\frac{I_E}{I_B}\) $$\Upsilon=\frac{I_E}{I_B}$$

Relationship between the \(\alpha,\beta\) and \(\gamma\):

We have amplification factor \(\alpha,\beta,\gamma\) in CB,CE and CC configuration are as,

$$ \alpha=\frac{I_C}{I_E},\beta=\frac{I_C}{I_B},\gamma=\frac{I_E}{I_B}$$Now,

$$\gamma=\frac{I_E}{I_B}=\frac{I_B+I_C}{I_B}=1+\beta$$ $$\gamma=1+\beta$$Also,$$\beta=\frac{\alpha}{1-\alpha}$$

$$\therefore \gamma=1+\frac{\alpha}{1-\alpha}=\frac{1}{1-\alpha}$$

$$\gamma=1+\beta=\frac{1}{1-\alpha}$$Such that \(\gamma>\beta>\alpha\)

Characteristics of CC-mode:

• Input characteristics:

  

The variation of base current with collector base voltage for different constant value of C-E voltage (\(V_{CE}\) in CC mode is called as input characteristics. The input characteristics for different constant value of \(V_{CE}\)(\(V_{CE}=2v,4v\)) is shown in above figure. The input characteristics of CC mode is different from that of CB and CE mode of transistor. Which is due to the fact that the input voltage (\(V_{CB}\) is; extremely dependant on output voltage \(V_{CE}\).

From above figure $$ V_{CE}=V_{CB}+V_{BE}$$ $$V_{CB}=V_{CE}-V_{BE}$$For normal operation of transistor \(V_{BE}\) should be greater than or equal to 0.7v(\(V_{BE}\geq0.7v for si)\) and hence the increases of \(V_{CB}\) is possible by decreasing value of \(V_{BE}\). Which reduces the base current (\(I_B\)). Thus, on increasing \(V_{CB}\) the base current \(I_B\) decreases slowly. Hence, the input characteristics is straight line with negative slope.

• Output characteristics:

It is the variation of emitter current with collector-emitter voltage for different constant values of base current as in figure. Since,\(I_C\) is nearly equal to

Since,\(I_C\approx I_E\) as \(I_C= \alpha I_E(\alpha\approx 1)\). Thus, the output characteristics is similar to that in CE mode.

• Transfer characteristics:

  

The transfer characteristics is variation of emitter current with base current for constant value of collector to emitter voltage. This is also similar to that of transfer characteristics in CE mode as,\(I_c\approx I_E\).

Comparison of three transistor configuration:

CE:

Most commonly used in general purpose amplifier designs. It provides high gain and high input impedance. The drawbacks with this simple configuration are limited bandwidth due to Miller effect of collector –base capacitance and limited load driving capability due to high output impedance.

CC:

Most commonly used as unity gain buffer. It provides the high input impedance, low output impedance and high bandwidth. Perfect for driving heavy loads. The drawbacks is no gain; gain is close to but less than one. Therefor, it is typically used in conjunction with an amplifier and not instead if an amplifier.

CB:

Most commonly used as a cacode stage to isolate output voltage signal feeding back to input eliminating Miller effect from amplifier to increase bandwidth. It provides low input impedance and close to unity current gain It is not typically used as the standalone amplifier due to low input impedence.

References:

(1)Theraja, B.L. Basic Electronics. N.p.: S.Chand, n.d. Print.

(2)C.L.Arora. Refresher Course in Physics. Vol. II and III. N.p.: S.Chand, 2006. Print.

(3)Malvino. Electronic Principles. N.p.: Tata McGraw-Hill, n.d. Print.

(4)N.Nelkon and P.Parker. Advanced Level Physics. 5th ed. N.p.: Arnold Heinemann, n.d. Print.

(5)Priti Bhakta Adhikari,Diya Nidhi Chaatkuli, Ishowr Prasad Koirala. A Textbook of Physics (2nd Year). N.p.: Sukunda Pustak Bhawan, 2070. Print.