Transistor input,output and transfer characteristics in common mode configuration

Transistor input, output and transfer characteristics in different configuration:

Depending upon the common terminal that are taken between input and output circuit three different circuit combination of transistor. Thus, there are three modes of transistor.

(1)Common base mode :

In this configuration base region is connected in between input and output supply as shown in figure below. Here, the emitter current \(I_E\) is input current and \(V_{EE}\) is input voltage.

Whereas current \(I_C\) is output current with output voltage \(V_{CB}\) in reverse biased condition.

The current amplification factor is the ratio output current with respect to its input current .The current amplification factor in CB mode is denoted by \(\alpha\) and is the ratio of collector current with respect to emitter current.

i.e= Input current=\(I_E\)

Output current =\(I_C\)

CFor ac signal the amplification factor is defined as the ratio of change in collector current to that of change in emitter current.

i.e.\(\alpha_{ac}=\frac{\Delta I_C}{\Delta I_E}\)Since,$$I_E=IB+I_C$$But\( I_B\neq 0\) $$\therefore I_C<I_E$$

\(\Rightarrow\alpha<1\)

Since, value of \(\alpha\) is nearly equal to 1 as \(I_B\) is very low,

\(\Rightarrow=0.95 to 0.99\)

Leakage current :

Even when emitter is open small amount of current is flowing across collector base junction is always reverse bias so that minority charge gets accelerated producing small current called as Leakage current.The Leakage current in CB mode is denoted by \(I_{CBO}\) is current from collector to base region when emitter is open.This Likage current is extremely temperature dependent.For every \(6^\circ c\) rise in temperature the leakage current doubles for germanium and that occurs for silicon for every \(10^\circ c\) rise in temperature.

When E-B junction is made close then total collector current (including leakage current) is ,$$I_C=\alpha {I_E}+I_{CBO}$$

Characteristics:

Depending upon the operation of transistor the characteristics are of

• Input characteristics
• Output characteristics
• Transistor characteristics

Input characteristics:

It is the variation of input current( \(I_E\)) with B-E voltage (\(V_{BE}\)) at constant votage (\(V_{BE}\)).The input characteristics is shown in above figure.

The input characteristics of C-B transistor is similar to that of forward characteristics of P-N junction diode.If \(V_{BE}<V_k\) then no emitter current appears when \(V_{BE}\) exceed \(V_k\) the emitter current flows rapidly but the variation is non-linear. The variation doesn’t obey the Ohm’’s law. This characteristics is also used to find the input resistance of the transistor which is given by reciprocal of slope of forward characteristics(very slow).

$$R_{in}=\frac{1}{slope}=\frac{1}{\frac{\Delta I_E}{\Delta V_{BE}}}=\frac{\Delta V_{BE}}{\Delta I_E}$$

Output characteristic:

It is the variation of output current (\(I_C\))with respect voltage \(V_{CB}\) at constant input current (\(I_E\)).The output characteristics for different constant value of emitter current(\(I_E\)) is shown in figure above.

• The collector current increases with increasing value of C-B voltage(\(V_{CB}\)) up to certain value and then further increase in\(V_{CB}\) causes slight increase (almost constant ) of collector current as shown in figure after certain safe value. The junction breakdown occurs with large increase of constant current (\(I_C\)) due to slight increase of \(V_{CB}\) as in figure.
• For \(I_E=0\), a small collector current flows through the transistor which is due to flow if minority charge carrier producing leakage current. This region is called cut off region where the transistor is switch off.
• When \(V_{CB}=0\);their appears collector current through transistor to injection to electron from emitter to base and are received by the collector due to junction potential across C-B.T make \(I_C=0\) some value of forward potential \(V_{CB}\) should be given. This region is called as saturation current.
• The output resistance is very high (\(\sim500k\Omega\)) and is given by reciprocal of slope of output characteristics,

i.e. \(R_{out}=\frac{1}{slope}=\frac{1}{\frac{\Delta V_{CB}}{\Delta I_C}}\)

Transfer characteristics:

It is the variation of output current with respect to input current at constant output voltage (\(V_{CE}\)). The transfer characteristics of C-B transistor is shown in figure. The transfer characteristics is the straight line having slope =\(\alpha\)=current amplification factor and intercept equal to leakage current(\(I_{CBO}\)).

Since,\(I_C\neq0\) for , \(I_E=0\) which is due to flow minority charge carrier in C-B junction. It also gives the amplification factor as, $$\alpha_{ac}=\frac{\Delta I_C}{\Delta I_E}=1\approx 1$$(slight less than 1)

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.