Question

Draw the circuit arrangement and also explain the characteristics curves for any common emitter configuration transistor. Also make the figures of curves and write the formula for voltage gain and current gain.

Answer 1

Common Emitter Configuration:

In this configuration emitter (E) input circuit and output circuit both are common. The potential difference between base and emitter is called potential difference input voltage whereas potential difference between collector and emitter is called output voltage. The basic arrangement for this configuration is shown in the figure. Here base current I_B is input current and collector current I_C is output current.

For a PNP transistor when used to draw common emitter characteristic curve the circuit is shown in the figure. With the help of battery V_BB and the potential breakdown arrangement R₁ forward biasing VBE is provided to the base-emitter junction. The measure of V_BE is done by the voltmeter present between base and emitter ends. Here, the value of base current I_B is very low(~ A) hence it is measured by micro-ammeter. The potential difference V_CE between the collector and emitter due to the battery V_CC and potential breakdown arrangement R₂ is measured by the voltmeter and collector current I_C is measured by milliammeter.

Here, this question is necessary that how does the base-collector junction is provided forward biasing, since there is no battery between them? To answer this question see figure carefully.

Here, both base and collector are connected to the negative ends of the circuits of their batteries (V_BB and V_CC ). If the magnitude of V_CE is higher than V_BE then base (A-type) will be at negative potential than collector (P-type).

In other words, A-type base is at a higher positive potential than P-type collector, clearly this junction will be reverse biased.

Input Characteristics: Here output voltage V_CE is kept constant and the changes in input current I_B are studied related to the input voltage V_BE. For this with the help of R₂. V_CE is kept at a constant value. Now with the help of R₁ the value of V_BE is increased from zero and changed to discrete values and the value of I_B related to increased. A graph is drawn between I_B and V_BE. The same process is repeated for other values of V_CE . The graph of curves so obtained are called the input characteristics of common emitter configuration. Figure shows the characteristic curves for three constant values.

Dynamic Input Resistance:

The dynamic input resistance for common emitter configuration of transistor is given by following formula:

The value of this approximately of 100 Ω. If this value is compared with common base circuit input resistance R_ib then, R_ie > R_ib.

Output Characteristic: Here, input current I_B is kept constant and the changes in output current I_C related to output voltage V_CE is studied. To calculate this with the help of R₂ the value of V_CE is changed and hence the value of I_C increases. A graph is drawn between V_CE and I_C . This process is repeated for other values of I_B. Hence, the group of curves so obtained are called output characteristics. These are shown in the figure.

At any constant value of I_B, V_CE when increased from zero, I_C very fast gets saturated. After this the value of IQ increases slowly. The line OA in figure is called saturated line and the area between I_C and this line is called saturated region. When I_B = 0 then also I_C is not zero, meaning when input current is zero still some output current flows.

The region between I_B = o and V_CE axis is called cut off region.
The remaining region apart from the cut off region and saturated region is called the active region. Here, the emitter-base junction is transistor is forward biased and the base-collector junction is reverse biased.
In active region I_C is not dependent on V_CE .
The dynamic output resistance R_oe for this configuration is defined as follows:

Since, I_C does not change much with V_CE (leaving the initial values of V_CE). Therefore, R_oe is of 50-100 kΩ.

Current Amplification Factor:

This is defined as the ratio of the change in collector current to the change in base current at a constant collector emitter voltage (V_CE) when the transistor is in active state.

This is also known as small signal current gain and its value is very large.
β = \(\frac{I_{C}}{I_{B}}\)
Since, I_C >> I_B and ΔI_C >> ΔI_B. Therefore, (β _dc and β both are grater than 1 meaning β >> 1.