Types of Configuration

TYPES OF CONFIGURATION

When a transistor is to be connected in a circuit, one terminal is used as an input terminal, the other terminal is used as an output terminal and the third terminal is common to the input and output. Depending upon the input, output and common terminal, a transistor can be connected in three configurations. They are (i) Common Base (CB) Configuration (ii) Common Emitter (CE) and (iii) Common Collector (CC) Configuration.

Common Base Configuration

This is also called grounded base configuration. In this configuration, emitter is the input terminal, collector is the output terminal and base is the common terminal.

The circuit for determining the static characteristics curve of an NPN transistor in the common base configuration is shown in Fig.1.37.

Input Characteristics

To determine the input characteristics, the collector base voltage V_CB is kept constant at zero volt and the emitter current I_E is increased from zero in suitable equal steps by increasing V_EB. This is repreated for higher fixed values of V_CB. A curve is drawn between emitter current I_B and emitter-base voltage V_EB at constant collector-base voltage V_CB. The input chracteristics thus obtained are shown in Fig. 1.38.

When V_CB is equal to zero and the emitter-base junction is forward biased as shown in the characteristics. The junction behaves as a forward biased diode so that emitter current I_E increases rapidly with small increase in emitter-base voltage V_EB

When V_CB is increased keeping V_EB constant, the width of the base region will decrease. This effect results in an increase of I_E. Therefore, the curves shift towards the left as V_CB is increased.

Output Characteristics

To determine the output characteristics, the emitter current I_E is kept constant at a suitable value by adjusting the emitter-base voltage V_EB. Then V_CB is increased in suitable equal steps and the collector current I_C is noted for each value of I_E. This is repeated for different fixed values of I_E. Now the curves of I_C versus V_CB are plotted for constant values of I_E and the output characteristics thus obtained is shown in Fig. 1.39.

From the characteristics, it is seen that for a constant value of I_E, I_C is independent of V_CB and the curves are parallel to the axis of V_CB. Further, I_C flows even when V_CB is equal to zero. As the emitter-base junction is forward biased, the majority

carriers i.e., electrons from the emitter are injected into the base region. Due to the action of the internal potential barrier at the reverse baised collector-base junction, they flow to the collector region and give rise to I_C even when V_CB is equal to zero.

(i) Input Impedance (h_ib)

It is defined as the ratio of the change in (input) emitter voltage to the change in (input) emitter current with the (output) collector voltage V_CB kept constant. Therefore,

It is the slope of CB input characteristics I_E versus V_EB as shown in Fig. 1.38. The typical value of hit ranges from 20 to 50 Ω.

(ii) Output Admittance (h_ob)

It is defined as the ratio of the change in (output) collector current to the corresponding change in the (output) collector voltage with the (input) emitter current I_E kept constant. Therefore,

It is the slope of CB output characteristics I_C versus V_CB as shown in Fig. 1.39. The typical value of the parameter is of the order of 0.1 to 10 μhos.

(iii) Forward Current Gain (h_fb)

It is detected as a ratio of the change in the (output) collector current to the corresponding change in the (input) emitter current keeping the (output) collector voltage V_CB constant. Hence,

It is the slope of I_C versus I_E curve. Its typical value varies from 0.9 to 1.0.

(iv) Reverse Voltage Gain (h_rb)

It is defined as the ratio of the change in the (input) emitter voltage and the corresponding change in (output) collector voltage with constant (input)emitter current, I_E. Hence,

It is the slope of V_EB Vs V_CB curve. Its typical value is the order of 10^-5 to 10^-4.

Common Emitter Configuration

Input Characteristics

To determine the input characteristics the collector to emitter voltage is kept constant at zero volt and base current is increased from zero in equal steps by increasing V_BE in the circuit shown in Fig. 1.40.

The value of V_BE is noted for each settling of I_B. This procedure is repeated for higher fixed values of V_CE, and the curves of I_B Vs V_BE are drawn. The input characteristics thus obtained are shown in Fig. 1.41.

When V_CE = 0, the emitter base junction is forward biased and the junction behaves as a forward biased diode.

Hence the input characteristics for V_CE = 0 is similar to that of a forward biased diode. When V_CE is increased the width of the depletion region at the reverse biased collector base junction will increase. Hence the effective width of the base will decrease. This effect causes a decrease in the base current I_B. Hence to get the same value of I_b as that for V_CE = 0, V_BE should be increased. Therefore, the curve shifts to the right as V_CE increases.

Output Characteristics

To determine the output characteristics, the base current I_B is kept constant at a suitable value by adjusting base-emitter voltage, V_BE. The magnitude of collector- emitter voltage V_CE is increased in suitable equal steps from zero and the collector current I_C is noted for each setting V_CE. Now the curves of I_C versus V_CE are plotted for different constant values of I_B. The output characteristics are obtained shown in Fig. 1.42.

For larger values of V_CE, due to Early effect, a very small change in a is reflected in a very large change in β.

The output characteristics of CE configuration show a larger slope when compared with CB configuration.

The output characteristics have three regions namely, saturation region, cutoff region and active region.

The region of curves to the left of the line OA is called the saturation region, and the line OA is called the saturation line. In this region, both junctions are forward biased and an increase in the base current does not cause a corresponding large change in I_C. The ratio of V_CE(sat) to I_C in this region is called saturation resistance.

The region below the curve for I_B = 0 is called the cut-off region. In this region, both junctions are reverse biased. When the operating point for the transistor enters the cutoff region, the transistor is OFF. Hence the collector current becomes almost

zero and the collector voltage almost equals V_CC, the collector supply voltage. The transistor is virtually an open circuit between collector and emitter.

The central region where the curves are uniform in spacing and slope is called the active region. In this region, emitter-base junction is forward biased and the collector base junction is reverse biased. If the transistor is to be used as a linear amplifier it should be operated in the active region.

Transistor Parameters

The slope of the CE characteristics will give the following four transistor parameters. Since these parameters have different dimensions, they are commonly known as common emitter hybrid parameters or h-parameters.

(i) Output Admittance (h_oe)

It is defined as the ratio of change in the (output) collector current to the corresponding change in the (output) collector voltage with the (input) base current I_B kept constant. Therefore

It is the slope of CE output characteristic I_C Vs V_CE. The typical value of this parameter is of the order of 0.1 to 10 μ mhos.

(ii) Input Impedance (h_ie)

It is defined as the ratio of the change in (input) base voltage to the change in (input) base current with the (output) collector voltage V_CE kept constant.

It is the slope of CE input characteristics I_B versus V_BE, the typical value of h_ie ranges from 500 to 2000 Ω.

(iii) Forward Current Gain (h_fe)

It is defined as a ratio of the change in the (output) collector current to the corresponding change in the (input) base current keeping the (output) collector voltage V_CE constant. Hence

It is the slope of I_C versus I_B curve. Its typical value varies from 20 to 200.

(iv) Reverse Voltage Gain (h_re)

It is defined as the ratio of the change in the (input) base voltage and the corresponding change in (output) collector voltage with constant (input) base current, I_B. Hence,

It is the slope of V_BE Versus V_CE curve. Its typical value is of the order of 10^-5 to 10^-4.

Common Collector Configuration

The circuit diagram for determining the static characteristics of an NPN transistor in the common collector configuration is shown in Fig. 1.43.

Input Characteristics

To determine the input characteristics, V_EC is kept at a suitable fixed value. The base-collector voltage V_BC is increased in equal steps and the corresponding increase in I_B is noted. This is repeated for different fixed values of V_EC. Plots of V_BC versus I_B for different values of V_EC shown in Fig. 1.44 are the input characteristics.

Output Characteristics

The output characteristics shown in Fig. 1.45 are same as those of the common emitter configuration.