CB, CE and CC Transistor Configurations

CB, CE and CC Transistor Configurations

AU:Feb.-02,09,Dec.-02,03,04,05,06,07,08,09,10,13,17May 03,05,06,07,09,11,12,13,14,17

• The transistor can be connected in a circuit in the following three configurations.

1. Common base configuration.

2. Common emitter configuration.

3. Common collector configuration.

Key Point : Regardless of circuit configuration, the base emitter junction is always forward biased while the collector-base junction is always reverse biased, to operate transistor in active region.

 

1. Common Base Configuration

• To under stand complete electrical behaviour of a transistor it is necessary to study the interrelation of the various currents  and voltages. These relationships can be plotted graphically which are commonly known as the characteristics of transistor.

• The most important characteristics of transistor in any configuration are input and output characteristics.  

• The Fig. 2.4.1 shows the common base configuration. As shown in Fig. 2.4.1, in this configuration input is applied between emitter and base and output is taken from the collector and base.

• Here, base of the transistor is common to both input and output circuits and hence the name common base configuration.

• Common base configurations for both npn and pnp transistors are shown in Fig. 2.4.1 (a) and 2.4.1 (b), respectively.

a. Current Relations in CB Configuration

• In common base configuration, the collector current I_C is given by,

I_C = I_{C (INJ)} + I_CBO ... (2.4.1)

I_{C (INJ)} : It is an injected collector current due to number of electrons crossing the collector base junction.

I_CBO : It is the reverse saturation current flowing due to the minority carrier between collector and base when the emitter is open. ICBO is negligible as compared to IC(INJ)   therefore we have

I_C ≈ I_C(INJ)

However, when emitter is open

I_C = I_CBO

Key Point : The reverse saturation current, ICBO, is temperature sensitive and it doubles for every 10 °C rise in temperature.

Current Application Factor (ɑ_dc)

ɑ_dc : It is defined as the ratio of the collector current resulting from carrier injection to the total emitter current.

ɑ_dc = ɑ ≈ I_C(INJ) / IE …(2.4.2)

• Since I_C < I_E the value of adc is always less than unity. It ranges from 0.95 to 0.995 depending on the thickness of the base region; larger the thickness of the base, smaller is the value of ɑ_dc. It represents the current gain in the CB configuration.

• Substituting the value of I_C(INJ)  in equation (2.4.2) we have

I_C ≈ ɑ_dc I_E

Current amplification factor in CB ɑ_dc = I_C / I_E

 

Ex. 2.4.1 Given IE = 2.5 mA, ɑ = 0.98 and I_CBO = 10 µA, calculate I_B and I_C.

Sol. :

b. Input Characteristics (Base Curves)

• It is the curve between an input voltage VEB (emitter-base voltage) and input current IE (emitter current) at constant collector-base voltage VCB- The emitter current is taken along Y-axis and emitter base voltage along X-axis.

• Fig. 2.4.3 shows the input characteristics of a typical transistor in common-base configuration.

From this characteristics we can observe the following important points :

1. The input resistance is a ratio of change in emitter-base voltage (ΔV_EB) to the resulting change in emitter current (Δ I_E) at constant collector-base voltage (V_CB). 

It is given by 

2. After the cut-in voltage (barrier potential, normally 0.7 V for silicon and 0.3 V for germanium), the emitter current (IE) increases rapidly with small increase in emitter-base voltage (VEB). Thus, the input resistance is very small.

3. It can be observed that there is slight increase in emitter current (lE) with increase in VCB. This is due to change in the width of the depletion region in the base region under the reverse biased condition.

4. VCB is positive for npn transistor and it is negative for pnp transistor. On the other hand VEB is negative for npn transistor and it is positive for pnp transistor.

c. Output Characteristics (Collector Curves)

• It is the curve between collector current IC and collector base voltage VCB at constant emitter current IE. The collector current is taken along Y-axis and collector-base voltage magnitude along X-axis. Fig. 2.4.4 shows the output characteristics of a typical transistor in common base configuration.

From this characteristics we observe following points :

1. The output characteristics has three basic regions : Active, cut-off and saturation.

2. Active region :

• For the operation in the active region, the emitter-base junction (JE) is forward biased while collector base junction (Jc) is reverse biased.

• In this region, collector current IC is approximately equal to the emitter current (IE) and transistor works as an amplifier.

• In the active region, the collector current is essentially almost constant.

• The Dynamic output resistance is the ratio of change in collector base voltage (AVCB) to the resulting change in collector current (AIC) at constant emitter current (IE). It is given by

• The collector current IC is almost independent on collector-base voltage VCB and the transistor can be said to work as constant-current source. This provides very high dynamic output resistance.

3. Saturation region : In this region, the emitter-base junction (JE) and collector base junction (Jc) both are forward biased (VCB is negative). Here, the Ic is independent of IE. IC decreases rapidly as VCB becomes more negative.

4. Cut-off region : The region below the curve IE = 0 is known as cut-off region, where the collector current is nearly zero and the collector-base (Jc) and emitter-base (JE) junctions of a transistor are reverse biased.

5. DC current gain :

ɑ_dc = ɑ = I_C/I_E

where I_C and I_E are the values of collector current and emitter current at any point on the curve.

6. AC current gain :

ɑ_dc = ΔI_C / ΔI_E | V_{CB = constant}

d. Early Effect

• When reverse bias voltage VCB increases, the width of depletion region also increases, which reduces the electrical base width. This effect is called as Earty effect' or Base width modulation'.

• This decrease in base width has two consequences.

1. There is less chance for recombination within the base region. Hence the transport factor β*, and also a, increase with an increase in the magnitude of the collector junction voltage.

2. The charge gradient is increased within the base, and consequently, the current of minority carriers injected across the junction increases. This increases emitter current slightly. Refer Fig. 2.4.5.

 

2. Common Emitter Configuration

• In this configuration input is applied between base and emitter, and output is taken from collector and emitter.

• Here, emitter of the transistor is common to both, input and output circuits, and hence the name common emitter configuration.

• Common emitter configurations for both npn and pnp transistors are shown in Fig. 2.4.6 (a) and 2.4.6 (b), respectively.

• The input voltage in the CE configuration is the base-emitter voltage (VBE) and the output voltage is the collector-emitter voltage (VCE). The input current is IB and the output current is Ic.

a. Current Relations in CE Configuration

In CB configuration we have seen that

• The β_dc is the ratio of output current I_C and input current I_B in common emitter configuration. It is common emitter amplification factor or current gain.

• It is given by,

β_dc = I_C / I_B

We know that, β = I_C / I_B

We have, I_E = I_C + I_B

i.e., I_B  = I_E – I_C

β = I_C / I_E - I_C

I_B = I_E – I_C

• Dividing the numerator and denominator of R.H.S. of above equation by IE, we get,

• Dividing the numerator and denominator of R.H.S. of above equation by IB, we get,

 

Ex. 2.4.2 Calculate the values of I_C and I_E for a BJT with ɑ_dc = 0.97 and I_B = 50 µA. Determine β_dc for the device.

Sol.:

• The terms (1 + Pdc) ICBO in equation (2.4.5) is denoted as ICEO and Is the reverse saturation current for the CE configuration.

 

Ex. 2.4.3 For a transistor in common emitter configuration, the reverse leakage current is 21 [iA, whereas when the same transistor is connected in common base configuration, it reduces to 1.1 µA. Calculate values of ɑ_dc and β_dc of the transistor.

Sol.:

 

Ex. 2.4.4 calculate the ɑ_dc and β_dc for the given transistor for which IC =  mA, IB = 0 µA and ICD = 1µA

Sol.:

b. Input Characteristics (Base Curves)

• It is the curve between and input voltage VBE (base-emitter voltage) and input current IB (base current) at constant collector-emitter voltage, VCE- The base current is taken along Y-axis and base emitter voltage V_BE is taken along X-axis.

• Fig. 2.4.8 shows the input characteristics of a typical transistor in common-emitter configuration.

From this characteristics we observe the following important points :

1. The input resistance is the ratio of change in base-emitter voltage (ΔVBE ) to the resulting change in base current (ΔIB) at constant collector emitter voltage VCE. It is given by,

2. The value of r in CE configuration is greater than the value of r in CB configuration.

3. As the input to a transistor in the CE configuration is between the base-to-emitter junction, the CE input characteristics resembles a family of forward biased diode curves.

4. After the cut-in voltage, the base current (IB) increases rapidly with small increase in base-emitter voltage (VBE ). Thus the dynamic input resistance is small in CE configuration.

5. For a fixed value of VBE, IB decreases as VCE is increased.

6. Voltages VBE and VCE are positive for npn transistor and they are negative for pnp transistor.

c. Output characteristics (Collector Curves)

From this characteristics we observe the following important points :

1. This characteristics shows the relation between the collector current IC and collector voltage VCEz for various fixed values of IB. This characteristics is often called collector characteristics. A typical family of output characteristics for an n-p-n transistor in CE configuration is shown in Fig. 2.4.9.

2. The value of β_dc of the transistor can be found at any point on the characteristics by taking the ratio I_C to I_B at that point, ie- β_dc = IC /IB. This is known as D.C. beta for the transistor.

3. From the output characteristics, we can see that change in collector-emitter voltage (ΔV_CE) causes the little change in the collector current (ΔI_C) for constant base current IB. Thus the output dynamic resistance is high in CE configuration.

4. The output characteristics of common emitter configuration consists of three regions : Active, Saturation and Cut-off.

5. Active region :

• For the operation in the active region, the emitter-base junction (J_E) is forward biased while collector base junction (J_C) is reverse biased.

• The collector current rise more sharply with increasing VCE in the linear region of output characteristics of CE transistor.

6. Saturation region :

• In this region, the emitter-base junction (JE) and collector base junction (Jc) both are forward biased. In this region, IC does not depend upon the input current IB.

• The saturation value of VCE, designated VCE (sat), usually ranges between 0.1 V to 0.3 V.

7. Cut-off region : The region below IB = 0 is the cut-off region of operation for the transistor. In this region, both the junctions of the transistor are reverse biased.

 

3. Common Collector Configuration

• In this configuration, input is applied between base and collector and output is taken from emitter and collector.

• Here, collector of the transistor is common to both input and output circuits and hence the name common collector configuration. It is also known as emitter follower configuration. 

• Common Collector connection for both npn and pnp transistors are shown in Fig. 2.4.10(a) and 2.4.10(b), respectively.

a. Current Relations in CC Configuration

b. Common Collector Input Characteristics

The input characteristics of CC configuration is a graph of input current Ig (base current) versus input voltage VCB (collector base voltage) at constant VCE

• The base current is taken along Y-axis and collector base voltage VCB is taken along X-axis.

Fig. 2.4.11 shows the input characteristics of a typical transistor in common-collector configuration.

• The common collector input characteristics are quite different from either common base or common emitter input characteristics. This difference is due to the fact that the input voltage VCB is largely determined by the level of collector to emitter voltage VCE.

• Looking at Fig. 2.4.11 we can write,

• In CC configuration input junction is BC and it is reversed biased so input resistance in CC configuration is very high.

c. Common Collector Output Characteristics

• It is the curve between emitter current IE and collector to emitter voltage VCE at constant base current IB.

• The emitter current is taken along Y-axis and collector to emitter voltage along X-axis.

• Fig. 2.4.12 shows the output characteristics of a typical transistor in common collector configuration.

• Since, IC is approximately equal to IE, the common collector output characteristics are practically similar to those of the common emitter output characteristics.

 

Ex. 2.4.5 A transistor has a = 0.9. If IE = 10 mA, find the values of β, ɤ, IB and Ic.

Sol .:

 

4. Comparison of CB, CE and CC Configuration

CE configuration is most widely used in amplifier circuits

• The common-emitter configuration is widely used amongst three transistor configurations because :

1. Provides both voltage gain and current gain : The CE configuration is the only configuration which provides both voltage gain as well as current gain greater than unity. In case of CB configuration, current gain is less than unity and in case of CC configuration, voltage gain is less than unity.

2. Provides high power gain : The power gain is a product of voltage gain and current gain. CE configuration provides voltage gain nearly equal to voltage gain provided by CB configuration (voltage gain is maximum in CB) and current gain nearly equal to current gain provided CC configuration (current gain is maximum in CC). Thus the power gain of the CE amplifier is much greater than the power gain provided by the other two configurations (voltage gain in CC and current gain in CB are less than unity).

3. Can be cascaded efficiently : In a common emitter circuit, the ratio of output resistance to input resistance is small, may range from 10 Ω to 100 Ω. This makes configuration an ideal for coupling between various transistor stages. However, in other connections, the ratio of output resistance to input resistance is very large and hence coupling becomes highly inefficient due to large mismatch of resistance.

Note : Maximum power is transferred from stage 1 to stage 2, when output resistance of stage 1 is equal to the input resistance of stage 2.

 

5. Conditions for Regions

• In the previous section we have seen that to bias a transistor in a particular operating region we have to bias the transistor junctions as shown in Table 2.8.1. Apart from this we can observe certain conditions to find the region of operation of a transistor. These conditions are :

Review Questions

1. Draw the neat circuit configuration of CB.

AU : Dec.-lO, May-11, 13, Marks 2

2. Sketch and explain the input characteristics of transistor in CB mode.

AU : May-05, 06, 07, 09, 14, Dec.-08, Feb.- 09

3. With a neat diagram explain the output characteristics of transistor in CB mode.

AU ; Feb.-02, 09, Dec.-08, May-05, 06, 07, 09, 14, Marks 8

4. What is early effect ?

5. Draw the neat circuit configuration of CE.

AU : Dec.-lO, 13, May-11, 13, Marks 2

6. Sketch and explain the input characteristics of transistor in CE mode.

AU : Dec.-03, 04, 07, 08, 10, 13, May-11, 13, Marks 5

7. With a neat diagram explain output characteristics of npn transistor in CE-configuration.

AU : Dec.-03, 04, 07, 08, 10, May-11, 13, Marks 5

8. Explain the operation of NPN transistor in CE configuration with it's input and output characteristics. Also define active, saturation and cut-off regions.

9. Draw the neat circuit configuration of CC.

AU : Dec.-lO, May-11, Marks 2

10. Draw and explain the input characteristics of common collector configuration.

11. Draw and explain the output characteristics of common collector configuration.

12. Define current gain of CC configuration.

13. Compare CB, CE and CC transistor configurations.

AU : Dec.-02,05,13,May-03,06,12, Marks 8

14. Write the relation between Ic, ft IB and ICBO in a BJT

AU : May-05, Marks 2

15. Explain the three regions of output characteristics of a transistor in CE configuration.

AU : Dec.-06, Marks 8

16. Explain the input I output characteristics of BJT in common base configuration.

AU : May-14, Marks 11

17. Compare CE, CB and CC configurations of BJT.

AU : May-14, Marks 5

18. Compare the performances of CE and CC configuration.

19. Compare the characteristics of CB, CE and CC amplifiers.