CB, CE and CC Transistor Configurations

CB, CE and CC Transistor Configurations

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 understand 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.

• Fig. 4.4.1 shows the common base configuration. As shown in Fig. 4.4.1, 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. 4.4.1 (a) and 4.4.1 (b), respectively.

i. Current Relations in CB Configuration

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

I_C = I_C(INJ) + I_CBO

           ... (4.4.1)

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. I_CBO is negligible as compared to I_C(INJ) and therefore we have

 I_C = I_C(INT)

However, when emitter is open

 I_C = I_CBO

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

Current Application Factor (α_dc)

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

 α_dc = α = I_C(INJ) / I_E

              ……………..(4.4.2)

Since I_C < I_E the value of α_dc 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 α_dc. It represents the current gain in the CB configuration.

Substituting the value of I_C(INJ) in equation (4.4.2)

we have

I_C = α_dcI_E +I_CBO

               ………..(4.4.3)

However, I_CBO is negligibly small

I_C = α_dcI_E

Current amplification factor in CB

α_dc = I_C / I_E

I_B in terms of α_dc

We know that

I_E = I_B + I_C

 I_B = I_E‒I_C

 = I_E ‒ (α_dcI_E + I_CBO)

= I_E ‒ α_dcI_E

 I_CBO → 0

I_B = (1 ‒ α_dc) I_E

Ex. 4.4.1: Given I_E = 2.5 mA, α = 0.98 and I_CBO = 10 μA calculate I_B and Ic.

Solution:

I_C = αI_E + I_CBO

= 0.98 × 2.5 × 10^‒3 + 10×10^‒6 = 2.46 mA

I_B  = I_E‒I_C = 2.5×10³ − 2.46×10⁻³ = 40 μA

 

ii. Input Characteristics (Base Curves)

• It is the curve between an input voltage V_EB (emitter‒base voltage) and input current I_E (emitter current) at constant collector‒base voltage V_CB. The emitter current is taken along Y‒axis and emitter base voltage along X‒axis.

• Fig. 4.4.3 shows the input characteristics of a typical transistor in common‒base configuration.

Note: While plotting input characteristics the magnitudes of voltage and current are considered. Practically the voltage and current polarities are opposite for pnp and npn transistors

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 (ΔI_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 (I_B) increases rapidly with small increase in emitter‒base voltage (V_EB). Thus, the input resistance is very small.

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

4. V_CB is positive for npn transistor and it is negative for pnp transistor. On the other hand V_EB is negative for npn transistor and it is positive for pnp transistor.

Note: While plotting output characteristics the magnitudes of voltage and current are considered. Practically the voltage and current polarities are opposite for pnp and npn transistors

 

iii. Output Characteristics (Collector Curves)

• It is the curve between collector current I_C and collector base voltage V_CB at constant emitter current I_E. The collector current is taken along Y‒axis and collector‒base voltage magnitude along X‒axis. Fig. 4.4.4 shows the output characteristics of a typical transistor in common base configuration.

From this characteristics we observe following points :

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

• 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.

• In this region, collector current I_C is approximately equal to the emitter current (I_E) 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 (ΔV_CB) to the resulting change in collector current (ΔI_C) at constant emitter current (I_E). It is given by

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

• Saturation region : In this region, the emitter‒base junction (J_E) and collector base junction (J_C) both are forward biased (V_CB is negative). Here, the I_C is independent of I_E. I_C decreases rapidly as V_CB becomes more negative.

• Cut‒off region: The region below the curve I_E = 0 is known as cut‒off region, where the collector current is nearly zero and the collector‒base (J_C) and emitter‒base (J_E) junctions of a transistor are reverse biased.

• DC current gain :

 α_dc = α = 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.

• AC current gain :

 

iv. Early Effect

• As shown in Fig. 4.4.5, the width of the base region occupied by charge particles is known as electrical width or physical width of the base region.

• Since doping in the base is ordinarily substantially smaller than that of the collector, the penetration of the transition region into the base is much larger than into the collector. Hence the base depletion region is large.

• Here, when reverse bias voltage V_CB increases, the width of depletion region in base region also increases, which reduces the electrical base width (W'_B).

• Due to reduction of the electrical base width, now there are more charge particles per unit area. In other words, due to reduction of the electrical base of width, concentration the charge gradient increases in the base region. This increase in concentration of charge carriers causes more diffusion of electrons from n‒type emitter to p‒type base increasing emitter current slightly.

• The increase in reverse bias voltage V_CB, the width of depletion region increases, which reduces the electrical base width. This effect is called as 'Early Effect' or 'Base width modulation'. This effect can be explained with the help of equation

 V = [ e. N_A . W² ] / 2ε

where

V = Reverse bias voltage.

and

W = Width of the space‒charge region/depletion region.

• As we know, the depletion width is more on the lightly doped region, i.e. base region, this effect is more in the base region reducing the effective (electrical) base width.

• 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 α_r 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.

 

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. 4.4.7 (a) and 4.4.7 (b), respectively.

• The input voltage in the CE configuration is the base‒emitter voltage (V_BE) and the output voltage is the collector‒emitter voltage (V_CE). The input current is I_B and the output current is I_C.

 

i. Current Relations in CE Configuration

• In configuration we have seen that

I_C = α_dcI_E + I_CBO

I_C‒I_CBO = α_dcI_E

               ... (4.4.4)

• 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]

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

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

α = β / [1 + β]

 

Ex. 4.4.2: Calculate the collector and emitter current levels for a BJT with α_dc = 0.99 and I_B = 20 μA

Solution: :

B_dc = α_dc / (1 ‒ α_dc)

= 0.99 / (1‒0.99)

= 99

I_C = β_dc I_B = 99×20 μA = 1.98 mA

I_E = I_C+I_B = 1.98 mA + 20 μA = 2 mА

 

Ex. 4.4.3: Calculate the values of I_C and I_E for a BJT with α_dc = 0.97 and I_B = 50 μA. Determine β_dc for the device.

Solution: :

B_dc = α_dc / (1 ‒ α_dc)

= 0.97 / (1‒0.97)

= 32.33

I_C = β_dc I_B = 32.33 × 50 μA = 1.6165 mA

I_E = I_B + I_C = 50 μA + 1.6165 mA

= 1.6665 mA

I_CEO

From equation (4.4.4) we have,

I_C = [α_dc / 1‒α_dc ]I_B + (I_CBO / 1‒α_dc )

I_C = β_dcI_B + (1+β_dc)I_CBO

            ………….(4.4.5)

Since

1+ β_dc = 1 + (α_dc / 1‒α_dc)

 = [ 1 ‒α_dc + α_dc ] / [ 1‒α_dc ]

= 1 / ( 1‒α_dc )

The terms (1 + β_dc) I_CBO in equation (4.4.5) is denoted as I_CEO and is the reverse saturation current for the CE configuration.

     (1+β_dc)I_CBO = I_CEO

 I_C = β_dcI_B + I_CEO

 

Ex. 4.4.4: The reverse leakage current of the transistor when connected in CB configuration is 0.2 μA and it is 18 μA when same transistor is connected in CE configuration. Calculate α_dc and β_dc of the transistor. (Assume I_B = 30 mA)

Solution:

Given: I_CBO = 0.2 μA, I_CEO = 18 μA

 I_CEO = (1 + β_dc)I_CBO

 (1 + β_dc) = I_CEO / I_CEO = 18 μA / 0.2 μA = 90

β_dc = 89

α_dc = β_dc / (1 + β_dc)

=  89 / (1+89) = 0.989

 

Ex. 4.4.5: Calculate the α_dc and β_ac for the given transistor for which I_C=5 mA, I_B =50 μA and I_CO = 1 μΑ.

Solution:

 I_C=5mA, I_B = 50 μA, I_CO =I_CBO = 1 μA

I_C = β_dcI_B +(1+β_dc)I_CBO

 5×10^‒3 = β_dc × 50×10⁻⁶ + (1+β_dc)×1×10^‒6

5×10^‒3 ‒ 1×10^‒6 = 51×10^‒6 β_dc

 β_dc = 4.999×10^‒3 / 51×10^‒6 = 98

 α_dc = β_dc / (1+ β_dc) = 98 / (1+98) = 0.9899

 

Ex. 4.4.6: A transistor has β = 150, find the collector and base current, if I_E = 10 mA.

Solution:

 I_B = I_E / (1+β) =  10mA / (1+150) = 66.225 μA

 I_C = βI_B = 150×66225 μA = 9.934 mA

 

Ex. 4.4.7: A transistor with I_B = 100 μA and I_C = 2 mA find

1) β of the transistor

2) α of the transistor

3) Emitter current I_E

4) If I_B changes by 25μA and I_C changes by 0.6 mA. Find the new value of β.

Solution: :

1) β = I_C/I_B

= 2×10^‒3 / 100×10^‒6

= 20

2) α = β / (1+β)

 = 20 / (1+20) = 0.9524

3) I_E = I_B + I_C = 100×10^‒6 + 2×10^‒3 = 2.1 mA

4) New value of I_B = 100 + 25 = 125 mA

New value of I_C = 2 + 0.6 = 2.6 mA

New value of β = 2.6×10^‒3 / 125×10^‒6

= 20.8

 

ii. Input Characteristics (Base Curves)

• It is the curve between and input voltage V_BE (base‒emitter voltage) and input current I_B (base current) at constant collector‒emitter voltage, V_CE: The base current is taken along Y‒axis and base emitter voltage V_BE is taken along X‒axis.

• Fig. 4.4.9 shows the input characteristics of a typical transistor in common‒emitter configuration.

From characteristics we observe the following important points:

1. The input resistance is the ratio of change in base‒emitter voltage (ΔV_BE) to the resulting change in base current (ΔI_B) at constant collector emitter voltage V_CE. It is given by,

Note: While plotting input characteristics the magnitudes of voltage and current are considered. Practically the voltage and current polarities are opposite for pnp and npn transistors

2. The value of r₁ in CE configuration is greater than the value of r₁ in CB configuration.

3. As the input to 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 (I_B) increases rapidly rapidly with small increase base‒emitter voltage (V_BE). Thus the dynamic input resistance is small in CE configuration.

5. For a fixed value of V_BE, I_B decreases as V_CE is increased.

6. Voltages V_BE and V_CE are positive for npn transistor and they are negative for pnp transistor.

 

iii. Output Characteristics (Collector Curves)

From this characteristics we observe the following important points:

1. This characteristics shows the relation between the collector current I_C and collector voltage V_CE, for various fixed values of I_B. 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. 4.4.10.

Note: While plotting output characteristics the magnitudes of voltage and current are considered. Practically the voltage and current polarities are opposite for pnp and npn transistors

2. The value of B_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, i.e. β_dc = I_C/I_B. 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 I_B. 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.

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 V_CE in the linear region of output characteristics of CE transistor.

Saturation region:

In this region, the emitter‒base junction (J_E) and collector base junction (J_C) both are forward biased. In this region, I_C does not depend upon the input current I_B.

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

Cut‒off region:

The region below I_B = 0 is the cut‒off region of operation for the transistor. In this region, both the junctions of the transistor are reverse biased.

For saturation: I_B > I_C/β_dc

For active region: V_CE > V_{CE (sat)}

DC current gain: β_dc = β = I_C/I_B

AC current gain:  

 

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 connections for both npn and pnp transistors are shown in Fig. 4.4.11 (a) and 4.4.11 (b), respectively.

1. Current Relations In CC Configuration

We know that

I_E = I_B + I_C

= I_B + α_dc1_E + I_CBO

I_E(1‒α_dc) = I_B +I_CBO

 I_E = I_B/(1‒α_dc) + I_CBO/(1‒α_dc)

We know that

 β_dc = α_dc  / (1‒α_dc)

1+ β_dc = 1+ [ α_dc  / (1‒α_dc) ]

= [ 1‒α_dc + α_dc ] / (1‒α_dc)

 = 1 / (1‒α_dc)

I_E = I_B(1+β_dc) + I_CBO(1+β_dc)

Neglecting I_CBO we have

 I_E = I_B (1+ β_dc)

Current gain in CC configuration is given by

 γ = I_E/I_B = (1+ β_dc)

 = 1 +  α_dc/(1‒α_dc)

= 1 / (1‒α_dc)

ii. Common Collector Input Characteristics

• The input characteristics of CC configuration is a graph of input current I_B (base current) versus input voltage V_CB (collector base voltage) at constant V_CE.

• The base current is taken along Y‒axis and collector base voltage V_CB is taken along X‒axis.

• Fig. 4.4.12 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 V_CB is largely determined by the level of collector to emitter voltage V_CE.

Fig. 4.4.12 Input characteristics of transistor in CC configuration

• Looking at Fig. 4.4.12 we can write,

V_CE = V_CB ‒ V_BE

Or

V_CB = V_CE + V_BE

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

 

iii. Common Collector Output Characteristics

• It is the curve between emitter current I_E and collector to emitter voltage V_CE at constant base current I_B.

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

• Fig. 4.4.13 shows the output characteristics of a typical transistor in common collector configuration.

Fig. 4.4.13 Output characteristics of the transistor in CC configuration

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

 

Comparison of CB, CE and CC Configurations

 

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.

Review Questions

1. Draw the neat circuit configuration of CB.

2. Define I_C(INJ) and I_CBO.

3. Define current amplification factor or current gain (α_dc).

4. Derive the expression for I_β.

5. Sketch and explain the input characteristics of transistor in CB configuration.

6. What is early effect? How can it account for the CB input characteristics?

7. Write a short note on early effect.

8. With relevant expressions and figures, describe early effect.

9. With a neat diagram explain the output characteristics of transistor in CB configuration.

10. Explain the transfer characteristics of CB configuration.

11. Draw the neat circuit configuration of CE.

12. Define βac.

13. Derive the relationship between α_dc and β_ac.

14. Define reverse leakage current (I_CEO) in CE configuration.

15. Sketch and explain the input characteristics of transistor in CE configuration.

16. With a neat diagram explain output characteristics of npn transistor in CE‒configuration.

17. Draw and explain the characteristics of PNP transistor in CB configuration.

18. Explain the transfer characteristics of CE configuration.

19. Draw the neat circuit configuration of CC.

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

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

22. Define current gain of CC configuration.

23. Explain the transfer characteristics of CC configuration.

24. Compare CB, CE and CC transistor configurations.

25. Why CE configuration is widely used in amplifier circuits?

26. Explain the characteristics of BJT in CC, CE, CB configurations compare the performance of a transistor in different configurations.

27. What is known as current amplification factor? Derive the relationship between the amplification factor of CE, CB and CC configuration.

28. Construct and demonstrate the working mechanism of CE configuration of BJT.

29. Construct and demonstrate the working mechanism of CB configuration of BJT.