VARIATIONS IN THE DESIGN OF RC-COUPLED CE AMPLIFIER

EDITOR: B. SOMANATHAN NAIR

NOTE: This article is an extension of the previous blog “RC­-COUPLED  COMMON-EMITTER AMPLIFIER–A SYSTEMATIC APPROACH TO DESIGN”.

1. TO FIX THE Q-POINT EXACTLY IN THE MIDDLE OF THE ACTIVE REGION

The design explained in the previous blog on RC-coupled amplifier design fixes the Q-point only approximately. Because of the large variations existing in the characteristics of transistors, data manuals cannot give us the complete range of the active region accurately. For determining this, at first, we have to plot the characteristics of the transistor experimentally. This is a tedious and time-consuming job. So, normally, this is not resorted to. Instead, we use a potentiometer in series with the R₁/R₂ biasing network, as shown in Fig. 1 for adjusting V_BEQ accurately. The choice of the potentiometer to be added in series with R₁ and R₂ is based on the criterion that

                                                            R_x + R_y + R_z = R₁ + R₂             (1)

where R_x and R_z are fixed resistors, and R_y is a potentiometer. In our design problem, R₁ + R₂ = 80 kΩ.  Therefore, we want

                                                            R_x + R_y+ R_z = 80 kΩ               (2)

Since we are not sure about the actual position of the Q-point, we may choose R_x = R_z = 4.7 kΩ each, and the potentiometer R_y = 68 kΩ. Note that a potentiometer of large value is chosen purposefully. This is to accommodate for the wide range of variation required in fixing the Q-point in the middle of the active region. The fixed resistors R₁ and R₂ are used to avoid the shorting that may take place due to the wiper arm of the potentiometer accidentally touching its lower or upper terminal if these resistors are not used.

Now, by carefully adjusting the pot, we look for equal voltages across V_CE and V_RC. It can be seen that when the Q-point is exactly in the middle of the active region, V_RC = V_CE. In our design, this is exactly 4.5 V. It can now be seen that if V_RC = V_CE = 4.5 V, then V_RE will be 1 V, and I_C will be 1 mA exactly, proving the validity of our design. In all practical electronic circuits, precise adjustments of this kind using pots are required to fix operating points. These finer adjustments are protected against tampering by applying wax or lacquer that prevents the wiper arm of the potentiometer from physically moving.

2.  DESIGN FOR DIFFERENT CURRENT SWINGS

The Standard Amplifier design given above was based on a current swing of 1 mA and a voltage swing of 4.5 V. In practice, these requirements may be suitably modified to get the design for any current and voltage swings. Table 1 gives the values of R_C, R_E, R₁ and R₂ for various current swings and the same voltage swing of 4.5 V. The values in respect of the Standard Amplifier are shown using bold letters and numbers.

                                                                      Table 1

IC	RC Ω	RE Ω	R1 Ω	R2 Ω	Current-multiplying factor	Resistance-multiplying factor	Wattage rating
1 mA	4.7 M	1 M	12 M	68 M	0.0001	1000	1/8 W
10 mA	470 k	100 k	1.2 M	6.8 M	0.01	100	1/8 W
100 mA	47 k	10 k	120 k	680 k	0.1	100	1/8 W
1 mA	4.7 k	1 k	12 k	68 k	1	1	1/8 W
10 mA	470	100	1.2 k	6.8 k	10	0.1	½ W
100 mA	47	10	120	680	100	0.01	10 W
1 A	4.7	1	12	68	1000	0.001	40 W

From Table 1, when the current I_C is multiplied by a factor of 10, a division using the same factor gives the corresponding resistor. But, if the current is divided by a factor of 10, then the corresponding resistor will be obtained through a multiplication by 10.

In fact, if I_C gets multiplied or divided by a certain constant a, then the resistors get divided or multiplied by the same constant a, provided b_min, V_OP, and V_RE remain constant. We now make the following observations from Table 1.

·         Wattage rating of resistors for values of collector current less than 1 mA is very low. Usually, in all such cases, we use (1/8)-W resistors, as they are freely available in the market. However, the picture changes quickly as I_C increases above 1 mA. For high currents, the power rating becomes very high.

·         When I_C = 1 A, we find that the emitter resistor R_E = 1 Ω only. However its power rating is about 4 W. Usually it is very difficult to get low-value resistors, such as 1 Ω. Also, it can be seen that a lot of power is wasted in this resistor. Therefore R_E is usually avoided in power amplifiers. When I_C = 1 A, we have R₁ = 12 ohms. This is a very low value, and hence, will load the input heavily. Therefore, R₁ is usually replaced with diode biasing across the base and emitter.

·         Amplifiers with as low a collector current as 1 mA have been designed and tested by the author. There is a little amount of practical difficulty in constructing and testing an amplifier with collector current less than 1 mA. One of the major problems encountered in this case is the non-availability of resistors of values greater than 10 MΩ in the market. Another problem is regarding the stability of the amplifier at very low collector currents. At very low values of I_C, collector-to-base leakage current I_CO may become prominent and comparable with I_C.  In such cases, special care should be taken to ensure the stability of the amplifier.

Caution: Usually, for power transistors, b is very small (typically, less than 20). Therefore, the values of resistors given in Table 5.1 for I_C = 1 A are only of theoretical interest. Practically, since the power requirement is very high, it is quite common to use the push-pull configuration for power amplifiers. In such situations, we normally avoid using high-wattage resistors as they unnecessarily waste power.