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 VBEQ accurately. The choice of the
potentiometer to be added in series with R1
and R2 is based on the
criterion that
Rx + Ry + Rz = R1
+ R2
(1)
where Rx and Rz are fixed resistors, and Ry is a potentiometer. In our design problem, R1 + R2 = 80 kΩ.
Therefore, we want
Rx + Ry+ Rz = 80 kΩ
(2)
Since we are not sure
about the actual position of the Q-point,
we may choose Rx = Rz = 4.7 kΩ each, and the
potentiometer Ry = 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 VCE
and VRC. It can be seen
that when the Q-point is exactly in
the middle of the active region, VRC
= VCE. In our design, this
is exactly 4.5 V. It can now be seen that if VRC = VCE
= 4.5 V, then VRE will be
1 V, and IC 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 RC, RE, 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
|
|
|
|
|
0.0001
|
1000
|
1/8 W
|
10 mA
|
470 k
|
100 k
|
|
|
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
|
|
4.7
|
1
|
12
|
68
|
1000
|
0.001
|
40 W
|
From Table 1, when the current IC
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 IC gets
multiplied or divided by a certain constant a,
then the resistors get divided or multiplied by the same constant a, provided bmin, VOP, and VRE 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 IC increases above 1 mA. For
high currents, the power rating becomes very high.
·
When IC = 1 A , we find that the emitter
resistor RE = 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 RE
is usually avoided in power amplifiers. When IC = 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 IC,
collector-to-base leakage current ICO
may become prominent and comparable with IC. 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 IC = 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.