This tutorial actually intended to explain the practical importance of well-known transistor theories, characteristic graphs and use them to design a high gain amplifier circuit using a single transistor and a minimum number of other components.
We will look at certain transistor characteristics and theories, summarize them, and by the end of this tutorial, we will be designing a high-gain amplifier on our own.
Common Emitter Configuration:
Let's consider a common emitter, fixed bias class A amplifier circuit
Figure 1.
The hybrid equivalent of the same circuit is given below
Figure 2.
Here hi is the impedance encountered by any device connected at the input, hoe is the admittance at the output.
( hfe * ib ) forms a current-dependent current source where hfe is the direct current transfer rate and ib is the base current. (hre * Vce) forms a voltage dependent voltage source, the value is too low for us to ignore it. hre is the reverse voltage ratio.
Therefore, we can reduce the equivalent circuit as follows:
Figure 3.
From figures 2 and 3, we can see that the BJT is a current-driven device.
Or, information 1.
The change in input current causes a corresponding change in the output current
We can also observe in the figure above that the output current (collector current) is directly proportional to the input current (base current).
Or we can explain the relationship using the equation
Ic = hfe * ib ……………………………………………………………….….. equation 1.
Transistor characteristics
Transistor Features :
Now, if we plot the output current (ic) Vs output voltage (Vce), for the different values of the input current (ib)
life = 100
Figure 4.
life = 200
Figure 5
Saturation region:
The saturation region is the region in which the output current increases nonlinearly with the output voltage for a specific input current. .
Cutting region:
The cutoff region shows the output current for zero input current.
Active region:
The active region is the region in which the output current varies almost linearly with the output voltage for all values of the input current.
From the above figures (4 and 5), it is clear that as the life increases twice from figure 4 to figure 5, the same set of input current produces twice the output current as the previous one.
We can see the effect more clearly in the following figure.
So we have the following information,
For a large variation in output current corresponding to the typically small variation in input current, a transistor with a high lifetime is required.
information 2
As life increases, the change in output current corresponding to the change in input current also increases.
So far we have seen graphs with changes in output current corresponding to changes in input current. Now let's see how changing the input current affects the output voltage.
For the above purpose, we use the Load line.
The load line is nothing more than a straight line connecting the maximum possible output current on the Y-axis to the maximum possible output voltage on the X-axis.
On the load line we can find the output voltage corresponding to a given input current.
From the figure above we have the following information:
information 3.
The maximum value of the output current is VCC/Rc and the minimum value is almost 0
information 4.
The maximum value of the output voltage is VCC and the minimum value is almost 0
information 5.
As the input current increases, the output voltage decreases
Now let's apply the load line concept to the previous transistor graphs in figure 6.
Figure 8.
From the figure above, we can see that
information 6 .
As life increases, the variation in output voltage corresponding to variations in input current increases.
Now, what happens if we increase the slope of the load line?
Figure 9.
It can be seen that the same variation in input current produces even more changes in output voltage than the previous graphs, as we increase the slope of the load line.
The slope of the load line can be increased by increasing the value of Rc.
So, we found another important piece of information.
information 7.
Rc increases, the variation in output voltage corresponding to variations in input current increases.
Although the output swing increases with increasing slope, the input range has decreased significantly.
Therefore, we must keep in mind that,
If we increase the slope of the load line as much as possible, we must keep the initial input current (input current when there is no input signal applied) as small as possible to keep the device in the linear region during the full input swing, to get the corresponding distortion less output.
The input current can be reduced by increasing Rb. From the characteristics of the transistor it is clear that the small value of the input current allows a wide range of output voltage, that is, from almost zero to close to VCC.
So we can say that,
information 8 .
Rb must be kept extremely high so as to increase the linear range of oscillation of the output voltage corresponding to the normally small input signal current
Figure 10.
If we are using a device at the input that can increase or decrease the input current from an average value, for example a sinusoidal input. In these cases, to obtain a full output swing corresponding to the input swing, we must maintain the initial output current and initial output voltage at a middle position called the Q point, or quiescent point.
The Q point is the operating point at which we maintain the amplifier, while no input is applied, to obtain a full, undistorted output oscillation corresponding to an input oscillation, whenever an input signal within the expected range is applied. .
In figure 10, Vce max is almost equal to VCC and Vce min almost equal to 0. Thus, we can assume that the output can oscillate between 0 and VCC and therefore it is appropriate to keep the output voltage initially at half VCC. Similarly, ib min is almost equal to 0. We can assume that the input current oscillates between ib min and ib max and therefore it is appropriate to keep the input current initially at ib max/2.
Although there is no entry present,
VceQ must be VCC/2, ibQ must be ib max/2.
Or,
information 9.
The value of Rc must be set so that Vceq = VCC/2 to obtain maximum output voltage swing
information 10 .
The value of Rb must be set so that ibQ = ibmax/2 to allow maximum input oscillation
Summary:
For a high gain amplifier,
1) Choose a transistor with as long a life as possible to obtain large variations in output current corresponding to variations in input current.
2) Rb must be kept extremely high so as to increase the linear range of oscillation of the output voltage corresponding to the normally small input signal current, also the value of Rb must be selected so as to keep the point Q exactly in the middle of the expected entry current balance.
3) Rc must be highly valued to obtain large variations in the output voltage corresponding to variations in the output current, caused by variations in the input current, which in turn are caused by variations in the applied input signal voltage.
Designing a High Voltage Gain Amplifier
Designing a high voltage gain amplifier:
So, let's start designing the circuit shown in figure 1.
We chose the BC109 transistor, as it has a lifespan of around 300.
We assume Vbe to be 0.7 V for a typical forward biased junction
Therefore VRb = 5 – 0.7V = 4.3 V
Let's fix the operating current at 1uA, for this we have to calculate the value of Rb as,
Rb = 4.3 / (1 * 10-6) = 4.7 Mohms
Now ic = hfe * ib
ic = 300 * ( 1 * 10-6 )
IC = 0.3 mA
Now we can calculate the value of Rc as,
Rc = 2.5 / (3 * 10-4) = 8.2 Kohms.
If we need only variations in the input to appear amplified at the output, we can use coupling capacitors at both ends. The value of the coupling capacitors needs to be calculated based on the frequency of the signals we use.
For audio amplifications, values below 100mfd will provide reasonable performance.
This circuit can be used as a single stage audio amplifier in which the input is powered by a microphone and on the output side we can use a common 8ohm speaker. We can use a resistance value of 1k to 20K to pull the microphone depending on its type.
