Common Base configuration of BJT (Voltage amplification)

In this type of configuration we have, the Base terminal of BJT is common with both input and output. Where input is provided to the Emitter and output is collected from the collector. The circuit of the configuration is shown in the figure below. This configuration is used for the voltage gain and current buffer. And we have an input impedance that is low and output impedance is high.

Application

Common base the configuration is not used for the low-frequency input signal and for low frequency operating circuit. Though it is used when there is low input impedance is connected or present. Such as preamplifier, in a where we have low signal and we have to strengthen it out. So there we use a common base configuration.
While mainly it is used for very high frequency and ultra-high frequency. It is because its input capacitance does not affect by the amplification process. Due to which high frequency does not degrade or change.

Circuit:

As you can see in the figure, that base is common with input and output, while Emitter is connected to the input, and the collector is connected to the output. There is a battery connected between base and emitter, to forward-biased base-emitter junction. To forward bias we need to connect the positive side of the battery to the P side of BJT and negative side of the battery to the N side of BJT (In PNP case emitter is P side and Base is N side). There is another battery connected between collector and base to reverse biased the collector-base junction. To do this we have connected the positive side with the base and negative side with the collector.

Working:

Current Gain

Let’s first see the current flow in this configuration. We all know that current flows from the positive terminal of the battery to the negative terminal of the battery. So, in this case, current I_E flows from V_BE towards the emitter, from there some of the current flows towards the base region which is I_B and remaining all the current goes towards the collector. So we can recall the equation,

I_E = I_B + I_C

I_E is the input current and I_C is the output current. So this equation shows that output current I_C can never be greater than input current I_E. So current gain could never be greater than 1. It will be “1” or “less than 1”.

Current Gain equation will be

$$ α= I_C/I_E , where\ "α"\ is \ current\ gain $$

Using this equation we can find out the current gain of a circuit.

Voltage gain

Now let’s see how it provides us the voltage gain (amplification).

As we have the current gain$$  α=I_C/I_E $$ 
And resistance gain              $$ = R_L/R_I_N , $$ So using Ohm’s law(V=IR), We get,
Voltage gain= Current gain x Resistance gain
Voltage gain = $$ I_C/I_E × R_L/R_I_N = {I_C R_L}/{I_E R_I_N} $$

So this equation shows that we can get the desired voltage gain by changing the amount of input and output resistance.

Input characteristics

Now let’s see what the input characteristics of the common base configuration are. Means when we change input voltages V_BE what effects does it has on input current I_E. And keeping the output voltage V_CB constant. We will plot a graph having V_BE on the x-axis as we are changing V_EB that’s why it is on the x-axis. While I_E will be on the y-axis. We will see that there is no current across input when V_EB is 0 to 0.6V. Because the starting voltage of the diode is 0.7V. So when V_EB cross 0.6 V. The I_E will start to grow up and will grow larger with small increase in voltage V_EB. So one case was with V_CB voltage was kept constant at 5V. Now observe another case with V_CB at 7V, and then with 10V. And at last plot a graph of all 3 cases as shown in the figure below.

Output characteristics

Now observe the output characteristics. We will change the output voltage V_CB and observe the change in output current I_C, with input current I_E will remain constant. So V_CB is on the x-axis because we will be changing it and I_C on the y-axis because it will be changed. We will observe that when we keep I_E constant at 0 and will change V_CB, it will have no effect on the I_C. I_C will remain zero at every value of V_CB (0V, 3V, 5V etc). While when we keep I_E constant at 1mA and change V_CB, then we will see I_C will come near to 1mA will remain constant for further increase in V_CB. Now keep I_E constant at 2mA, we will see that after changing V_CB, I_C will come near to I_E, means near to 2mA and will go constant for further increase in V_CB.Will draw plot for all the experiments. And we observe that I_C will never be greater than I_C. As equation 1 tells that.

Bipolar Junction Transistor(BJT) as a Amplifier

Amplification

As we have discussed the other uses of BJTs in previous posts, now there is another main use or advantage of Bipolar transistor is that it can amplify the input signal whether it is voltage or current. As some time we need to amplify the current and sometimes voltage. We provide an input signal to the BJT, and it produces the amplified signal at the output. The gain of the amplifier can be calculated by the ratio of output to the input value of current or voltage. The amplifier does not change the waveform or frequency. It just affects the current and voltage.

How to make BJT an Amplifier:

To make BJT work as amplifier we need to do some different types of configuration of it, in a circuit. There are three configurations, which we can do,

•        Common Base Configuration (only has voltage gain).
•       Common Emitter Configuration (has both voltage and current gain)
•       Common Collector Configuration (only has current gain)

Bipolar Junction Transistor As a Switch

Switching:

The Bipolar junction transistors can also be used for switching in a circuit. It means that we can use BJT as for current controlling switch. The current in the base will decide whether the current will flow from emitter to collector or not, that’s how it can be used as switch. BJT in circuit used for switching the high voltage or high power circuit with the very low power control variable. We will try to figure it out with the help of example.

Example of BJT as Switch:

As shown in figure below simple lamp circuit with a battery. As switch is open then lamp is off as in fig (1), while when switch is closed, the circuit is closed and lamp is glowing in fig (2).

Fig (1)

Fig (2)

Now when we used bipolar junction transistor in place of switch in above circuit and connect a 1 K resistor with base to provide some current for the base. NPN type of Bipolar junction transistor is being used in this circuit. PNP could also be used, it is just difference of polarity in connections. Keep in mind that the direction of flow of current should be opposite to the direction of arrow on emitter of BJT. The direction of current flow is shown by red arrows in fig (3).

Fig (3)

The BJT do switching in between two state regions; cut off region and saturation region. When there will be no resistor connected with base or if there is open circuit at base of BJT, then the flow of current from emitter to collector will not occur and transistor would be in its “cut off” state. And thus lamp will not glow.

While when there will be resistance connected with the base, some current will flow through base (where we have studied before base current is controlling current). When there will be current through base then current will pass through collector and lamp will glow up and transistor would be in "saturated" state.

So we have seen that we can do switching of high level with the very small amount of current in base. That’s a big advantage of having a BJTs.

Applications:

Due to this switching property of BJT, it has many application in industry and smart production.

1. In a thermocouple circuit, we can use BJT as switch while taking heat as a source for base current. So that with heat on switch on.

2. In automatic door, with the presence of a person near the door, will initiate the base and switch will open.

Like these it has many application in this era.