Sunday, 10 May 2020

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 IE flows from VBE towards the emitter, from there some of the current flows towards the base region which is IB and remaining all the current goes towards the collector. So we can recall the equation,



IE = IB + IC


IE is the input current and IC is the output current. So this equation shows that output current IC can never be greater than input current IE. 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 VBE what effects does it has on input current IE. And keeping the output voltage VCB constant. We will plot a graph having VBE on the x-axis as we are changing VEB that’s why it is on the x-axis. While IE will be on the y-axis. We will see that there is no current across input when VEB is 0 to 0.6V. Because the starting voltage of the diode is 0.7V. So when VEB cross 0.6 V. The IE will start to grow up and will grow larger with small increase in voltage VEB. So one case was with VCB voltage was kept constant at 5V. Now observe another case with VCB 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 VCB and observe the change in output current IC, with input current IE will remain constant. So VCB is on the x-axis because we will be changing it and IC on the y-axis because it will be changed. We will observe that when we keep IE constant at 0 and will change VCB, it will have no effect on the IC. IC will remain zero at every value of VCB (0V, 3V, 5V etc). While when we keep IE constant at 1mA and change VCB, then we will see IC will come near to 1mA will remain constant for further increase in VCB. Now keep IE constant at 2mA, we will see that after changing VCB, IC will come near to IE, means near to 2mA and will go constant for further increase in VCB.Will draw plot for all the experiments. And we observe that IC will never be greater than IC. As equation 1 tells that.





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