Transistor as an Amplifier, An Easy Understanding

Amplification Definition

Amplification meaning in electronics is the process of increasing the amplitude of a signal. In simple words we make the signal more stronger. See figure below

 

In above figure you can observe the weak signal is made more stronger after applying amplification process.

Amplification Factor

The amplification factor tells how much an amplifier can increase the strength or amplitude of a signal. It is measured in decibels (dB)

Transistor as Amplifier

In this lecture we will try to understand transistor as amplifier working working principle. As we know there are three types of transistor configurations i.e common emitter, common collector and common base. Usually common emitter configuration is used as amplifier because it’s gain is better than common base and common collector. Consider the following figure for common emitter configuration

Fig-1: Common Emitter Transistor

The above figure shows emitter is common to both base and the collector. I_E,I_B and I_c represent emitter, base and collector currents respectively. R_B and R_C are base and collector resistances. V_BB is the voltage applied between emitter and base whereas V_CC is the voltage between emitter and collector. V_BE is the base to emitter junction voltage (which is usually constant i.e 0.7 for Si)

Loop 1: 

Apply KVL (Kirchhoff Voltage Law) in loop 1 or input loop. We get

V_BB = I_BR_B + V_BE

For instance ignore V_BE, then we get

V_BB = I_BR_B

as R_B is constant, so according to Ohm’s law

 V_BB ∝  I_B                                     (1)

Loop 2:

Again apply KVL in loop 2 or output loop. We get

V_CC = I_CR_C + V_CE

V_CE = V_CC –  I_CR_C                          (2)

where V_CC and R_C are contants

and

I_C =βI_B                                          (3)

where β is gain of the transistor ranges from 20 to 200 and more.

From (1), (2) and (3) we conclude the followings

• If V_BB increases then I_B increases and if V_BB decreases then I_B also decreases
• If I_B increases then I_C increases and if I_B decreases then I_C also decreases
• If I_C increases then V_CE decreases and if I_C decreases then V_CE increases or in other words when I_C becomes more positive then V_CE becomes more negative and vice versa

Now consider the following figure:

Fig-2: NPN Transistor as Amplifier

 

Here we just added a variable signal (V_S ) at the output and we are taking output as V_C.

The capacitors at both sides (input and output) do not allow direct current to flow towards mic and speaker side. But they will allow any variable current to flow towards the other side. Now recall the following concept.

Fig-3: Addition of Signals

If we add any variable value to the constant we get the result as shown above. The signal is being shifted depending upon the amplitudes of both signals at the same time. Similar is the case with the amplifier. When we apply any variable signal at the input it adds to the  V_BB , as a result I_B varies. When I_B changes then I_C varies largely as compared to I_B because of β (see eq. 3 above) and eventually V_CE changes largely (see eq. 2 above).

After this combination of  V_BB , I_B , I_C and V_CE we get an amplified but inverted output.

DC Biasing

It is important to note that transistor works as an amplifier in the active mode (Read modes of transistor). So we have to keep the transistor in the active mode otherwise the output signal will be distorted. Because if input current I mean IB increases or decreases to certain levels a transistor can go into saturation or cutoff mode.

For this we need to set proper DC operating point or Q point of a transistor. In other words we need to set VBB and hence IB . The following figure shows how a signal can be clipped off if transistor is not biased properly.

Fig-4: Non-linear Operation of Transistor Amplifier

What is DC operating point (Q-point) of transistor Amplifier?

A transistor biasing should be adequately done so it can perform well as a linear amplifier. This biasing establish a DC operating point which is also known as Q-point (quiescent point).

What can happen if we don’t set proper DC operating point?

If we don’t set a Q-point properly, then transistor can go into saturation or cutoff mode after applying the input signal. Hence we will not get the desired amplification or part of the signal will be distorted.

Graphical Analysis of Transistor Amplification

In above discussion you observe in loop 1 and loop 2 there is a nexus between four parameters i.e VBB, IB,IC and VCE. The Q point can be set on the output characteristics curves of transistor amplifier. These curves are drawn between IC and VCE for different values of IB. 

Suppose in Fig-1 VBB varies between 0-5V, β=100, RC = 220 ohm and VCC = 12V. Then Fig-5 shows the output characteristic curves.

Fig-5: Collector Characteristics Curves

DC Load Line

Graphically the transistor operation can be described using DC load line. This is drawn on the characteristic curves from cut off point on the x-axis to saturation point on y-axis.

Cut-off Point on Collector Characteristics Curves

On the x-axis IC=0, so put this value in equation (2)

VCE=VCC-ICRC = VCC-0

VCE=VCC = 12V

Saturation Point on Collector Characteristics Curves

On the y-axis VCE = 0, so no put this value in equation (2)

0 = VCC-ICRC

By rearranging the above equation

IC= VCC/RC = 12/220 = 54.5 mA

 

Fig-6: DC Load Line

 

To setQ-point lets we take three values of IB and subsequently calculate values of IC and VCE. 

For IB = 250µA

From equation (3)

IC = 100x 250µA = 25mA

And from equation (2)

VCE = 12 – (25mA)(220)= 6.5V

The first Q-point is shown in Fig-7

 

Fig-7: 1st Q-point

For IB = 350µA

IC = 100x 350µA = 35mA

And

 VCE = 12 – (35mA)(220)= 4.3V

 

Fig-8: 2nd Q-point

For IB = 450µA

IC = 100x 450µA = 45mA

And

 VCE = 12 – (45mA)(220)= 2.1V

Fig-9: 3rd Q-point

Now combine figures (7), (8) and (9)

 

So from the above figure you can observe we have to select Q2. So in case VCE increases or decreases due to VBB transistor will not go into cutoff or saturation region.

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