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.
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. IE,IB and Ic represent emitter, base and collector currents respectively. RB and RC are base and collector resistances. VBB is the voltage applied between emitter and base whereas VCC is the voltage between emitter and collector. VBE is the base to emitter junction voltage (which is usually constant i.e 0.7 for Si)
Apply KVL (Kirchhoff Voltage Law) in loop 1 or input loop. We get
VBB = IBRB + VBE
For instance ignore VBE, then we get
VBB = IBRB
as RB is constant, so according to Ohm’s law
VBB ∝ IB (1)
Again apply KVL in loop 2 or output loop. We get
VCC = ICRC + VCE
VCE = VCC – ICRC (2)
where VCC and RC are contants
IC =βIB (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 VBB increases then IB increases and if VBB decreases then IB also decreases
- If IB increases then IC increases and if IB decreases then IC also decreases
- If IC increases then VCE decreases and if IC decreases then VCE increases or in other words when IC becomes more positive then VCE becomes more negative and vice versa
Now consider the following figure:
Here we just added a variable signal (VS ) at the output and we are taking output as VC.
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 VBB , as a result IB varies. When IB changes then IC varies largely as compared to IB because of β (see eq. 3 above) and eventually VCE changes largely (see eq. 2 above).
After this combination of VBB , IB , IC and VCE we get an amplified but inverted output.
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
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
VCE = 12 – (35mA)(220)= 4.3V
Fig-8: 2nd Q-point
For IB = 450µA
IC = 100x 450µA = 45mA
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.