Transistor As An Amplifier With Circuit Diagram
An amplifier is an electronics device which raises the strength of a weak signal. In this article we will study how a transistor can function as an amplifier.
The figure below shows the basic circuit of a transistor amplifier in CE arangement.
The weak input signal is applied between the emitter and base terminals and the output is taken across the load RC connected in the collector circuit.
In order to achieve faithful amplification, the input circuit should always be forward biased. To do so, a d.c. voltage VEE is applied between the base and emitter in addition to to the signal to be amplified.
This d.c. voltage is called as bias voltage and its magnitude is such that it always keeps the input circuit forward biased regardless of the polarity of the signal to be amplified.
Transistor Amplifier Working
During the positive half-cycle of the signal, the forward bias across the base-emitter junction is increased. Hence, more electrons flow from the emitter to the collector through the base.
This causes an increase in emitter current as well as collector current.
Now the increased collector current flows though the the high load resistance RC produces a large voltage across it.
During the negative half-cycle of the signal, the forward bias across the emitter-base junction is decreased. Hence, collector current also decreases. This results in decreased voltage drop (in opposite direction) across the load resistance.
Thus, a weak signal applied in the input circuit is amplified in the collector circuit. This way a transistor acts as an amplifier.
Transistor Load line Analysis
In the analysis of a transistor circuit, it is required to determine the collector current for various collector-emitter voltage.
One of the methods used for this is to plot the output characteristics and determine the collector current for any desired value of collector-emitter voltage.
However, there exist a more convenient method to do so, known as load line analysis method, which is quite easy and frequently used in the analysis of transistor applications.
D.C. Load Line
Let us consider a common emitter npn transistor circuit as shown in figure below.
In the above circuit, no signal is applied in the input circuit, hence d.c. condition prevail in the circuit.
The output characteristics of this circuit are shown in figure below .
The collector-emitter voltage at any instant is given by :
As VCC and RC are fixed value,it is a first degree equation and can be represented by a straight line on the output characteristics. This is known as d.c. load line.
To draw the load line, we need to know two end points of the straight line. These two points can be determined as follows :
(i) Let the collector current IC = 0, then the collector-emitter voltage attains the maximum value and is equal to VCC . i.e.
(ii) Now let the collector-emitter voltage VCE = 0 , the collector current is maximum and is equal to VCC /RC i.e.
This gives the second point A, where OA =VCC /RC , on y-axix (collector current axis) as shown in figure below.
By joining these two points A and B, we will get the load line AB.
The zero signal value of IC and VCE on the load line are known as the operating point.
When a signal is applied to the input, the variations in IC and VCE take place around this point, hence it is called the operating point.
It is also known as quiescent (silent) point or Q-point, as it is the point on the output characteristics when the transistor is silent i.e. without any input signal.
The Q-point is determined by the point where d.c. load line intersects the proper base current curve or output characteristics at a constant base current.
Cut off and Saturation point
(i) Cut off point
The point where the load line intersects the output characteristics at IB=0 , is known as cut off point.
At this point, IB = 0 and a small collector current which is the leakage current flows.
At cut off, the base – emitter junction no longer remains forward biased and the transistor does not operate in normal mode.
The collector-emitter voltage at this instant is equal to VCC. Hence,
(ii) Saturation Point
The point where the load line intersects the output characteristics at IB =IB(Sat), is called saturation point.
At this point, the base current is maximum and hence the collector current is also maximum.
At saturation, collector-base junction no longer remains reverse biased and the transistor lost its normal operation.
(iii) Active Region
The region between cut off and saturation point is known as active region.
In this region, the base-emitter junction is forward biased and collector-base junction is reverse biased. Hence, the transistor operates in normal mode in this region.
We normally bias the transistor (ref : Transistor biasing) to ensure that it operates in the active region only.