A vital part of an electronic circuit is a transistor . One which is
familiar to you is Bipolar Junction Transistor(BJT). Just take a tour
through the necessary points. Here include the basic equations related to transistor. Also explain the concept of biasing and Q-point with the help of an amplifier circuit.
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| fig 1 |
Ic = β IB
Ic = αIE
α = β /1+β
IE = Ic + IB
Recollect functions of collector, emitter and base and related currents.Ok, then move to amplifier section.
The below circuit is very familiar in your first year.
RC coupled amplifier(CE amplifier)
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| fig 2 BJT amplifier |
This
is an amplifier circuit. If we give a small input(sine wave) as Vs, we
will get an amplified output in the output side Vo. To understand this
circuit, it can be divided into 3 sections:
1) biasing section(inside red dots)
It consists of resistors R1,R2,Rc,RE, dc voltage Vcc and ground.This is the first section to be wired while setting up an amplifier for lab experiments. Value of the dc source Vcc and value of the resistors in this section decide the position of operating point and region of operation of the transistor. While the transistor is working as an amplifier, it should be in the active region to ensure proper amplification. Otherwise some portions of the output sine wave will be cut off.
1) biasing section(inside red dots)
It consists of resistors R1,R2,Rc,RE, dc voltage Vcc and ground.This is the first section to be wired while setting up an amplifier for lab experiments. Value of the dc source Vcc and value of the resistors in this section decide the position of operating point and region of operation of the transistor. While the transistor is working as an amplifier, it should be in the active region to ensure proper amplification. Otherwise some portions of the output sine wave will be cut off.
2) input section
It consists of input signal Vs, resistor in the source side Rs and
capacitor C1. This section provides the input signal which to be
amplified.
3) output side
It consists of capacitor C2 and sometimes a resistor Ro(not in figure). Amplified output can be obtained from here.
Circuit also consists of a bypass capacitor CE.
Why a biasing section is included in amplifier circuit:
As
i already said, a biasing section should we wired before input and
output sections in an amplifier. This biasing circuit ensures initial dc
values of IB, Ic and VcE. After ensuring this initial value, if we apply the input sine wave, then new variations in the values of IB, Ic and VcE are around its initial dc values.
IB = Base current, Ic = Collector current and VcE = Voltage across Collector and Emitter
From fig 2, VcE = Vcc - Ic(Rc+RE) ---(1)
Minimum value of VcE is zero and if we apply this to (1),
Ic =Vcc/(Rc+RE) -of course this is the maximum value that Ic can be obtained and represented as Ic(sat).
Maximum value of VcE is Vcc and this happens when Ic = 0 in (1). Therefore minimum value of Ic = 0.
- With these values of Ic and Vcc, if a graph is drawn , it is known as D C load line for an amplifier.
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| fig 3 |
Load line represents every possible values of (Ic , VcE )
combinations. It means if we measure values of Ic and VcE of an amplifier at various instants, these points (Ic, VcE )
will be on the load line (dotted line in above figure).
- After familiarized with load line, we will go to Q-point(operating point).
Consider the biasing section in fig 2.(At this point we are taking that no input sine wave is applied - only Vcc is given). If Ic and VcE are measured at this point, they have specific dc values. These values of Ic and VcE can be represented as a point (Ic , VcE ) and it will corresponds to a particular point on dc load line. This point is known as Q-point. (Note that we are dealing with a zero input condition only bias voltage Vcc is applied).
Need for biasing:
In fig 3, Ic(sat) represents saturation current for the circuit. At
this point transistor in the amplifier circuit is in saturation and VcE is zero (A
transistor is said to be in saturation when collector(output) current
is maximum and at this condition there is a short circuit from Emitter
to Collector to provide maximum current. Therefore voltage across
Emitter and Collector(VcE) is zero.).
When VcE = Vcc, Ic is zero(from graph) and an open circuit from Emitter to Collector. Therefore transistor is in cut off.
Taking these two conditions in fig 3,
i) if (Ic , VcE ) point on the load line is near to Y-axis, there is a chance for the transistor to be in saturation.
ii) if (Ic , VcE ) point on the load line is near to X-axis, there is a chance for the transistor to be in cut off.
During
the above conditions, transistor cannot properly amplify the input sine
wave. Some parts of the amplified output may get lost. To avoid this, (Ic , VcE ) point must be at the center of the load line (to reduce chances of saturation and cut off).
For proper amplification, it is desirable to have Q-point centered on the load line. This is ensured by a proper biasing circuit (as in fig 2).
After proper biasing, if we connect the input section and apply an input signal, then Ic and VcE both will vary. This variation is around their Q-point values which is already set by biasing circuit at the center of the load line.
For proper amplification, it is desirable to have Q-point centered on the load line. This is ensured by a proper biasing circuit (as in fig 2).
After proper biasing, if we connect the input section and apply an input signal, then Ic and VcE both will vary. This variation is around their Q-point values which is already set by biasing circuit at the center of the load line.