Introduction Of Transistor Biasing
The basic function of transistor is to do amplification. The weak signal is given to the base of the
transistor and amplified output is obtained in the collector circuit. One important requirement during amplification is that only the magnitude of the signal should increase and there should be no change in signal shape. This increase in magnitude of the signal without any change in shape is known as faithful amplification. In order to achieve this, means are provided to ensure that input circuit (i.e. base-emitter junction) of the transistor remains forward biased and output circuit (i.e. collector base junction) always remains reverse biased during all parts of the signal. This is known as transistor biasing. In this artical, we shall discuss how transistor biasing helps in achieving faithful amplification.
The process of raising the strength of a weak signal without any change in its general shape is known as faithful amplification.
The theory of transistor reveals that it will function properly if its input circuit (i.e. base-emitter
junction) remains forward biased and output circuit (i.e. collector-base junction) remains reverse
biased at all times. This is then the key factor for achieving faithful amplification. To ensure this, the
following basic conditions must be satisfied :
(i) Proper zero signal collector current
(ii) Minimum proper base-emitter voltage (VBE) at any instant
(iii) Minimum proper collector-emitter voltage (VCE) at any instant
The conditions (i) and (ii) ensure that base-emitter junction shall remain properly forward biased
during all parts of the signal. On the other hand, condition (iii) ensures that base-collector junction
shall remain properly reverse biased at all times. In other words, the fulfillment of these conditions
will ensure that transistor works over the active region of the output characteristics i.e. between
saturation to cut off.
(i) Proper zero signal collector current.
Consider an NPN transistor circuit shown in Fig. 9.1 (i). During the positive half-cycle of the signal, base is positive w.r.t. emitter and hence base emitter junction is forward biased. This will cause a base current and much larger collector current to flow in the circuit. The result is that positive half-cycle of the signal is amplified in the collector as shown. However, during the negative half-cycle of the signal, base-emitter junction is reverse biased and hence no current flows in the circuit. The result is that there is no output due to the negative half cycle of the signal. Thus we shall get an amplified output of the signal with its negative half-cycles completely cut off which is unfaithful amplification.
Now, introduce a battery source VBB in the base circuit as shown in Fig. 9.1 (ii). The magnitude
of this voltage should be such that it keeps the input circuit forward biased even during the peak of
negative half-cycle of the signal. When no signal is applied, a d.c. current IC will flow in the collector circuit due to VBB as shown. This is known as zero signal collector current IC. During the positive half-cycle of the signal, input circuit is more forward biased and hence collector current increases. However, during the negative half-cycle of the signal, the input circuit is less forward biased and collector current decreases. In this way, negative half-cycle of the signal also appears in the output and hence faithful amplification results. It follows, therefore, that for faithful amplification, proper zero signal collector current must flow. The value of zero signal collector current should be at least equal to the maximum collector current due to signal alone i.e.
Zero signal collector current ≥ Max. collector current due to signal alone
Illustration. Suppose a signal applied to the base of a transistor gives a peak collector current of
1mA. Then zero signal collector current must be at least equal to 1mA so that even during the peak of negative half-cycle of the signal, there is no cut off as shown in Fig. 9.2 (I). If zero signal collector current is less, say 0.5 mA as shown in Fig. 9.2 (ii), then some part (shaded portion) of the negative half-cycle of signal will be cut off in the output.
(ii) Proper minimum base-emitter voltage.
In order to achieve faithful amplification, the base-emitter voltage (VBE) should not fall below 0.5V for germanium transistors and 0.7V for Si transistors at any instant.
The base current is very small until the *input voltage overcomes the potential barrier at the base-emitter junction. The value of this potential barrier is 0.5V for Ge transistors and 0.7V for Si transistors as shown in Fig. 9.3. Once the potential barrier is overcome, the base current and hence collector current increases sharply. Therefore, if base-emitter voltage VBE falls below these values during any part of the signal, that part will be amplified to lesser extent due to small collector current. This will result in unfaithful amplification.
(iii) Proper minimum VCE at any instant.
For faithful amplification, the collector-emitter voltage VCE should not fall below 0.5V for Ge transistors and 1V for silicon transistors. This is called knee voltage (See Fig. 9.4).
When VCE is too low (less than 0.5V for Ge transistors and 1V for Si transistors), the collectorbase junction is not properly reverse biased. Therefore, the collector cannot attract the charge carriers emitted by the emitter and hence a greater portion of them goes to the base. This decreases the
collector current while base current increases. Hence, value of β falls. Therefore, if VCE is allowed
to fall below Vknee during any part of the signal, that part will be less amplified due to reduced β. This will result in unfaithful amplification. However, when VCE is greater than Vknee, the collector-base junction is properly reverse biased and the value of β remains constant, resulting in faithful amplification.
What Is Transistor Biasing?
It has already been discussed that for faithful amplification, a transistor amplifier must satisfy three
basic conditions, namely :
- proper zero signal collector current,
- proper base-emitter voltage at any instant and
- proper collector-emitter voltage at any instant. It is the fulfillment of these conditions which is known as transistor biasing.
The proper flow of zero signal collector current and the maintenance of proper collector-emitter voltage during the passage of signal is known as transistor biasing. The basic purpose of transistor biasing is to keep the base-emitter junction properly forward biased and collector-base junction properly reverse biased during the application of signal. This can be achieved with a bias battery or associating a circuit with a transistor. The latter method is more efficient and is frequently employed. The circuit which provides transistor biasing is known as biasing circuit. It may be noted that transistor biasing is very essential for the proper operation of transistor in any circuit.
Example 9.1. An npn silicon transistor has VCC = 6 V and the collector load RC= 2.5 kΩ. Find : (i) The maximum collector current that can be allowed during the application of signal for faithful amplification. (ii) The minimum zero signal collector current required.
Solution. Collector supply voltage, VCC = 6 V
Collector load, RC = 2.5 kΩ
(i) We know that for faithful amplification, VCE should not be less than 1V for silicon transistor.
∴ Max. voltage allowed across RC = 6 − 1 = 5 V
∴ Max. allowed collector current = 5 V/RC = 5 V/2.5 kΩ = 2 mA
Thus, the maximum collector current allowed during any part of the signal is 2 mA. If the collector current is allowed to rise above this value, VCE will fall below 1 V. Consequently, value of β will fall, resulting in unfaithful amplification.
(ii) During the negative peak of the signal, collector current can at the most be allowed to become zero. As the negative and positive half cycles of the signal are equal, therefore, the change in collector current due to these will also be equal but in opposite direction.
∴ Minimum zero signal collector current required = 2 mA/2 = 1 mA During the positive peak of the signal [point A in Fig. 9.5 (ii)],
iC= 1 + 1 = 2mA and during the negative peak (point B),
iC = 1 − 1 = 0 mA
Example 9.2. A transistor employs a 4 kΩ load and VCC = 13V. What is the maximum input signal if β = 100? Given Vknee = 1V and a change of 1V in VBE causes a change of 5mA in collector current.
Inherent Variations of Transistor Parameters
In practice, the transistor parameters such as β, VBE are not the same for every transistor even of the same type. To give an example, BC147 is a silicon NPN transistor with β varying from 100 to 600 i.e. β for one transistor maybe 100 and for the other, it maybe 600, although both of them are BC147.
This large variation in parameters is a characteristic of transistors. The major reason for these variations is that transistor is a new device and manufacturing techniques have not too much advanced. For instance, it has not been possible to control the base width and it may vary, although slightly, from one transistor to the other even of the same type. Such small variations result in large change in transistor parameters such as β, VBE etc. The inherent variations of transistor parameters may change the operating point, resulting in unfaithful amplification. It is, therefore, very important that biasing network be so designed that it should be able to work with all transistors of one type whatever may be the spread in β or VBE. In other words, the operating point should be independent of transistor parameters variations
The collector current in a transistor changes rapidly when
(i) the temperature changes,
(ii) the transistor is replaced by another of the same type. This is due to the inherent variations
of transistor parameters. When the temperature changes or the transistor is replaced, the operating point (i.e. zero signal IC and VCE) also changes. However, for faithful amplification, it is essential that operating point remains fixed. This necessitates to make the operating point independent of these variations. This is known as stabilisation.
The process of making operating point independent of temperature changes or variations in transistor parameters is known as stabilisation. Once stabilisation is done, the zero signal IC
and VCE become independent of temperature variations or replacement of transistor i.e. the operating point is fixed. A good biasing circuit always ensures the stabilisation of operating point.
Need for stabilisation
Stabilisation of the operating point is necessary due to the following reasons:
(i) Temperature dependence of IC
(ii) Individual variations
(iii) Thermal runaway
Temperature dependence of IC
The collector current IC for CE circuit is given by:
IC= β IB + ICEO = β IB + (β + 1) ICBO
The collector leakage current ICBO is greatly influenced (especially in germanium transistor) by temperature changes. A rise of 10°C doubles the collector leakage current which may be as high as 0.2 mA for low powered germanium transistors. As biasing conditions in such transistors are generally so set that zero signal IC = 1mA, therefore, the change in IC due to temperature variations cannot be tolerated. This necessitates to stabilise the operating point i.e. to hold IC constant inspite of temperature variations.
The value of β and VBE are not exactly the same for any two transistors even of the same type. Further, VBE itself decreases when temperature increases. When a transistor is replaced by another of the same type, these variations change the operating point. This necessitates to stabilise the operating point i.e. to hold IC constant irrespective of individual variations in transistor parameters.
The collector current for a CE configuration is given by :
IC= β IB + (β + 1) ICBO
The collector leakage current ICBO is strongly dependent on temperature. The flow of collector
current produces heat within the transistor. This raises the transistor temperature and if no stabilisation is done, the collector leakage current ICBO also increases. It is clear from exp. (i) that if ICBO increases, the collector current IC increases by (β + 1) ICBO. The increased IC will raise the temperature of the transistor, which in turn will cause ICBO to increase. This effect is cumulative and in a matter of seconds, the collector current may become very large, causing the transistor to burn out. The self-destruction of an unstabilised transistor is known as thermal runaway.
In order to avoid thermal runaway and consequent destruction of transistor, it is very essential that operating point is stabilised i.e. IC is kept constant. In practice, this is done by causing IB to decrease automatically with temperature increase by circuit modification. Then decrease in β IB will compensate for the increase in (β + 1) ICBO, keeping IC nearly constant. In fact, this is what is always aimed at while building and designing a biasing circuit.
Essentials of a Transistor Biasing Circuit
It has already been discussed that transistor biasing is required for faithful amplification.
The biasing network associated with the transistor should meet the following requirements :
- It should ensure proper zero signal collector current.
- (ii) It should ensure that VCE does not fall below 0.5 V for Ge transistors and 1 V for silicon transistors at any instant.
- (iii) It should ensure the stabilization of operating point.
It is desirable and necessary to keep IC constant in the face of variations of ICBO (sometimes represented as ICO). The extent to which a biasing circuit is successful in achieving this goal is measured by stability factor S. It is defined as under :
The rate of change of collector current IC w.r.t. the collector leakage current *ICO at constant β
and IB is called stability factor i.e.
The stability factor indicates the change in collector current IC due to the change in collector leakage current ICO. Thus a stability factor 50 of a circuit means that IC changes 50 times as much as any change in ICO. In order to achieve greater thermal stability, it is desirable to have as low stability
factor as possible
The ideal value of S is 1 but it is never possible to achieve it in practice. Experience shows that values of S exceeding 25 result in unsatisfactory performance. The general expression of stability factor for a C.E. configuration can be obtained as under: