Polarizations of the transistor
pp 95-100
4.3 polarizations of the transistor
An essential prerequisite for the operation of a transistor in the active region is the proper application to polarization-emitting diode and the diode reverse bias collector. To achieve this objective requires the application of appropriate voltages between the terminal base-emitter (VBE) and collector-emitter (VCE). In Fig 4.2.2 used two external sources VBB and VCC and currents in each loop confined by means of resistors RB and RC. The use of two sources of bias a transistor is termed direct polarization basis and is the simplest but not the most effective way of wiring polarity CE. Before considering other more efficient ways of polarization will be determined and studied the operating point of transistor connectivity to CE.
To determine the operating point of the transistor alter the voltage VBB and RB resistance in the circuit sch.4.2.2. This will result in a change in collector current IC and therefore the change in voltage collector - emitter VCE. The set of pairs (IC, VCE) provides, in chart features current collector, the DC load line.
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Figure 4.3.1 Straight burden and options of the transistor operating point
The desired operating point of a transistor Q in the continuum defined by the pair (IC, VCE) straight onto the tape.
When operating a transistor and varies the base current of the operating point Q is constantly changing position on the moving line load, satisfying the relations:
VCE = VCC-IC · RC
I = b · I
'C PDC' B
When the current increases the base point Q moves to the left and approaches the saturation region and the base when the current decreases, the point Q moves to the right and approaching the cutoff region. Because the transistor is required to operate in the active region, the operating point Q should not enter in the saturation region cutoff. Thus, on the load line is defined by an assessment endpoint QH, which corresponds to a high collector current and another endpoint QL, which corresponds to a low collector current (see sch.4.3.1).
O determination of the elements operating a transistor can be done easily by following these simple options:
(VCC
• Determination of the load directly from the coordinates t? -, 0
n RC
and (0, VCC), assuming that the saturation voltage collector-emitter is
0 minutes
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• Assessment of job QH and QL
• Installing the operating point (resting) Q midway QH and QL
• Identify (IC, VCE) from the typical collector current
• Calculate the current through the base, dc and
• Selection of VBB and RB so that the pad diareetai the current required
A major problem encountered in bipolar transistors is that, dc varies from transistor to transistor. So
meet fluctuations in 3:1 ratio up to the same type transistors, so the above method is difficult to implement. Furthermore, the emitter diode of the same thermal problems common to all channels, ie the correct polarity of the voltage across reduced by approximately 2,2 mV / ° C. The latter has resulted in a continuous increase in the power base and therefore the collector current with increasing temperature. The end result is the slipping point Q to the right (Fig. 4.3.1) with
culminating in saturation. When the temperature decreases
Sia, the path Q is contrary culminating in
cutoff.
These problems and the desire to reduce the number of sources are treated with the circuit sch.4.3.2, called a voltage divider bias.
The connection sch.4.3.2 polarization reduces the number of sources
from two to one. This is achieved using the voltage divider
the resistors R1 and R2. Furthermore, the resistance RE, which is inserted in the emitter circuit makes the circuit of independent, dc transistor and reduces thermal drift.
As for the design and operation of the line load, the
voltage divider circuit with different coordinates and the function is determined as follows:
Vr.
and (0, VCC)
CC, 0
VRC + RE
and the point of operation through the following simple steps:
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4.3.2 Shape polarization voltage divider and the emitter resistance
A. Calculation of the base voltage through the voltage divider:
4.3.1
B. Calculation of emitter voltage (VBE = 0,7 V for silicon transistors):
VE = VB-VBE
C. Calculation of current emissions:
I = VlE RE4.3.2
4.3.3
D. Calculation of collector current assuming that the collector current is practically equal to the emitter current of:
IC »IE
E. Calculation collector voltage:
VC = VCC-Ic 'RC
st.Ypologismos collector emitter voltage of VC
VCE = VC-VE
4.3.4
4.3.5
4.3.6
As observed by, DC, not shown in the calculations so that the assembly is independent of, DC. The only if we can calculate the base current of IB.
RR2
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Example 4.3.1
In the circuit given sch.4.3.2 VCC = 20 V, R1 = 82 KO, R2 = 18 KO,
RE = 1 KO and RC = 2,2 KO. To calculate the currents and voltages
nodes of the circuit if, DC = 100.
Solution
Following in the footsteps of the preceding paragraph calculated
gizoume voltage base
R2 18KO
V = 'Vrr = x 20V =
B R, + R2 cc + 82KO 18KO
18
emitter voltage,
VE = VB - VBE = 3,6 V - 0,7 V = 2,9 V
current of the emitter,
VE 2,9 V
IE = RE = ^ fK9V = 2,9 mA
the collector current,
IC »IE = 2,9 mA
the collector voltage,
VC = VCC - IC · RC = 20V - (2,9 mA) x (2.2 KO) = 20V - 6,38 V = 13,62 V
the collector-emitter voltage,
VCE = VC - VE = 13,62 V - 2,9 V = 10,72 V
and the current base
2,9 mA
DC
B =-W = 2,9 JA
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x 20V = 0,18 x 20V = 3,6 V
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SUMMARY 4.3
• The straight line drawn in the load characteristic (IC, VCE) a transistor
• The load line defines the set of values which receives the collector current and collector-emitter voltage of a transistor.
• The operating point of a transistor Q out onto the line on which load and moving during operation
• Problems such as dispersion, DC, slipping the operating point with temperature and reducing the number of sources resolved by the biasing voltage divider with
