Chapter 3. Bipolar Junction Transistors Outline: Fundamental of Transistor Common-Base Configuration Common-Emitter Configuration Common-Collector Configuration
Introduction The transistor is a three-layer semiconductor device consisting of two n- and one p-type layers of material or two p- and one n-type layers of material. The former is called an npn transistor, and the latter is called a pnp transistor.
The terminals have been indicated by E for emitter, C for collector and B for Base. They have different thickness and degrees of doping. This three-terminal device is often referred to as bipolar junction transistor. The term bipolar reflects the fact that holes and electrons involve in the current flow.
Figure: Types of Transistor
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Transistor Operation The biasing of the two types of transistor has been illustrated in the figures. The operation of npn and pnp transistors are same if the roles played by the electrons and holes are interchanged. One p-n junction of a transistor is reversebiased, whereas the other is forward-biased.
Just like the role played by external potential to diode, the applied voltages influence the depletion regions of the transistors. Thus the flow of holes and electrons has been manipulated. So for both types of transistors, we obtain: I E = I C +I B
I E I B I C Figure: Biasing of pnp transistor
I E I B I C Figure: Biasing of npn transistor
Common-Base Configuration The common-base configuration with pnp and npn transistors are shown in the figures. The term common-base is derived from the fact that the base is common to both the input and output sides of the configuration. The arrow in the symbol defines the direction of emitter current through the device.
The applied biasing are such as to establish current in the direction indicated for each branch. That is, direction of I E is the same as the polarity of V EE and I C to V CC. Also, the equation I E = I C + I B still holds.
I E I C I E I C I B I B Figure: Common-base configuration of pnp transistor
I E I C I E I C I B I B Figure: Common-base configuration of npn transistor
Input characteristics The driving point or input parameters are shown in the figure. An input current (I E ) is a function of an input voltage (V BE ) for various of output voltage (V CB ). This closely resembles the characteristics of a diode.
As an approximation, the change due to changes in V CB can be ignored. The characteristics can be shown in orange curve. If piecewise-linear approach is applied, the blue curve is obtain. Furthermore, ignoring the slop of the curve and the resistance results the red curve.
It is this red curve that is used in the dc analysis of transistors. Once a transistor is in on state, the b-e voltage is assumed to be 0.7V. And the emitter current may be at any level as controlled by the external network.
Figure: Input characteristics for common-base transistor
Figure: Equivalent model for b-e junction
Output characteristics The output set relates an output current (I C ) to an output voltage (V CB ) for various of level of input current (I E ). There are three regions of interest: Active region In the active region, the b-e junction is forward-biased, whereas the c-b junction is reverse-biased.
The active region is the region normally employed for linear amplifier. Also, in this region, Cutoff region I C I E The cutoff region is defined as that region where the collector current is 0A.
In the cutoff region, the b-e and c-b junctions of a transistor are both reverse-biased. Saturation region: It is defined as that region of the characteristics to the left of V CB = 0 V. In saturation region, the b-e and c-b junctions of a transistor are both forwardbiased.
Active Region Saturation Region Cutoff Region Figure: Output characteristics for common-base transistor
Alpha (α) In the dc mode, the levels of I C and I E at the operation point are related by: Normally, α 1. α dc = I C / I E For practical devices, α is typically from 0.9 to 0.998.
Common-Emitter Configuration The common-emitter configuration with npn and pnp transistors are shown in the figures. The term common-emitter is derived from the fact that the emitter is reference to both the input and output terminals. The current relations are still applicable, i.e., I E = I C + I B and I C =α I E
Figure: Common-emitter configuration of npn transistor
Figure: Common-emitter configuration of pnp transistor
Input characteristics An input current (I B ) is a function of an input voltage (V BE ) for various of output voltage (V CE ). The characteristics of the input or baseemitter circuit is shown in the figure. The magnitude of I B is in μa and not as horizontal as I E in common-base circuit.
Figure: Input characteristics for common-emitter transistor
The output set relates an output current (I C ) to an output voltage (V CE ) for various of level of input current (I B ). There are three portions as shown: Active region Output characteristics The active region, located at upper-right quadrant, has the greatest linearity.
The curve for I B are nearly straight and equally spaced. In active region, the b-e junction is forward-biased, whereas the c-b junction is reverse-biased. The active region can be employed for voltage, current or power amplification.
Cutoff region The region below I B = 0μA is defined as cutoff region. For linear amplification, cutoff region should be avoided. Saturation region: The small portion near the ordinate, is the saturation region, which should be avoided for linear application.
Active Region Saturation Region Cutoff Region Figure: Output characteristics for common-emitter transistor
Beta (β) In the dc mode, the levels of I C and I B at the operation point are related by: β dc = I C / I B Normally, β ranges from 50 to 400. For ac situations, β is defined as ac I I C B V CE constant
The proper biasing is essential to place the device in the active region. A common-emitter amplifier of a pnp transistor is shown in the figure. Biasing 1. The first step is to indicate the direction of I E as established by the arrow in the transistor symbol.
2. The other current, I B and I C, are introduced, satisfying I C + I B = I E. 3. The supplies are introduced with polarities that will support the resulting directions of I B and I C. 4. If the transistor is a npn transistor, all the current and polarities would be reversed.
Figure: Biasing for common-emitter pnp transistor
Common-Collector Configuration The common-collector configuration with npn and pnp transistors are shown in the figures. It is used primarily for impedance-matching purpose since it has a high input impedance and low output impedance.
The load resistor is connected from emitter to ground. The collector is tied to ground and the circuit resembles common-emitter circuit. The output set relates an output current (I E ) to an output voltage (V CE ) for various of level of input current (I B ).
This is almost the same as the output characteristics of common-emitter circuit, which are the relations between I C and V CE for various of level of input current I B. Since that: I E I C. The input characteristic of commonemitter are sufficient for requirement of common-collector circuit.
Figure: Common-collector configuration of npn transistor
Figure: Common- collector configuration of pnp transistor
Figure: Common-collector circuit used for impedance-matching purpose
Summary of Chapter 3 Three-terminal devices, transistor Three types of configurations: common-base, common-emitter and common-collector. Proper biasing of the three configurations. Input and output characteristics of the three configurations.