Common Base Connection | Transistor Connections

What are Transistor Connections

There are three leads in a transistor viz., emitter, base and collector terminals. However, when a
transistor is to be connected in a circuit, we require four terminals; two for the input and two for the output. This difficulty is overcome by making one terminal of the transistor common to both input and output terminals. The input is fed between this common terminal and one of the other two terminals. The output is obtained between the common terminal and the remaining terminal.

Accordingly; a transistor can be connected in a circuit in the following three ways :


(i) common base connection (ii) common emitter connection


(iii) common collector connection


Each circuit connection has specific advantages and disadvantages. It may be noted here that regardless of circuit connection, the emitter is always biased in the forward direction, while the collector always has a reverse bias.

Common Base Connection

In this circuit arrangement, input is applied between emitter and base and output is taken from collector and base. Here, base of the transistor is common to both input and output circuits and hence the name common base connection. In Fig. 8.9 (i), a common base npn transistor circuit is shown whereas Fig. 8.9 (ii) shows the common base pnp transistor circuit.

Current amplification factor (α).

It is the ratio of output current to input current. In a common base connection, the input current is the emitter current IE and output current is the collector current IC. The ratio of change in collector current to the change in emitter current at constant collector base voltage VCB is known as current amplification factor i.e.

It is clear that current amplification factor is less than **unity this value can be increased (but not more than unity) by decreasing the base current. This is achieved by making the base thin and doping it lightly. Practical values of α in commercial transistors range from 0.9 to 0.99.

Expression for collector current

The whole of emitter current does not reach the collector. It is because a small percentage of it, as a result of electron-hole combinations occurring in base area, gives rise to base current. Moreover, as the collector-base junction is reverse biased,

therefore, some leakage current flows due to minority carriers. It follows, therefore, that total collector current consists of :


(i) That part of emitter current which reaches the collector terminal i.e. ***α IE
.
(ii) The leakage current I leakage. This current is due to the movement of minority carriers across
base-collector junction on account of it being reverse biased. This is generally much smaller than
α IE.

Relation (i) or (ii) can be used to find IC.

It is further clear from these relations that the collector current of a transistor can be controlled by either the emitter or base current. Fig. 8.11 shows the concept of ICBO. In CB configuration, a small collector current flows even when the emitter current is zero. This is the leakage collector current (i.e. the collector current when emitter is open) and is denoted by ICBO. When the emitter voltage VEE is also applied, the various currents are as shown in Fig. 8.11 (ii).


Note. Owing to improved construction techniques, the magnitude of ICBO for general-purpose and low-powered transistors (especially silicon transistors) is usually very small and may be neglected in calculations. However, for high power applications, it will appear in microampere range. Further, ICBO is very much temperature dependent; it increases rapidly with the increase in temperature. Therefore, at higher temperatures, ICBO plays an important role and must be taken care of in calculations.

Example 8.2. In a common base connection, IE = 1mA, IC = 0.95mA. Calculate the value of IB

Example 8.3. In a common base connection, current amplification factor is 0.9. If the emitter
current is 1mA, determine the value of base current.

Example 8.4. In a common base connection, IC = 0.95 mA and IB = 0.05 mA. Find the value of α.

Example 8.5. In a common base connection, the emitter current is 1mA. If the emitter circuit is
open, the collector current is 50 µA. Find the total collector current. Given that α = 0.92.

Example 8.6. In a common base connection, α = 0.95. The voltage drop across 2 kΩ resistance which is connected in the collector is 2V. Find the base current. Solution. Fig. 8.12 shows the required common base connection. The voltage drop across RC (=2 kΩ) is 2V.

Example 8.7. For the common base circuit shown in Fig. 8.13, determine IC and VCB. Assume the transistor to be of silicon.


Solution. Since the transistor is of silicon, VBE = 0.7V. Applying Kirchhoff’s voltage law to the emitter-side loop, we get,

Characteristics of Common Base Connection

The complete electrical behaviour of a transistor can be described by stating the interrelation of the
various currents and voltages. These relationships can be conveniently displayed graphically and the curves thus obtained are known as the characteristics of transistor. The most important characteristics of common base connection are input characteristics and output characteristics.

input characteristic

It is the curve between emitter current IE and emitter-base voltage VEB at constant collector-base voltage VCB. The
emitter current is generally taken along y-axis
and emitter-base voltage along x-axis. Fig. 8.14
shows the input characteristics of a typical transistor in CB arrangement . The following points
may be noted from these characteristics :


(i) The emitter current IE increases rapidly with small increase in emitter-base voltage VEB. It means that input resistance is very small.


(ii) The emitter current is almost independent of collector-base voltage VCB. This leads to the conclusion that emitter current (and hence collector current) is almost independent of collector voltage.

Input resistance of common base connection

It is the ratio of change in emitter-base voltage (ΔVEB) to the resulting

In fact, input resistance is the opposition offered to the signal current. As a very small VEB is sufficient to produce a large flow of emitter current IE , therefore, input resistance is quite small, of the order of a few ohms. 2. Output characteristic. It is the curve between collector current IC and collector-base voltage VCB at *constant emitter current IE . Generally, collector current is taken along y-axis and collector-base voltage along x-axis. Fig. 8.15 shows the output characteristics of a typical transistor in CB arrangement. The following points may be noted from the characteristics :

(i) The collector current IC varies with VCB only at very low voltages ( < 1V). The transistor is never operated in this region.

(ii) When the value of VCB is raised above 1 − 2 V, the collector current becomes constant as indicated by straight horizontal curves. It means that now IC is independent of VCB and depends upon IE only. This is consistent with the theory that the emitter current flows almost entirely to the collector terminal. The transistor is always operated in this region.

(iii) A very large change in collector-base voltage produces only a tiny change in collector current. This means that output resistance is very high.

Output resistance

It is the ratio of change in collector-base voltage (ΔVCB) to the resulting change in collector current (ΔIC ) at constant emitter current i.e.

The output resistance of CB circuit is very high, of the order of several tens of kilo-ohms. This is
not surprising because the collector current changes very slightly with the change in VCB.

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