Bipolar Transister

The Common Base Configuration :

If the base is common to the input and output circuits, it is know as common base configuration as shown in fig. 1.

Fig. 1

For a pnp transistor the largest current components are due to holes. Holes flow from emitter to collector and few holes flow down towards ground out of the base terminal. The current directions are shown in fig. 1.

(I_E = I_C + I_B ).

For a forward biased junction, V_EB is positive and for a reverse biased junction V_CB is negative. The complete transistor can be described by the following two relations, which give the input voltage V_EB and output current I_C in terms of the output voltage (V_CB) and input current I_E.

V_EB = f₁(V_CB, I_E)
I_C= f₂(V_CB, I_E) 

The output characteristic:

The collector current I_C is completely determined by the input current I_E and the V_CB voltage. The relationship is given in fig. 2. It is a plot of I_C versus V_CB, with emitter current I_E as parameter. The curves are known as the output or collector or static characteristics. The transistor consists of two diodes placed in series back to back (with two cathodes connected together). The complete characteristic can be divided in three regions.

Figure 7.2

(1). Active region:

In this region the collector diode is reverse biased and the emitter diode is forward biased. Consider first that the emitter current is zero. Then the collector current is small and equals the reverse saturation current I_CO of the collector junction considered as a diode.

If the forward current I_B is increased, then a fraction of I_E ie. a_dcI_E will reach the collector. In the active region, the collector current is essentially independent of collector voltage and depends only upon the emitter current. Because a_dc is, less than one but almost equal to unity, the magnitude of the collector current is slightly less that of emitter current. The collector current is almost constant and work as a current source.

The collector current slightly increases with voltage. This is due to early effect. At higher voltage collector gathers in a few more electrons. This reduces the base current. The difference is so small, that it is usually neglected. If the collector voltage is increased, then space charge width increases; this decreased the effective base width. Then there is less chance for recombination within the base region.

(2). Saturation region:

The region to the left of the ordinate V_CB = 0, and above the I_E = 0, characteristic in which both emitter and collector junction are forward biased, is called saturation region.

When collector diode is forward biased, there is large change in collector current with small changes in collector voltage. A forward bias means, that p is made positive with respect to n, there is a flow of holes from p to n. This changes the collector current direction. If diode is sufficiently forward biased the current changes rapidly. It does not depend upon emitter current.

(3). Cut off region:

The region below I_E = 0 and to the right of V_CB for which emitter and collector junctions are both reversed biased is referred to cutoff region. The characteristics I_E = 0, is similar to other characteristics but not coincident with horizontal axis. The collector current is same as I_CO. I_CBOCO. It means collector to base current with emitter open. This is also temperature dependent. is frequently used for I

The Input Characteristic:

In the active region the input diode is forward biased, therefore, input characteristic is simply the forward biased characteristic of the emitter to base diode for various collector voltages. fig. 3. Below cut in voltage (0.7 or 0.3) the emitter current is very small. The curve with the collector open represents the forward biased emitter diode. Because of the early effect the emitter current increases for same VEB. (The diode becomes better diode). When the collector is shorted to the base, the emitter current increases for a given VEB since the collector now removes minority carriers from the base, and hence base can attract more holes from the emitter. This mean that the curve VCB= 0, is shifted from the character when VCB = open.

Fig. 3

Equivalent circuit of a transistor: (Common Base)

In an ideal transistor, adc= 1. This means all emitter electrons entering the base region go on to the collector. Therefore, collector current equals emitter current. For transistor action, emitter diode acts like a forward bias diode and collector diode acts like a current source. The equivalent circuits of npn and pnp transistors are shown in fig. 4. The current source arrow points for conventional current. The current source is controlled by emitter current. Common Base Amplifier: The common base amplifier circuit is shown in Fig. 1. The VEE source forward biases the emitter diode and VCC source reverse biased collector diode. The ac source vin is connected to emitter through a coupling capacitor so that it blocks dc. This ac voltage produces small fluctuation in currents and voltages. The load resistance RL is also connected to collector through coupling capacitor so the fluctuation in collector base voltage will be observed across RL. The dc equivalent circuit is obtained by reducing all ac sources to zero and opening all capacitors. The dc collector current is same as IE and VCB is given by VCBCCCC. = V - I R Fig. 1 These current and voltage fix the Q point. The ac equivalent circuit is obtained by reducing all dc sources to zero and shorting all coupling capacitors. r'e represents the ac resistance of the diode as shown in Fig. 2. Fig. 2 Fig. 3, shows the diode curve relating IE and VBE. In the absence of ac signal, the transistor operates at Q point (point of intersection of load line and input characteristic). When the ac signal is applied, the emitter current and voltage also change. If the signal is small, the operating point swings sinusoidally about Q point (A to B). Fig .3 If the ac signal is small, the points A and B are close to Q, and arc A B can be approximated by a straight line and diode appears to be a resistance given by If the input signal is small, input voltage and current will be sinusoidal but if the input voltage is large then current will no longer be sinusoidal because of the non linearity of diode curve. The emitter current is elongated on the positive half cycle and compressed on negative half cycle. Therefore the output will also be distorted. r'e is the ratio of ΔVBE and Δ IE and its value depends upon the location of Q. Higher up the Q point small will be the value of r' e because the same change in VBE produces large change in IE. The slope of the curve at Q determines the value of r'e. From calculation it can be proved that. In general, the current through a diode is given by Where q is he charge on electron, V is the drop across diode, T is the temperature and K is a constant. On differentiating w.r.t V, we get, The value of (q / KT) at 25°C is approximately 40. Therefore, or,       To a close approximation the small changes in collector current equal the small changes in emitter current. In the ac equivalent circuit, the current �iC' is shown upward because if �ie' increases, then �iC' also increases in the same direction. Voltage gain: Since the ac input voltage source is connected across r'e. Therefore, the ac emitter current is given by ie = Vin / r'e or,        Vin = ie r'e The output voltage is given by Vout = ic (RC || RL) Under open circuit condition vout = ic Rc

Fig. 4

Example-1

Find the voltage gain and output of the amplifier shown in fig. 4, if input voltage is 1.5mV.

Fig. 4

Solution:

The emitter dc current I E is given by

Therefore, emitter ac resistance =

or,           A_V= 56.6

and, V_out = 1.5 x 56.6 = 84.9 mV

Example-2

Repeat example-1 if ac source has resistance R s = 100 W .

Solution:

The ac equivalent circuit with ac source resistance is shown in fig. 5.

Fig. 5

The emitter ac current is given by

or,              

Therefore, voltage gain of the amplifier =

    

and,                       V_out = 1.5 x 8.71 =13.1 mV


Common Emitter Curves:

The common emitter configuration of BJT is shown in fig. 1.

Fig. 1

In C.E. configuration the emitter is made common to the input and output. It is also referred to as grounded emitter configuration. It is most commonly used configuration. In this, base current and output voltages are taken as impendent parameters and input voltage and output current as dependent parameters

V_BE = f₁ ( I_B, V_CE )
I_C = f₂( I_B, V_CE )

Input Characteristic:

The curve between I_B and V_BE for different values of V_CE are shown in fig. 2. Since the base emitter junction of a transistor is a diode, therefore the characteristic is similar to diode one. With higher values of V_CEBE is zero and I_B is also zero. collector gathers slightly more electrons and therefore base current reduces. Normally this effect is neglected. (Early effect). When collector is shorted with emitter then the input characteristic is the characteristic of a forward biased diode when V

Fig. 2