Input and Output Characteristic Curves of CB Transistor

Here in this article we will going to discuss about characteristic curves of CB transistor like static input and static output characteristic curve of CB transistor (Common Base).

Characteristic Curves of Transistor in Common Base Configuration

Figure 1 shows a PNP transistor connected in Common Base (CB) configuration. Base is the terminal common to the input side and the output side and this terminal has been grounded. Then we are left with two voltage variables namely V_EB and V_CB. Further the current I_B is ignored since it is nothing but -(I_E + I_C). Then we are left with two current variables namely V_EB, V_CB, I_E and I_C any two may be selected as the independent variables and the other two as the dependent variables. In any transistor we select input current and output voltage as the independent variables and output current and input voltage as the dependent variables. Then, I_C and V_EB may be expressed in terms of V_CB and I_E as per following equation:

$I_C = f_1(V_{CB},I_E)$ ………..(1)

$V_{EB} = f_2(V_{CB},I_E)$ ……….(2)

Hence, for a CB transistor we may be obtained the following two families of static characteristic curves plotting equations (1) and (2).

Static output characteristic curves plotting equation (1). Thus, we plot I_C against V_CB with I_E as parameter i.e. plot curves for different values of I_E.
Static input characteristic curves plotting equation (2). Thus, we plot V_EB against I_E with V_CB as the parameter i.e. plot a set f curves for different values of V_CB.

Circuit Setup | Input and Output Characteristic Curves of CB Transistor

Figure 2 gives the basic circuit set up for determining the static characteristic curves of a PNP transistor. Current I_E may be varied by potentiometer R₁. But, V_BE is low typically less than 1 volts. Hence, we include a series resistor R_S (about 1 k-ohm) in the emitter circuit to keep I_E low.

Voltage V_CB may be varied by potentiometer R₂. Millimeter and voltmeters are connected as shown in figure 2 to read I=E_,I_C, V_EB and V_CB.

Static Output Characteristics of a PNP CB Transistor | Input and Output Characteristic Curves of CB Transistor

Figure 3 shows the static output characteristic curves of a typical PNP Ge grown junction transistor in CB configuration. Experimental procedure consists in setting upto circuit as shown in figure 2, fixing I_E initially to zero, increasing the magnitude of V_CB from zero in steps of say 1 volt, noting the corresponding I_C and plotting I_C against V_CB. The procedure is repeated for higher values of I_E say 10 mA, 20 mA etc. Thus, we get the curves shown in figure 3. In fact, these curves are nothing but plot of equation (2).

Active Region | Input and Output Characteristic Curves of CB Transistor:

For active region operation. J_E is forward biased whereas J_C is reverse biased. Consider the operation with I_E = 0. Then, the transistor behaves as a reverse biased diode constituted by the order of microamperes in Ge transistor and nanoamperes in Si transistor. I_CO remains constant for all value of collector voltage V_CB below the breakdown potential. In pnp transistor, I_CO flows from base to collector and hence is considered negative since assumed positive direction of I_CO is taken to be the same as that of I_C i.e. into the device. In npn transistor, I_CO flows from collector to base and is, therefore, considered positive.

Next let a small emitter current I_E (say 10 mA) flow in the emitter circuit. then a fraction $\alpha$ I_E of this current reaches the collector and contributes $-\alpha$ I_E to the total collector current I_C as given by equation $I_C = I_{CO}-\alpha I_E$ . In the active region, both components I_CO and $-\alpha I_E$ , collector current I_C almost equals $-\alpha I_E$ .

Output characteristics for I_E = 20 mA, 30 mA etc. are similar to the characteristic for I_E = 0 but shifted upward by $-\alpha I_E$ . However, because of the Early effect, the output characteristics have a small upward slope. This slight upward slope of the output characteristic results in a finite output conductance instead of zero output conductance i.e. very large output resistance instead of finite output resistance.

Saturation Region | Input and Output Characteristic Curves of CB Transistor

In saturation region both J_E and J_C are forward biased. Hence, in pnp transistor, the saturation region lies to the left of ordinate V_CB = 0 and above the characteristic for I_E = 0. In this region, the voltage has dropped near the bottom of the characteristic with V_CB = 0 and hence bottoming is said to take place. In a PNP transistor, V_CB is actually slightly in this region i.e. J_C is forward biased. Hence, collector current I_C increases exponentially with V_CB as per basic diode relationship. With J_C forward biased, hole current flows from p-type collector region across J_C to n-type base region. As a result, the net hole current magnitude drops rapidly as shown in figure 4. If the forward bias is J_C is further increased, I_C may even become positive.

Cutoff Region | Input and Output Characteristic Curves of CB Transistor

The output characteristics are almost parallel. But only the characteristic for I_E = 0 passes through the origin. This output characteristic for I_E = 0 does not coincide with the voltage axis. Rather I_C equals I_CO, a few $\mu A(n-A)$ for Ge (Si) transistor. Since I_CO is very small, the characteristic appears to coincide with the voltage axis. The region below and to the right of characteristic for I_E = 0 is called the cutoff region and in the region both J_E and J_C are reverse biased.

Static Input Characteristic of CB Transistor | Input and Output Characteristic Curves of CB Transistor

Figure 4 gives the static input characteristic of a typical CB pnp Ge transistor. The experimental procedure consists in setting up the circuit as shown in figure 2, fixing V_CB = 0, increasing the magnitude of I_E from in steps of say 5mA, nothing the corresponding V_EB and platting V_EB against I_E. the procedure is repeated for other values of V_CB say -10 volts, -20 volts etc. and also with collector junction open. Thus, we get the curve shown in figure 4. In fact, these curves are nothing but plot of equation 2.

These input characteristics are simply the forward characteristics of J_E for various values of V_CB. Thus, with V_CB open, the input characteristic is nothing but the forward characteristic of J_E acting as a pn diode. With V_CB = 0, i.e. with collector shorted to base, the collector sweeps away the minority carrier holes from base and the base therefore, attract more holes from the emitter. Hence, the characteristic for V_CB = 0 is shifted downward relative to that for V_CB open. The cut-in voltage V_V is approximately 0.1-volt (0.5 volt) for Ge (Si) transistor for V_CB = 0.

For a given value of V_EB, any increases in |V_CB|, as per Early effect, cause reduction in the effective base width, increases in the gradient of the minority carrier concentration in the base region and hence increases in I_E. As a result, with increases of |V_CB|, input characteristic curves shift downward as shown in figure 4.

It may be seen that a small increase in V_EB causes large increases in I_E. Thus, the dynamic input resistance $\dfrac{\partial V_{EB}}{\partial I_E}$ of CB transistor is quite low. This is true for both pnp and npn transistors.