Common-Emitter (C-E) Characteristics

A transistor can be arranged in one of three ways or modes in circuits. Figure 39.11 shows the common-base (C-B) mode, so-called because the base is common to the input (emitter-base) and output (collector-base) circuits. Figure 39.12 shows the common-emitter (C-E) mode. A third arrangement is a common-collector (C-C) mode. As we explain later, the common-emitter provides satisfactory current amplification and is widely used in a.f. amplifiers.

Figure 39.12 shows a circuit for obtaining the characteristics of a n-p-n transistor in the common-emitter mode. X and Y may be batteries of 1^.5 V and 4^.5 V respectively, connected to potentiometers P and Q of 1 kOhm and 5 kOhm. This enables the base-emitter p.d., V_BE or V _B) and the collector-emitter p.d., усе or V_c to be varied. The p.d. is measured by high resistance voltmeters, prefer­ably d.c. solid state voltmeter types capable of measuring p.d. in steps such as 50 mV. The meter for base current, I _B, should be a microammeter and for the collector current, I_c, a milliammeter. Typical results are shown in Fig. 39.13 (i), (ii) and (iii).

Output characteristic (I_c — I_c, with I_B constant). The 'knee' of the curves shown in Fig. 39.13 (i) correspond to a low p.d. of the order of 0^.2 V. For higher p.d. the output current I_C varies linearly with V _c for a given base current. The linear part of the characteristic is used in a.f. amplifier circuits, so that the output voltage variation is then undistorted.

The output resistance r₀ is defined as D V_C /D I _C, where the changes take place on the straight part of the characteristic. r₀ is an a.c. resistance; it is the effective resistance in the output circuit for an a.c. signal input. It should be distinguished from the d.c. resistance, V_C/I_C, which is not required in amplifier circuit analysis.

The small gradient of the straight part of the characteristic shows that r₀ is high. For example, suppose D V _С = 2 V and D I_C = 0^.2 mA = 2 x 10 ⁵ A. Then r₀ - 2/(2 x 10^-5) = 100000 Ohm. If a varying resistance load is used in the output or collector circuit, the high value of r₀ relative to the load shows that the output current is fairly constant. So the output voltage is proportional to the load resistance.

Transfer characteristic (I_c - ib, V_c constant). The output current I c varies fairly linearly with the input current I _B, Fig. 39.13 (ii). The current transfer ratio b, or current gain, is defined as the ratio D I _с/ DI_B under a.c. signal conditions It should be distinguished from the d.c. current gain, I _C/ I _B. From Fig. 39.13 (ii),

Input characteristic (I_B — V _B, V_c constant). The input resistance r _i is defined as the ratio D V _В/D I _В. As the input characteristic in Fig. 39.13 (iii) is non-linear, then r₁ varies. At any point of the curve, r _iis equal to the gradient of the tangent to the curve and is of the order of kilohms.

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