Amplification in Transistor:

Current Amplification in Transistor – We already know that a transistor can be used for current amplification. A small change in the base current (ΔI_B) produces a large change in collector current (ΔI_C) and a large emitter current change (ΔI_E), [see Fig. 4-16(a) and (b)]. Rewriting Eq. 4-5, the current gain from the base to collector can be stated in terms of current level changes

The increasing and decreasing levels of input and output currents may be defined as alternating quantities. In this case, small (lower-case) letters are used for the subscripts. Thus, I_b is an ac base current, I_c is an ac collector current, and I_e is an ac emitter current. The alternating current gain from base to collector may now be stated as,

As in the case of dc current gain, two parameter symbols are available for common-emitter ac current gain; β_ac and h_fe. Either symbol may be used, but, once again, h_fe is the symbol employed on transistor data sheets.

Voltage Amplification:

Refer to the circuit in Fig. 4-17(a) and assume that the transistor (Q₁) has β_dc = 50. Note that the 0.7 V dc voltage source (V_B) forward biases the transistor base-emitter junction. An ac signal source (υ_i) in series with V_B provides a ± 20 mV input voltage. The transistor collector is connected to a 20 V dc voltage source (V_CC) via the 12 kΩ collector resistor (R₁).

If Q₁ has the I_B/V_BE characteristic shown in Fig. 4-17(b), the 0.7 V level of V_B produces a 20 μA base current. This gives,

The dc level of the transistor collector voltage can now be calculated as,

The V_C and I_C levels are shown on Fig. 4-17(a).

While the ac input voltage (υ_i) is zero, the transistor collector voltage remains at 8 V. When υ_i causes a base voltage variation (ΔV_B) of ±20 mV, the base current changes by ±5 μA, as shown in Fig. 4-17(b). The I_B change produces a collector current change.

Figure 4-18(a) shows that ΔI_C causes a change in the voltage drop across R₁, and thus produces a variation in the transistor collector voltage.

The circuit ac input is the base voltage change (ΔV_B), and the ac output is the collector voltage change (ΔV_C). Because the output is greater than the input, the circuit has a voltage gain; it is a voltage amplifier. The voltage gain (A_υ) is the ratio of the output voltage to the input voltage.

As already discussed for current changes, the increasing and decreasing levels of voltage can be referred to as no quantities. The ac signal voltage (υ_i) produces the ac base current (I_b), and this generates the ac collector current (I_C) which produces the ac voltage change across R₁, [see Fig. 4-18(b)]. The equation for ac voltage gain is,

Substituting the ac quantities for Fig. 4-17(a) into Eq. 4-8 gives A_υ = 150 once again.