Transistor as an Amplifier

An AMPLIFIER is a device that increases the amplitude of an alternating signal -- voltage, current, or power. A transistor achieves this in the common emitter configuration by superimposing a small sinusoidal AC input signal $v_i$ on top of the transistor's steady DC bias (Fig. 16.25): the resulting small sinusoidal variations in base current $I_B$ produce correspondingly larger sinusoidal variations in collector current $I_C$, and hence in the output voltage $V_O$ across the load resistor $R_L$ placed in the collector circuit. A coupling capacitor in the output circuit blocks the DC component of the collector voltage, passing through only its AC-varying part as the useful output.\n\nWith no AC signal applied, Kirchhoff's voltage law gives, for the output (collector) loop, $V_{CC}=V_{CE}+I_CR_L$ --- (16.9), and for the input (base) loop, $V_{BB}=V_{BE}+I_BR_B$ --- (16.10). When the AC signal $v_i$ IS applied, it produces a small change in the base-emitter voltage and hence in the emitter (and collector) current; from Eq. (16.9), since $V_{CC}$ is fixed, any change $\Delta I_C$ in collector current must be balanced by an equal and opposite change in $V_{CE}$: $0=\Delta V_{CE}+R_L\,\Delta I_C$, i.e. $\Delta V_{CE}=-R_L\,\Delta I_C$. This changing $V_{CE}$ IS the amplified output signal, $v_o=\Delta V_{CE}=-\beta_{AC}R_L\,\Delta I_B$, where the AC current gain is defined as $\beta_{AC}=\dfrac{\Delta I_C}{\Delta I_B}$ (numerically almost identical to $\beta_{DC}$ for normal operating voltages). The VOLTAGE GAIN of the amplifier is then $A_v=\dfrac{v_o}{v_i}=-\dfrac{\beta_{AC}R_L}{r_i}$; the negative sign shows the output signal is exactly OUT OF PHASE (180$^\circ$) with the input, and the magnitude of $A_v$ is what is normally quoted, $|A_v|=\beta_{AC}R_L/r_i$. Combining current gain and voltage gain gives the POWER GAIN, $A_p=\beta_{AC}A_v$. A transistor genuinely amplifies POWER (since $\beta_{AC}\gg1$ an …