AC Load Line of BJT

AC Equivalent Circuits – Capacitors behave as short-circuits to ac signals, so in the ac equivalent circuit for a transistor circuit all capacitors must be replaced with short-circuits. Power supplies also behave as ac short-circuits, because the dc supply voltage is not affected by ac signals. Also, all power supplies have large-value capacitors at the output terminals, and these will offer short-circuits to ac signals. Substituting short-circuits in place of the power supply and all capacitors in the circuit in Fig. 6-6(a) gives the ac equivalent circuit in Fig. 6-6(b). If R_L is present, as shown, it appears in parallel with R_C in the ac equivalent circuit of AC Load Line of BJT.

Once the ac equivalent circuit is drawn, the circuit ac performance can be investigated by drawing an AC Load Line of BJT, and by substituting a transistor model in place of the device.

AC Load Lines:

The dc load for the circuit in Fig. 6-6(a) is (R_C + R_E), consequently, the dc load line is drawn for a total resistance of (R_C + R_E). Because the emitter resistor is capacitor bypassed in Fig. 6-6(a), resistor R_E is not part of the circuit ac load. If external load R_L were not present, the circuit ac load would simply be R_C. With R_L capacitor-coupled to the circuit output, the ac load is R_C||R_L. An AC Load Line of BJT may now be drawn to represent the circuit ac performance.

When there is no input signal, the transistor voltage and current conditions are exactly indicated by the Q-point on the dc load line. An ac signal causes the transistor voltage and current levels to vary above and below the Q-point. Therefore, the Q-point is common to both the ac and dc load lines. Starting from the Q-point, another point is found on the AC Load Line of BJT by taking a convenient collector current change (usually ΔI_C = I_CQ) and calculating the corresponding collector-emitter voltage change (ΔV_CE) as shown in Fig. 6-8.