**Capacitor Coupled Class AB Output Stage:**

The basic circuit of a Class-AB amplifier using a complementary emitter follower output stage and a Capacitor Coupled Class AB Output Stage load is shown in Fig. 18-18. The circuit is termed a complementary symmetry amplifier. Transistor Q_{1} and resistors R_{1}, R_{2}, R_{C}, and R_{E1} comprise a common emitter amplifier stage that produces all of the circuit voltage gain. The output of Q_{1} is developed across R_{C} and applied to the bases of Q_{2} and Q_{3}. Capacitor C_{o} ac couples R_{L}, and dc isolates R_{L} to keep it from affecting the circuit bias conditions.

The total voltage drop (V_{B}) across diodes D_{1} and D_{2} and resistor R_{B} forward biases the base-emitter junctions of Q_{2} and Q_{3} to avoid cross-over distortion. Emitter resistors R_{E2} and R_{E3} help limit the quiescent current through Q_{2} and Q_{3}. Adjustment of the bias voltage (V_{B}) is provided by variable resistor R_{B}. The diodes have voltage drops (V_{D}) that approximately match the, output transistor V_{BE} levels. Also, V_{D} does not change significantly when the diode current changes, so, the diodes behave like bypassed resistors. The diodes and output transistors can be **thermally-coupled** by mounting D_{1} and Q_{2} on a single heat sink, and D_{2} and Q_{3} on a single heat sink. In this case, V_{D} follows the V_{BE} level changes with temperature, thus stabilizing the transistor bias conditions over a wide temperature range. The junction of R_{E2} and R_{E3} must be biased to V_{CC}/2, so that the output voltage to C_{o} can swing by equal amounts in positive- and negative-going directions.

**Class-AB Capacitor-Coupled Amplifier Design:**

Design of the type of circuit shown in Fig. 18-18 is largely a matter of selecting appropriate resistor voltage drops and current levels, and then applying Ohm’s law to calculate the resistor values. The peak output voltage (V_{P}) and peak output current (I_{P}) can be determined from earlier Equations, respectively. Those equations were developed for the power delivered to a transformer primary, but they apply equally to power delivered to any load resistor.

The voltage drops across R_{E2} and R_{E3} when the peak output current is flowing are typically selected as 5% to 10% of the peak output voltage. This is illustrated in Fig. 18-19(a) and (b) where the output capacitor is represented as an ac short-circuit. So,

It should be remembered that R_{E2} and R_{E3} are included to help stabilize the transistor quiescent currents at a level that eliminates cross-over distortion in the output waveform. For the type of amplifier circuit in Fig. 18-18, without overall negative feedback, it is best to select the emitter resistors as large as possible. Smaller emitter resistors can be used in circuits with dc and ac negative feedback.

When the output is at its negative-going peak, V_{CE1 }should be 1 V minimum, to ensure that Q_{1} does not go into saturation. Also, V_{E1} should typically be 3 V. So, the minimum level of V_{C1} is typically 4 V, [Fig. 18-19(b)]. Similarly, when the output is at its positive-going peak, there must be an appropriate minimum voltage drop across resistor R_{C}, [Fig. 18-19(a)]. It is not acceptable to set a 1 V minimum for V_{RC}, because the current through R_{C} would be too Small for the required peak base current to Q_{2}. So, it is best to select,

The minimum current through (I_{RC(min)}) should typically be selected 1 mA larger than the peak base current for the output transistors. R_{C} is calculated from V_{RC(min)} and I_{RC(min)}.

Referring to Fig. 18-19(a), the supply required to produce the positive output peak voltage is

Also, from Fig. 18-19(b), the negative output peak requires,

So, the total supply voltage is,

The voltage drop across the diodes and R_{B} should just bias Q_{2} and Q_{3} on for Class-AB operation, (see Fig. 18-20). The current through R_{B} is the Q_{1} quiescent current (I_{CQ1}), and this is calculated from R_{C} and the dc voltage drop across R_{C}. V_{RC(dc)} equals V_{C1(dc)}, and the sum of them equals (V_{CC} – V_{B}), (see Fig. 18-20). So,

The resistance of R_{B} is now calculated from V_{RC(dc)} and I_{CQ1}. Resistors R_{1}, R_{2}, and R_{E1} are determined in the usual manner for an emitter current bias circuit.

The output transistors should be specified in terms of their maximum voltage, current, and power dissipation. The maximum V_{CE} for Q_{2} and Q_{3} (in Fig. 18-18) is the total supply voltage (V_{CC}). Maximum current for Q_{2} and Q_{3} is the peak load current plus the selected quiescent current (I_{Q23}). This normally 1.1 I_{p}. Transistor power dissipation is calculated by determining the dc power delivered to the output stage from the power supply, and then subtracting the ac load power. The remainder is halved to find the power dissipated in each transistor. Equation 18-12 applies; P_{T} = 0.5 (P_{i} – P_{o}). Recall that the transistors must be operated within the safe operating area of the characteristics.

With a capacitor-coupled load, current is drawn from the power supply during the positive half-cycle of the output, but not during the negative half cycle. The capacitor acts as an energy reservoir to supply load current when the output is negative-going. Consequently, the supply current has a half-wave rectified waveform (see Fig. 18-21), and so,

The dc supply power to the output stage is,

As always, with the exception of the capacitor selected to set the circuit low 3 dB frequency (f_{1}), each capacitor impedance is selected as one tenth of the impedance in series with the capacitor. Where there is no overall negative feedback, the capacitor with the lowest-value series-connected impedance is normally selected to set f_{1}. For the circuit shown in Fig. 18-18, the impedance looking into the emitter of Q_{1} is h_{ib1}, and this is in series with C_{E}. If h_{ib1} is smaller than R_{L}, then at f_{1};

Most power amplifiers typically have R_{L} = 8 Ω or 16 Ω. So, the load-coupling capacitor normally sets the circuit low 3 dB frequency.