Power Supply Decoupling:

Power Supply Decoupling – High-power amplifiers require high supply current levels, so unregulated power supplies are often employed to avoid the power wasted in a series regulator. The high ripple voltage that occurs with unregulated supplies can be amplified to appear at speaker outputs as very unpleasant power supply hunt Supply decoupling components C_D and R₁₅ are employed as shown in

Fig. 18-29(a) to combat hum. Capacitive impedance X_CD forms an ac voltage divider with resistor R₁₅, so that the ripple amplitude is attenuated, as illustrated. The resistance of R₁₅ is usually selected approximately equal to emitter resistor R_E1, and X_CD is made very much smaller than R₁₅ at the ripple frequency (f_r). If X_CD = R₁₅/100 at f_r, the, ripple voltage will be attenuated approximately by a factor of 100. The dc voltage drop across R₁₅ must be taken into account when calculating V_CC.

Figure I8-29(b) shows power supply decoupling capacitors (C_D1 and C_D2) without any series connected resistors. These are often used even when the supply voltages are regulated and hum is not a problem. When a sudden high output current is switched on or off the current change can produce short-lived (spike-type) voltage drops (transient) on the supply lines. These transients may be amplified to produce output distortion if they are allowed to appear at the supply lines for the first or second stages of a circuit. Capacitor C_D1 is a relatively high-capacitance component that might normally be expected to perform the necessary decoupling.

However, such capacitors usually offer a relatively high impedance to high-frequency variations or fast transients. Consequently, the high-frequency low-capacitance component C_D2 is required to ensure satisfactory decoupling. It is very important that decoupling capacitors be located right on circuit boards, as close as possible to the terminals of the circuit to be decoupled.

Increased Voltage Gain and Negative Feedback:

The Q₁ collector resistor (R_C) in the circuits discussed so far has its resistance limited by the need to supply base current to the npn output transistor. The resistance of R_C also dictates the Q₁ collector current level. This is shown by the I_C1(dc) calculation.

A larger resistance for R_C would give a greater voltage gain, which is desirable for negative feedback. It is shown that negative feedback reduces distortion and increases circuit bandwidth, so its use is necessary in all power amplifiers.

In Fig. 18-30 R_C is replaced by a constant current circuit constituted by transistor Q₇ together with emitter resistor R_E7, and base bias resistors R₁₃ and R₁₄. The minimum level of V_CE7 and the R_E7 voltage drop are selected to equal the Q₁ levels. This allows the voltage to the output stage to swing positively by the same amount as it can go negatively. The constant current circuit offers a high ac load resistance (1/h_oe7) for the Q₁ stage, to give the highest possible voltage gain.

Load Compensation:

All design and analysis calculations for power amplifiers assume a resistive load with a given resistance value (R_L). For audio amplifiers, the load is usually the coil of a speaker which, as illustrated in Fig. 18-31, combines coil inductance L_c and winding resistance R_C. The load impedance is, Z_L = R_c + j (2π f L_c), and clearly Z_L increases (from a low of R_c at dc) as the signal frequency increases. An 8 Ω speaker might offer an impedance of 8 Ω only at a frequency around 400 Hz. The fact that the load is inductive means that the load current lags the load voltage, and typically the phase angle could be as high as 60°.

Similarly, when capacitive loads are involved the load current can lead the load voltage. The phase difference between load current and voltage can put stress on the output transistors, so output compensating components are often included to minimize the phase difference.

Figure 18-32 shows the typical arrangement of the compensating components. Inductor L_x and its parallel-connected resistor R_x are usually recommended by device and IC manufacturers for isolating a capacitive load. Capacitor C_o and series-connected resistor R_o help to correct the lagging phase angle of an inductive load.