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A d v a n c e d P o w e r A m p l i f i e r D e s i g n Te c h n i q u e s arrangement is that the filtering capacitor can have its ground referenced to the ground of the circuit that will be employing the resulting power rail. The second approach is to design the individual amplifier stages to have higher PSRR out to higher frequencies. In many cases the PSRR of an amplifier stage will fall with increasing frequency. Differential circuits and cascode circuits often exhibit better power supply rejection up to higher frequencies.
Output Stage Power Supply Rejection Although the output stage is usually just an emitter follower or source follower with some drivers in front of it, it is not immune to power supply garbage. This is largely because of the Early effect in the output transistors (or its analogous effect in MOSFETs). This is a weak effect in an emitter follower or source follower arrangement, but the amplitude of the garbage on the rails at this point is often quite large. This is of special concern when the output swings close to the rails. This effect is reduced in designs employing cascoded output stages. Splitting the power supply reservoir capacitors and separating them by as little as 0.1 Ί can make a big improvement here.
13.6  Input Common Mode Distortion An often-neglected source of distortion results from the common mode signal swing on the input differential pair of a noninverting power amplifier. The common mode voltage is simply equal to the input signal swing, which may be on the order of a couple of volts. The distortion can result from the collector-base voltages of the LTP changing with signal, causing beta to change via the Early effect. It can also result from changes in the collector-base junction capacitances as a result of their voltage dependence on Vcb. Nonlinear output impedance of the tail current source may also create common mode distortion. Techniques for minimizing input common mode distortion were discussed in Chapter 7. An input stage that has better common mode rejection will tend to have smaller common mode distortion. Conversely, steps taken to reduce common mode distortion will often improve common mode rejection. Noise on power supply rails is often seen as common mode noise by the input stage. For this reason, input stages with good common mode rejection will tend to have better PSRR.
Testing for Common Mode Distortion One way to test for the presence of common mode distortion is to drive the amplifier through a resistor connected to the inverting input, with no signal applied to the conventional noninverting input. This forces the amplifier to operate in the inverting mode, where there is no common mode signal swing on the input LTP. If distortion is significantly reduced under these conditions, it is likely that there is common mode distortion.
13.7  Resistor Distortion Some resistors can change their value slightly as a function of the voltage across them or the current through them. These effects are quite small and very difficult to measure in most resistors of reasonable quality. In some cases carbon composition resistors will