Audio Amplifi ers 311in Figure 9.26(a) , and the JFET active load, shown in Figure 9.26(b) , act to increase
the dynamic impedance ofR 1 , although the DC bootstrap, which can, of course, be
constructed using complementary devices, has the advantage of offering a low output
impedance. The amplifi ed diode, shown as Figure 9.26(c) , is a device that is much used
as a means of generating the forward bias required for the transistors used in a push–
pull pair of output emitter followers, particularly if it is arranged so that Q 1 can sense
the junction temperature of the output transistors. It can also be used, over a range of
relatively low voltages, as an adjustable voltage source to complement the fi xed voltage
references provided by zener and avalanche diodes, band-gap references (IC stabilizers
designed to provide extremely stable low voltage sources), and the wide range of voltage
stabilizer ICs. Finally, when some form of impedance transformation is required, without
theVbe offset of an emitter follower, this can be contrived as shown by putting two
complementary emitter followers in series. This layout will also provide a measure of
temperature compensation.
9.13 Slew Rate Limiting ...............................................................................................
This is a potential problem that can occur in any voltage amplifi er or other signal
handling stage in which an element of load capacitance (which could simply be circuit
stray capacitance) is associated with a drive circuit whose output current has a fi nite
limit. The effect of this is shown in Figure 9.27. If an input step waveform is applied to
network (a), then the output signal will have a waveform of the kind shown at ‘ a ’ , and the
slope of the curve will refl ect the potential difference that exists, at any given moment,
between the input and the output. Any other signal that is present at the same time will
pass through this network, from input to output, and only the high-frequency components
will be attenuated.
However, if the drive current is limited, the output waveform from circuit 9.27 (b) will be
as shown at ‘ b ’ and the slope of the output ramp will be determined only by the current
limit imposed by the source and the value of the load capacitance. This means that any
other signal component that is present, at the time the circuit is driven into slew rate
limiting, will be lost. This effect is noticeable, if it occurs, in any high-quality audio
system and gives rise to a somewhat blurred sound—a defect that can be lessened or
removed if the causes (such as too low a level of operating current for some amplifying
stage) are remedied. It is prudent, therefore, for the amplifi er designer to establish the
possible voltage slew rates for the various stages in any new design and then to ensure