Handbook for Sound Engineers

Consoles 839

Pragmatists, who close down the bandwidth of an
amplifier as rapidly as possible outside the required
passband, maximizing stability phase margin and RF
neutrality. Then there are the Purists, who maintain
circuit gain as far out and as high as possible, walking
the tightrope of stability—usually in deference to the
in-band phase linearity.

The normal, easiest, and most flexible way to deter-
mine the closed loop roll-off of a circuit is by means of
a feedback phase-leading capacitor across the main
output-to-inverting-input feedback resistor. A typical
arrangement is shown in Fig. 25-18. Generally, the need
to properly define the bandwidth of a gain block by just
such a means automatically takes care of the matter,
although it’s dangerous design practice to assume that
the two requirements—phase-margin determination and
bandwidth limitation—are always mutually satisfiable.

A fairly common eroder of phase margin and
progenitor of instability is stray capacitance from the
inverting input of the amplifier to ground. This capaci-
tance, a combination of internal device, pinout, and
printed-circuit layout proximity capacitances, reacts
against the feedback impedance to increase the closed
loop gain at high frequencies. In normal circuits, even
the typical 5 pF or so is enough to tilt up the closed loop
gain parameters, threatening stability. Far worse is the
situation where the inverting input is extended quite
some distance along wiring, and worse yet, a bus—as in
a virtual-earth mixing amplifier—hundreds, and some-
times thousands, of picofarads may be lurking out there.
It can arise that despite a sizable time constant being
present in the feedback leg, none of the expected
high-frequency roll-off occurs since it is merely
compensating for the gain hike created by bus capaci-
tance. Ensuring required response and phase character-
istics using any virtual-earth mixer can only be done

properly with at least two orders of compensation

around the mix-amp and with the finished system up

and running completely, since any additional sources

modify the impedance presented by the bus.

To define just how much this unwanted gain can rise,

a small limiting resistor may be added as close to the

amplifier inverting input terminal as possible; this is at

the expense of the virtual-earth point now having a

minimum impedance based on the value of that resistor.

The resistor, incidentally, is also a measure of protection

against any radio-frequency signals on the bus being

rectified by the input stage junctions. Better yet, a small

(real!) inductance in series with the summing amplifier

input provides another means of out-of-band gain

reduction and RF immunity.

25.7.6 Time-Domain Effects

There is invariably a finite time taken for a signal

presented at the input of any amplifier to show an effect

at the output of the amplifier—the so-called transit time.

Every tiniest capacitance and consequent time constant

in the internal circuitry of the amplifier make this inevi-

table; electronics takes time to do things. This transit

time becomes an appreciably greater proportion of the

wavelength of the wanted signal as the frequency

increases, and as such it has to be taken into account.

Fig. 25-19 shows how the fixed transit time becomes

more relevant to increasing signal frequency. Ulti-

mately, of course, the transit time will become half the

time necessary for a wavelength of the signal frequency.

At that stage what emerges from the amplifier will be a

half wavelength or 180° out of phase. Before this point,

its detraction from phase margin with increasing

frequency can start to cause serious problems; at this

Figure 25-18. Feedback phase-leading stability compensation.

Feedback

network

resistors

Compensating

capacitor

A. Basic external feedback

compensation

B. Virtual-earth mixer with stray pinout and bus

capacitances.

Sources

Bus

Small

limiting

resistor

Comp C,

add to taste

Pinout & card

stray capacitance

Buss

capacitance

or wiring