148 Chapter 5
supply line through the constant current source, Q6 and Q7, because this has a very high
output impedance in comparison with the emitter impedance of Q1 and Q2, so any AC
ripple passing down this path would be very highly attenuated. However, there would be
no attenuation of rubbish entering the signal line via R5, so that, in a real-life amplifi er,
R5 would invariably be replaced by another constant current source, such as that arranged
around Q7 and Q8.
For the negative supply rail, the cascode connection of Q10 would give this device an
exceedingly high output impedance, so any signal entering via this path would be very
heavily attenuated by the inevitable load impedance of the amplifi er. Similarly, the output
impedance of the cascode-connected transistors Q3 and Q4 would be so high that the
voltage developed across the current mirror (Q5 and Q6) would be virtually independent
of any ve rail ripple voltage. In general, the techniques employed to avoid supply line
intrusion are to use circuits with high output impedances wherever a connection must
be made to the supply line rails. In order of effectiveness, these would be a cascode-
connected fi eld-effect transistor or bipolar device, a constant current source, a current
mirror, or a decoupled output, such as a bootstrapped load. HT line decoupling, by means
of an LF choke or a resistor and a shunt-connected capacitor, such as R2 and C2, was
widely used in valve amplifi er circuitry, mainly because there were few other options
available to the designer. Such an arrangement is still a useful possibility if the current
fl ow is low enough for the value of R2 to be high and if the supply voltage is high enough
for the voltage drop across this component to be unimportant. It still suffers from the
snag that its effectiveness decreases at low frequencies where the shunt impedance of C2
begins to increase.
5.8 Voltage Regulator Systems ...................................................................................
Electronic voltage regulator systems can operate in two distinct modes, each with their
own advantages and drawbacks: shunt and series. The shunt systems operate by drawing
current from the supply at a level that is calculated to be somewhat greater than maximum
value, which will be consumed by the load. A typical shunt regulator circuit is shown
in Figure 5.2(a) , in which the regulator device is an avalanche or Zener diode or a two-
terminal band-gap element for low current, high stability requirements. Such simple
circuits are normally only used for relatively low current applications, although high
power avalanche diodes are available. If high power shunt regulators are needed, a better