Environmental Engineering Reference
In-Depth Information
Chapter 6
FUNDAMENTAL FEEDBACK CONFIGURATIONS
In this chapter we will consider the four basic types of feedback
amplifier: the
series-shunt,
the
shunt-series,
the
shunt-shunt
and the
series-
series
configurations. These are used to realise voltage, current,
transresistance and transconductance closed-loop amplifiers, respectively,
and are capable of significantly reducing the dependence of forward transfer
characteristics on the ill-controlled parameters implicit to the open-loop
gain. Particularly, this chapter analyses first the low-frequency performance
of these architectures, which are normally realised by multi-stage topologies,
and subsequently gives frequency compensation guidelines. At this purpose,
the results derived in Chapter 2 and Chapter 5 are extensively exploited.
6.1 SERIES-SHUNT AMPLIFIER
The AC schematic diagram of a
series-shunt
feedback amplifier is
depicted in Fig. 6.1. In this circuit, the output voltage, is sampled by
feedback network composed of the resistances and The sampled
voltage is fed back in such a way that the closed-loop input voltage,
is
the sum of (the voltage across the input port of the amplifier) and
(developed across in the feedback subcircuit). Since the
output port of the feedback configuration can be viewed as connected in
series with the amplifier input port. On the other hand, output voltage
sampling constraints the net load current, to the algebraic sum of the
amplifier output port current, and the feedback network input current,
Accordingly, the output topology is indicative of a shunt connection of the
feedback subcircuit to the amplifier output port. The fact that the voltage is
fed back to a voltage-driven input port renders the driving point input
resistance,
of the closed-loop amplifier large, whereas the output
