Page 129 - The Art and Science of Analog Circuit Design

Analog Breadboarding

Figure 9-15,

ground aunvK nruR QH (NHk
to torn wamha*. f Iw* to

Where the HF currents of a circuit are mostly internal (as is the case
with many ADCs) it is sufficient that we short-circuit its supplies at HF
so that it sees its supplies as stiff voltage sources at all frequencies.
When it is driving a load, the decoupling must be arranged to ensure
that the total loop in which the load current flows is as small as possible.
Figure 9-15 shows an emitter follower without supply decoupling—the
HF current in the load must flow through the power supply to return to
the output stage (remember that Kirchoff's Law says, in effect, that cur-
rents must flow in circles). Figure 9-16 shows the same circuit with
proper supply decoupling.
This principle is easy enough to apply if the load is adjacent to the
circuit driving it. Where the load must be remote it is much more diffi-
cult, but there are solutions. These include transformer isolation and the
use of a transmission line. If the signal contains no DC or LF compo-

Figure 9-16.

Property decoupled with
local load

Constant
Current
Source Load

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