Specifications

Diamond buer
Back to the “Diamond” buer. Professor Baker developed
another circuit which he called “The “Diamond Circuit”
in an internal publication for MIT, but on the patent he
called it “A Gateable Bridge Network Having Power Gain”
(attached), with some properties that mad it sort of a
hybrid between a digital circuit and an analog circuit. In
the patent application, Baker drew it rather dierently
than it is normally drawn and one can now *easily* see
why he called it a “Diamond Circuit” - see diagram 3.
Diagram 3 is simply a rearranged version of the simplied
circuit above, but when drawn in this manner it is
obvious to see why Baker called it the “Diamond Circuit”.
In actual practice it was most useful as a “buer” -- that
is, a circuit with no voltage gain but a high current
gain, which is precisely what is needed to drive a low
impedance load. Hence it has become popularly known
as a “Diamond buer”.
It is commonly used in the output stage of “buer”
IC’s that have unity voltage gain, but boost the current
to drive heavier loads. Sometimes it is also used with
discrete circuits, where it requires careful matching of the
opposite polarities of parts (NPN and PNP) to minimize
the oset voltages.
It is typically only used in low power
stages, such as preampliers outputs
or things of that nature. After playing
around with it for quite a while I came
to understand why this is so. To work
properly, the driver stage needs to use an
identical transistor to the output stage.
An output transistor is a big beast that
typically draws a lot of current at idle and
also has large capacitances between the
terminals. So this means that the driver
stage also has to use a “big beast” and
this stage also needs to draw a lot of
current and is fairly dicult to drive due
to the large capacitances.
Using a diamond buer as an output stage has two
fairly signicant disadvantages:
1) It draws a lot of current and this adds to the total power
consumption (from the wall outlet) of the amplier, as well
as requires additional heatsinking to dissipate the power
drawn by the driver stage.
2) Since the input transistors of the diamond buer must be
identical to the output transistors of the diamond buer,
they are large and rather dicult to drive. This means that
one cannot drive them directly with the typical voltage
amplier used to provide the gain in a power amplier
circuit.
" What is dierent about the diamond buer from a typical output stage is
that after the signal has been split into two halves to send to each output
device, is that the two half signals are rejoined at ONE single point. There is
NOTHING in between these two half signals. "
t: 01727 865488 e: [e-mail] w: www.symmetry-systems.co.uk
Symmetry, Suite 5, 17 Holywell Hill, St Albans, Hertfordshire AL1 1DT
Page 7
The AX-5 Story
from Ayre Acoustics March 2014