gain appnote ucd PDF

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TM

Kattegat 8
9723 JP Groningen, The Netherlands
+31 50 526 4993
sales@hypex.nl

High-Efficiency Audio Power Amplifier

Voltage Gain of the UcD Modules
Hypex UcD modules are sold with the voltage gain
pre-set at 20 times (26dB). This is an arbitrary
choice. Topologically the module is an
instrumentation amplifier, so gain can be changed
by changing only a single resistor.

COLD IN
1.8k

~400

Rg
Rf

1.8k

8.2k

COLD OUT

HOT IN

Bare UcD stage
When we strip the UcD circuit of all its reactive components
(inductors and capacitors) and replace the comparator plus the
switching power stage with a linearised approximation we get
the DC equivalent model. It looks like this:
1.8k

8.2k

HOT OUT

~400
HOT IN

HOT OUT

Rf

Gain Structure

COLD IN

8.2k

1.8k

8.2k

COLD OUT

This is a difference amplifier. Gain is not quite 8.2k/1.8k because
of the finite gain of the switching output stage. It works out as
4.5 times.
The input impedance of this circuit is quite low. On the hot side
it’s 10k referred to ground, on the cold side it’s 10k referred to
output. Forget what is commonly said about the cold input
impedance being 1.8k referred to ground because it would be
1.8k referred to the virtual short, which in turn is a function of
the hot input voltage. This explains why it is not possible to
make a one-opamp difference amplifier with balanced
impedances – the input impedance is balanced, but each refers
to a different circuit node.
This in combination with the lowish impedance calls for a buffer
stage.
Differential buffer stage
The difference amplifier circuit almost begs to be used in an
instrumentation amp. A differential gain buffer is added:

AV=1+2*Rf/Rg

AV=4.5

The differential mode gain of the first stage is 1+2*Rf/Rg. The
common mode gain is 1 and no conversion from common mode
to differential mode takes place. The CMRR performance of the
difference stage (the UcD block) is directly improved by a factor
equal to the gain of the first stage. This is a very strong
argument in favour of making the gain of the UcD as low as
possible, just high enough that the buffer stage can still drive it
comfortably. A second argument lies in the fact that the input
transistors of the UcD comparator are not matched. Although
it’s perfectly possible to make a modulator with a gain of 30dB,
doing so exacerbates the effects of offset voltage. Again a good
reason to shift the burden of gain towards the differential buffer
stage, which is built with op amps.

Practical

R
A V = 4.5 ×  1+ 2 × f

R
g







Changing gain involves no more than changing Rg. Until
recently, all modules were stuffed with Rf=1k and Rg=560Ω. At
the time of writing this is being changed to 2.2k and 1.2k.
To verify which values are used on your module, the position
numbers for Rf and Rg on the various modules are:
Model
Rf
Rg
UcD700
R58, R59
R57
UcD400
R4, R14
R3
UcD180
R11, R12
R10
Bruno Putzeys



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