DZ HT-7, Assembly Manual, Page: 31 / 44

Page 31
by Rob Schenck, W2CU, on an
online AM forum.
From Rob’s description:
“This circuit is a variation of
the classic "Precision Half Wave
Rectifier" as described in many
op ‐ amp application notes as shown
below [in Figure 1]:
Figure 1. Half wave AM detector
“Its operation is based upon ba ‐
sic principles of operational am ‐
plifier circuits. An ideal op ‐ amp
has extremely high input imped ‐
ance, extremely low output imped ‐
ance, and extremely high gain.
Note the operative word
‘extremely’. The non inverting
input (+) of the op ‐ amp is con ‐
nected to ground. Feedback is
provided by the two paths R2+D1
and D2. The op ‐ amp will drive its
output pin to a voltage such that
the inverting input ( ‐ ) is at
ground voltage. There will be a
slight variation (a few mil ‐
livolts) from 0 volts as a result
of what is known as input offset
voltage, a property of real op ‐
amps. The inverting input is of ‐
ten called the summing junction,
as the sum of the currents in
that node will be zero. Since
virtually no current flows into
the inverting input ( ‐ ) of the
op ‐ amp itself, any current coming
in through the input resistor R1
will be met by an equal and oppo ‐
site current coming through ei ‐
ther of the two paths of R2+D1 or
D2.
“The I.F. input signal will pro ‐
duce an input current of:
Iin = Vin/R1
“During the negative half of the
input AC waveform, the op ‐ amp
output will drive positive such
that the current through R2+D1
will be equal and opposite the
input current, or:
Iin = Vin/R1 = ‐ ID1 = ‐ IR2
“The voltage developed across R2,
and hence the output voltage as
the left side of R2 is at ground
potential, will be:
V = Vout = ‐ IR2 * R2 = ‐ Vin/R1 *
R2
“By setting R1 = R2, the output
voltage will be:
Vout = Vin
“Similarly, the positive going
input cycle produces a feedback
current through D2. Any non lin ‐
ear voltages developed across the