The
Diode Rectifier
A Note
on behavior with input sinusoids with time-varying envelopes
This note is intended to help you learn more about the behavior of the
diode rectifier, especially when the diode rectifier is used to produce
a DC signal from an AC transducer. In those situations, the AC signal
being rectified can - and will - change. (That's the purpose of using
the sensor - to measure a changing physical quantity.) When the input
to the diode rectifier changes, that produces a situation different from
the one we have considered so far where the input is a constant.
We have two simulators available to help you understand what happens when
you have a sinusoidal signal with a time-varying amplitude. (You
should have gone through the
interactive problem available here.) In the first simulator,
the input signal is a sinusoidal signal that has an amplitude that increases
in a linear fashion. In the second simulator, the input signal is
a sinusoidal signal that has an amplitude that decreases in a linear fashion.
Both simulators use an ideal diode model in the simulations, i.e. there
is no forward voltage across the diode when it is conducting, and no current
leakage what it is back-biasesd.
-
Click
here for the simulator for the diode rectifier with a sinusoidal signal
with an up-ramp envelope.
-
Click
here for the simulator for the diode rectifier with a sinusoidal signal
with a down-ramp envelope.
Using these simulators you can learn about the behavior of the diode rectifier
circuit when you have a sinusoidal signal with a varying amplitude - and
after all the information is often contained in the amplitude. Here
are some things to do first.
-
Run the up-ramp simulator
using the pre-loaded paramters. Note the following:
-
The signal that you recover
does not increase linearly. At first it looks like a sequence of
steps. That's not quite what's there. You can see that better
if you change the time constant to 10 sec. or 5 sec. You can see
that there is more droop at the higher levels. Actually, the percentage
droop is the same for all cycles, but it produces a larger slope at the
higher levels.
-
Run the down-ramp simulator
using the pre-loaded paramters. Note the following:
-
The signal that you recover
does not decrease linearly. There is droop, and at first the droop
slope is larger than the slope of the down-ramp on the envelope of the
sinusoidal signal. Later, however, the droop slope cannot keep up
with the decrease in the sinusoidal signal and the recovered signal just
decays exponentially.
Now, when you want to use these circuits you need to be aware of the two
phenomena that you can see in the simulators. In particular there
are a few points to note.
-
For increasing signals,
the rectifier circuits does not interpolate between successive peaks.
You will get plateaus or decreasing signals between peaks. There's
not much you can do about that unless you go to a more complex circuit.
-
For decreasing signals,
the rectifier circuit does not follow successive peaks.
-
For peaks that do not
decrease by a large percentage, the rectifier output voltage may fall below
the next peak.
-
For peaks that decrease
by a large percentage, the rectifier output voltage will not fall fast
enough to keep up with the decrease in the input voltage.
-
Examples of the above
two statements can be seen in the simulator for a decreasing ramp envelope.
-
Use the default parameters,
and you can see that the initial part of the response is a situation where
the output peaks are not decreasing as fast percentage wise as they
are later. In this part of the response the rectifier output falls
below the signal peaks.
-
In the last portion of
the response, the output peaks are decreasing by a large percentage each
cycle, and the rectifier output cannot fall quickly enough. At that
point, the signal decreases to zero, but the rectifier output is decaying
exponentially to zero.
There is a dividing point between the two cases for the output signal.
-
The output voltage will
match the next peak exactly when:
-
e-t/t
= Next Peak/Current Peak