- [Instructor] A peak detector is a circuit
that measures the peak amplitude of a signal
to determine the highest voltage that a signal reaches.
One common use for peak detectors
is in audio processing systems.
Since the volume of an audio signal will vary
depending on it's voltage levels,
by monitoring the audio signal with a peak detector
to determine it's maximum value,
a sound system can appropriately scale it's output.
I can build a simple peak detector circuit
by connecting a diode in series with a capacitor.
When a positive input voltage is applied to this circuit,
current will pass forward through the diode
and charge the capacitor up to that value.
If the input voltage is then reduced,
that charge stored in the capacitor
doesn't have anywhere to go,
since current can't pass backwards through the diode.
If the input voltage is then reduced,
that charge stored in the capacitor
doesn't have anywhere to go
since current can't pass backwards through the diode.
So, the capacitor remains charged
and holds that higher voltage from before.
So, the output voltage across the capacitor
will correspond to the maximum input voltage
that the circuit has seen.
This is a bit of a simplification.
The output voltage won't be exactly the same
as the peak input voltage because the circuit
will lose about point six volts to the forward voltage drop
across that diode.
So, to determine the actual peak input voltage,
I'll need to account for that lost voltage from the diode
by adding it back to the value I measured at the output.
To demonstrate that, I've built a simple peak detector
circuit on my breadboard using a one in four one four eight
diode and a 100 microfarad capacitor.
I'm using an adjustable DC power supply
to provide the input voltage for the circuit.
And, I'm measuring that with channel one of my oscilloscope.
Channel two is measuring the output voltage
across the capacitor.
When I turn the input voltage, shown in yellow,
up to two volts, I see that the output voltage in blue
rises to around one point four volts,
as I expected due to that forward voltage drop of the diode.
Now, if I lower my input voltage back down to one volt,
I see that the output voltage remains at that peak value.
The components in that circuit are not perfect,
so over time, it will leak a small amount of current,
and the output voltage will slowly drift down toward zero.
And so, because of that, I don't want to wait too long
before measuring the output of the peak detector,
or else, I'll get a slightly lower value.
I'm not going to wait for that now,
so I'll crank the voltage back up to three volts,
which is higher than the previous maximum voltage.
And when I drop that signal back down again,
the output voltage remains near two point four volts,
which corresponds to the higher three volt
peak input voltage.
Now, what if I want the peak detector circuit
to reset it's output back down to zero
after a specific amount of time?
Maybe I only care about the peak value that occurred
within the past five seconds.
To accomplish that, I can insert a resistor
in parallel with the capacitor to provide an escape route
for the capacitor's stored charge.
When the input voltage drops below the peak,
current will flow through the resistor to more quickly
discharge the capacitor.
The amount of time it takes to discharge will depend
on the resistor and capacitor values.
To determine which component values I need,
I often use the rule of thumb that the amount of time
in seconds that it takes for a charged capacitor
to discharge through a resistor,
is roughly five times the resistance times the
capacitance values.
Let's say I want my peak detector to reset itself
within about five seconds after seeing a peak.
Since I'm only using a 100 microfarad capacitor,
I'll rearrange that equation to solve for the
necessary resistor value to achieve
my desired discharge time.
Five seconds divided by five times 100 microfarads
gives me a resistor value of 10 kilohms.
This is the same peak detector circuit
I demonstrated before, but now I've added a
10 kilohm resistor in parallel with the capacitor.
If I create a short spike on the input voltage,
by quickly raising it and then dropping it back down,
I can see that the output voltage in blue
quickly rises up to the corresponding peak
and then slowly decays as the capacitor discharges
through the resistor.