Why do you need a peak detector circuit? First, if you test much electronic equipment, you need one. And it’s because projects like this allow levels of radio frequency interference.
Initially, peak detectors explicitly showed the personal annoyance level that a listener experiences when exposed to impulsive interference. But these days, the device helps you determine the peak amplitude in a waveform.
But there’s more to this topic. And that’s why we focused on the peak detector, how it works, how to build a peak detector PCB, and more.
What’s a Peak Detector Circuit?
The peak detector circuit uses a capacitor and diode to observe the peaks of a related input signal. In other words, the peak detector is the perfect go-to if you cannot measure a signal that varies rapidly.
Also, it’s possible to use the circuit to measure rapid signals because it confines the maximum amplitude for a short time.
That said, there are two types of peak detector circuits: active and passive.
The active peak detectors are circuits that consist of active components like transistors. And thanks to its active ingredients, it tends to be more accurate.
On the other hand, the passive peak detector has passive components like capacitors. Plus, it’s not so accurate. And it’s because of the elements, especially the resistors, experience losses.
How Does a Peak Detector Circuit Work?
In this section, we refer to the working principle of an active peak detector circuit. And the course has a diode (D), capacitor (C), and op-amp. Also, Vo is the output voltage, while Vi is the input you apply to the op-amp’s non-inverting terminal.
So, your diode will come ON if your Vc (capacitor voltage) is less than the input voltage. Consequently, your capacitor will charge. When this happens, your circuit will start working like a non-inverting amplifier. And the Vc will follow the Vi.
Further, the diode will go OFF when the Vc is higher than the Vi. Also, the Vc will hold the initial maximum charged voltage till the Vi increases beyond the maximum voltage. Consequently, the op-amp will start operating in an open-loop configuration.
An Example of How the Circuit Works
If your Vc has an initial value of 0, your input voltage will apply to the non-inverting terminal. And this will make your diode come ON. Also, your Vc will accompany the input waveform till it gets to T2 (T represents Transistor).
So, at t2, Vi = V2. Hence, Vo = Vc = V2. But once the time exceeds T2, there will be a decrease in the input voltage. Then, the diode will go OFF when the anode voltage is less than the cathode voltage. Also, the capacitor will maintain the maximum voltage till the waveform reaches T3. And at this time, Vc will be less than Vi.
After t3, the Vc will go after the Vi till it reaches T4. When it gets to T4, Vi = V3. Consequently, Vo = Vc = V3. When the T4 elapses, the input voltage will start reducing. And the diode will go OFF when the anode voltage is less than the cathode voltage.
The capacitor will continue with the maximum voltage until Vc is more minor than Vi—at T5. So, after T5, the Vc goes with the Vi till they get to T6. At this period, Vi = V4.
Likewise, the capacitor charges to the V5. Then, it maintains the voltage until the next maximum voltage or peak arrives.
How to Build a Peak Detector Circuit
The components you need to build a peak detector circuit are:
- Transistor (optional)
- Resistor (10K Ohms)—optional
- An electrolytic capacitor (100mF)
The goal of this circuit is to have a device that allows current flow in only one direction. Your diode should have a voltage drop across—the barrier voltage. Also, your capacitor has to be large so that it will hold more charge.
Plus, your capacitor's voltage rating must match the voltage you’re using. For example, if your voltage won’t exceed 40V, you’ll need a 70V voltage capacitor.
That said, you can start by connecting your capacitor and diode in series with one another. Consequently, the current will move in one direction—since the diode is a one-way current device. Then, you can put a positive voltage source (the arrangement should be in series) alongside the forward-biased diode.
Also, the capacitor should be in series. As a result, you’ll notice a movement in current from the power source via the diode. When this happens, the capacitor charges. And the waveform will have a new peak.
Afterward, the capacitor will charge up to the level of the new peak. That is, the capacitor will follow the signal. So, when the capacitor charges to the recent rise, it holds the charges.
And this happens because the diode stops the charges from flowing out of the capacitor. So, the diode will be reverse biased to the capacitor.
In addition, the retained charges will help you read the capacitor’s voltage. With this, you can tell the signal’s peak amplitude fed into the circuit.
What Modifications Can You Add to the Circuit?
No doubt, the setup above can conveniently retain charges indefinitely. But the circuit may experience leakage current over time—due to the simple components. Hence, you can readjust your value to zero to get the peak value for new signals every period.
Also, you can wipe the capacitor’s entire charge. That way, the capacitor will discharge until it gets back to 0V.
How do you discharge your capacitor? You can make a parallel connection between your resistor and capacitor. Also, it’s vital to note that the time for a capacitor to charge depends on the value of two things: resistor and capacitor.
For example, if your capacitor’s total discharge time is 5RC, you can calculate how long it takes to discharge.
R (10k Ohms) is the resistance, while C is the capacitor (100mF). So, T = 5RC = 5(10)(100) = 5 seconds.
Therefore, it means that your circuit will reset in five seconds. But if that’s too short, you can increase your resistor and capacitor values to get more time.
Also another way you can build a peak detector that discharges itself periodically is to connect a transistor to the circuit. That is, replace the resistor with a transistor.
In addition, you can connect a microcontroller to your circuit—if you want it to discharge when you put high voltage at the gate terminal.
Improvement in Peak Detector Circuit
An improved peak detector circuit helps protect the signal's source from the capacitor. So, the enhanced peak detector has two op-amps, unlike the primary course, with one op-amp.
That said, the source offers the first op-amp a high impedance load. And the second op-amp conducts buffering action (between the load and capacitor). Also, it’s vital to ensure that the values of the two resistors (R1 and R2) are equal—to avoid an offset voltage.
Plus, to get stability against oscillations, you have to give the op-amp all the essential frequency compensation.
Applications of Peak Detector
You can use the peak detector in the following applications:
- Sound measuring devices
- Analysis of mass spectrometer
- Destructive testing
- Instrumentation measurement
The peak detector circuit helps you get the maximum voltage signal at its input. So, the negative peak detector receives the most damaging part of your input signal. And the positive peak detector gets the most positive point of your input signal.
Also, the circuit’s output tracks the input voltage until it reaches a peak. But it holds the value as the input reduces.
What are your thoughts about the peak detector? Or do you have questions or suggestions? Please feel free to contact us.