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Peak Detector Circuit
How to Design the peak detector circuit (Using Diode and capacitor) and its working
Now, many times in electrical and electronic circuits it is required to measure the peak value of the input signal.
And the peak detector circuit can be used for this purpose. So, let’s understand how we can design this peak detector circuit.
The simple peak detector circuit can be designed using just a diode and a capacitor. And the circuit over here will detect the positive peak value of the input signal. The negative peak of the signal can be detected by reversing the direction of the diode. So, let’s understand how this circuit can detect the peak value.
Now, let’s say some input signal is applied to this circuit. And for a moment let’s assume that this diode over here is an ideal diode. And initially, there is no charge across the capacitor.
Now, as soon as some positive input is applied to this circuit, the diode will start conducting. And the capacitor will get charged through this path. And it will get charged up to the peak voltage of the input signal. Now after reaching the peak value, this input signal starts reducing.
And at that time, this diode will become reversed biased. Because the voltage at the anode (of the diode)is less than the cathode terminal Peak Detector Circuit.
So, during that time, the capacitor will hold the peak value of the input signal. Now, this diode will again start conducting when the input voltage crosses the previous peak voltage.
So, up to this point, the capacitor will hold the voltage and then after once this diode becomes forward biased, this capacitor will get charged up to the new peak value. Now, after reaching the peak value, even if the input signal reduces, then also the capacitor will hold the peak value.
So, this is how this circuit detects the peak voltage. Now, so far we have assumed that the diode is an ideal diode. But actually, there will be some voltage drop across the diode. And because of that, the output will be slightly less than the expected value.
Precision peak detector Circuit
Now, this circuit is good for getting a rough approximation of the peak value of the signal.
But suppose if you want to accurately measure the peak value, then instead of this normal diode, the precision diode or super diode should be used Peak Detector Circuit.
And we have already talked about this circuit in the previous article of the precision rectifier. So, this circuit behaves like an ideal diode.
And just by connecting the capacitor at the output terminal, it will become peak detector circuit.
And it works exactly the same way as the previous circuit except for the fact that there is no voltage drop across the diode.
Effect of load on the peak detector circuit
Now, to measure the peak voltage across the capacitor some finite amount of load has to be connected at the output terminal Peak Detector Circuit.
Now this load can be either an external load resistor or the input impedance of the circuit which is just connected after this peak detector.
Or it can be an input impedance of some data acquisition system. So because of this load, the capacitor will get discharged through this resistor whenever the diode is non-conducting.
So, instead of holding the voltage, slowly this capacitor is discharging through this load resistor. To avoid that problem the RC time constant of the circuit should be very large.
If the input signal is a periodic signal, in that case, this RC time constant of the circuit should be at least 10 times more than the time constant of the signal.
Similarly, whenever the capacitor is charging, it should follow the input signal immediately. So the RC time constant during the charging of this capacitor should be much less than the time period of the signal. Or we can say that during that time the RC time constant should be at least less than the 1/10th of the time period of the signal.
Here Rd is the forward resistance of the diode. So, by following these conditions the discharging of the capacitor can be reduced. But many times it is not possible to control the load resistance, particularly whenever the output of the circuit is given to some other circuit.
Precision peak detector Circuit with Buffer
Because at that time the input impedance of the circuit is not known. So, in such instances, it is good to use the buffer circuit before connecting it to the load resistance. The buffer circuit provides very high input impedance and prevents the capacitor from discharging through the load resistor.
And suppose if you want to reset the circuit, the switch can also be connected across the capacitor to manually discharge it. Even transistors can be used as a switch to control the discharging of the capacitor.
And as shown in the figure, the switching action of the transistor can be controlled using the controller.
So, by using the buffer, we can avoid the discharging of the capacitor. but still, there is a problem with this circuit. Now, if you observe this circuit is that whenever the diode is not conducting at that time, the op-amp will operate in the open-loop condition. And that will lead the op-amp into negative saturation.
For example, let’s say the capacitor over here is holding the value of the signal. Now, if the input goes below this peak value then the diode will be reversed biased and op-amp will operate in the open-loop condition, as there is no feedback from output to the input side. Now the negative input terminal of the op-amp will be at peak voltage. And the positive input terminal is at input voltage which is slightly less than this peak voltage.
So, the op-amp will go into negative saturation. Now, even if this input voltage goes above this input voltage, this op-amp will take some time to come out of this saturation. And that basically depends on the slew rate of the op-amp.
So, that will limit the operating frequency range of this peak detector. And because of that, this circuit will not be able to detect or follow the fast-changing signal. And this problem can be avoided by modifying the peak detector. So let’s understand how this circuit works.
Modified Peak detector circuit for the fast signals (High-Speed Peak Detector)
Initially, this capacitor is fully discharged. So there is no voltage across this capacitor. And as it is connected to the buffer circuit, the same voltage will also appear at this terminal. And similarly, the same voltage will also appear at this terminal. Because initially, all the nodes are at zero potential.
Now, whenever some finite input voltage is applied to this op-amp at that time the output of the op-amp will become positive. So, this diode D1 will become reversed biased and this diode D2 will become forward biased. And the capacitor will get charged up to the peak voltage through this path.
Now, here as this diode D1 is non-conducting, and here we are assuming that no current is flowing into the op-amp terminals so no current will flow through this resistor R as well. So, whatever voltage appears at this node, the same voltage will also appear at this node.
And here through this resistor R, there is negative feedback in the given circuit. Now, let’s see the second case whenever the input voltage goes below this peak voltage. So, at that time, this diode D2 will become reversed biased. And the outer loop of this circuit will get broken. Now, the negative input terminal of the op-amp will be at peak voltage. While the positive input terminal over here is slightly less than the peak voltage. So, momentarily the output of the op-amp will become negative.
So, because of that, this diode D1 will become forward biased. And it provides the feedback to this op-amp circuit. And that prevents this op-amp from going into the negative saturation. So, whatever voltage is applied at the input terminal, the same voltage will appear at the output terminal. And in this way, the op-amp is not going into the negative saturation.
Summary/Last Word About Peak Detector Circuit
And at the same time, as this diode D2 is-conducting, it prevents the capacitor from discharging. So, in this way, this circuit will be able to respond to the fast signals. Now, of course, the response time of the circuit also depends on the response time of the diode as well as the response time of the op-amp. And moreover that it also depends upon the charging time of the capacitor.
But if these components are fast enough, in that case, this particular circuit will be able to respond to the fast signals. While the circuit which we have seen previously, will not able to respond to the fast signals.
And for very high-speed peak detection, the hotkey diodes are preferred over the signal diodes. Because these Schottky diodes have a very quick response time. So, this is all about the peak detector circuit.
So, I hope in this article you understood how to design the peak detector circuit. So, if you have any questions or suggestions do let me know in the comment section below.
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