Column: Circuit drill
Peak detector circuit frequency performance
By Dr. Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions T
he peak detector is a simple yet versatile circuit, used to detect the peak value of an input signal. In its simplest form it consists of a
diode and a capacitor, and it can be used in many applications, including radio communication, audio signal processing and power supply monitoring. T e diode clamps the input signal to a positive voltage, which means the capacitor will only charge when the input signal is above a certain threshold. T e capacitor stores the signal’s peak value; the output of the circuit is a DC voltage that represents the input signal’s peak value.
Applications T e peak detector circuit is non-linear; i.e., its output is not a linear function of
the input, which can be disadvantageous in some applications. T is circuit is suitable for detecting peak
amplitudes of an audio signal, useful for normalising a signal, or for compression or limiting. It can also be used to demodulate a radio signal or for automatic gain control. In power supplies, the peak detector can monitor the peak voltage, to protect the equipment from overvoltage conditions. Figure 1 shows a simple peak detector
circuit, which consists of an op-amp, two resistors and a capacitor. Here we will try to determine how the circuit behaves at various input signal frequencies.
Experiment set-up We are using 1V as circuit input for this experiment, with its frequency varied from 5kHz to 100kHz in 5kHz increments. T e output signal’s amplitude was measured at
each step, and plotted; see Figure 2. For the experiment, we assume the
following: • All circuit components are kept unchanged.
• T e ambient temperature is static at 25o
C.
• T e power supply voltage is kept at 9Vdc.
Unstable performance T e peak detector circuit does not deliver a stable output voltage, similar to the input signal voltage. T e output voltage only matches the input voltage momentarily, at a specifi c frequency. For example, when the input signal is at either 5kHz or 50kHz, the output voltage will match it. However, any slight change in the input signal voltage will raise or lower the output voltage compared to the input.
Figure 1: A typical peak detector circuit
08 October 2023
www.electronicsworld.co.uk
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