AM Diode Detector

- overview of the basic AM diode detector or demodulator used for converting AM signals into useable audio with circuits, application notes and theory

One of the advantages of amplitude modulation is that it is cheap and easy to build a demodulator circuit for a radio receiver.

The simplicity AM radio receivers AM is one of the reasons why AM has remained in service for broadcasting for so long. One of the key factors of this is the simplicity of the receiver AM demodulator.

Diode detector basics

A number of methods can be used to demodulate AM, but the simplest is a diode detector.

It operates by detecting the envelope of the incoming signal which it does by rectifying the signal. Current is allowed to flow through the diode in only one direction, giving either the positive or negative half of the envelope at the output.

If the detector is to be used only for audio detection it does not matter which half of the envelope is used, either will work equally well. Only when the detector is also used to supply the automatic gain control (AGC) circuitry will the polarity of the diode matter.

The AM detector or demodulator includes a capacitor at the output. Its purpose is to remove any radio frequency components of the signal at the output. The value is chosen so that it does not affect the audio base-band signal. There is also a leakage path to enable the capacitor to discharge, but this may be provided by the circuit into which the demodulator is connected.

Circuit diagram of a simple AM diode detector or AM diode demodulator used to recover the modulation from amplitude modulated signals
Simple AM diode detector circuit

This type of detector or demodulator is called a linear envelope detector because the output is proportional to the input envelope.

DC return required

In order for a diode detector to generate the required DC voltage, a DC return must be available within the circuit. supplied.

This can be achieved by placing an RF choke across the input to the detector diode. This appears like an open circuit to radio frequency signals, but acts as a DC return path for the audio and other signals appearing from the detector.

Circuit diagram of a simple AM diode detector with DC return path included
Diode detector with DC return

Often this DC return path may be within a transformer used to drive the diode detector. Alternatively a resistor may be used. It value will be the same at all frequencies and therefore its choice is a matter of compromise.

Diode detector advantages & disadvantages

The diode detector is widely sued, but it has several advantages and disadvantages:

Diode detector advantages

  • Simplicity:   The diode detector is very simple and is easy to construct. The circuit six very straightforward, consisting of a very few components.
  • Low cost:   Requiring so few components, and the fact that he components are not specialised, this form of detector is very cheap. Accordingly it is widely used in AM domestic radios.

Diode detector disadvantages

  • Distortion:   Although the diode detector is able to operate in a reasonably linear fashion over a reasonable range, outside this range high levels of distortion are introduced, and even within the more linear range, distortion levels are not particularly low. It is adequate for small low cost radios.
  • Selective fading:   These detectors are susceptible to the effects of selective fading experienced on short wave broadcast transmissions. Here the ionospheric propagation may be such that certain small bands of the signal are removed. Under normal circumstances signals received via the ionosphere reach the receiver via a number of different paths. The overall signal is a combination of the signals received via each path and as a result they will combine with each other, sometimes constructively to increase the overall signal level and sometimes destructively to reduce it. It is found that when the path lengths are considerably different this combination process can mean that small portions of the signal are reduced in strength. An AM signal consists of a carrier with two sidebands
  • Insensitive:   Semiconductor diodes have a certain turn-on voltage. Accordingly the voltage has to reach a certain level before the diode is able to operate reasonably efficiently.

Schottky diode detector

As standard silicon diodes have a turn on voltage of 0.6 volts and even germanium ones have a turn on voltage of around 0.3 volts, this is relatively high when small signals require demodulation.

To help overcome this issue, Schottky diodes are often used. These diodes are silicon based but have a turn on voltage of around 0.2 volts. This makes them far more effective than standard silicon diodes that may otherwise be used. Germanium ones are available, but not nearly as widely used. Germanium is not used much these days and therefore any components based on this semiconductor tend to be more expensive and therefore not suitable for the low cost applications for which they may be required.

Input matching

Often a diode detector like the ones described above will need to match to 50Ω. Unfortunately the basic circuit shown will never present a good match to this impedance.

Once a diode is in its on state, the circuit will appear to be less than 50Ω. To overcome this issue an impedance transformer can be used to step up its impedance where the impedance is an issue.

If the section of the signal that is removed falls in one of the sidebands, it will change the tone of the received signal. However if carrier is removed or even reduced in strength, the signal will appear to be over modulated, and severe distortion will result. This is a comparatively common occurrence on the short waves, and means that diode detectors are not suitable for high quality reception. Synchronous demodulation ( detection ) is far superior.

By Ian Poole

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