AM detector more sensitive than simple diode
The traditional diode AM detector is shown in Figure 1. This detector has to operate at zero DC potential, so if the source has a DC component, the R-C combination is needed to block the DC in the signal. The detector loads the source stage, possibly increasing its bandwidth. The output impedance of the detector is relatively high, which is undesirable. The volume control puts an AC load on the detector causing distortion in the audio. The diode used has to be a germanium type or a hot carrier diode that has a low forward conduction voltage.
Figure 1 Traditional AM detector
The Design Idea in Figure 2 shows an alternative circuit which solves all these problems at once. The source is shown as a 5 volt AM modulated signal in series with a 5 volt DC source (which can be ignored here). The AM signal is a 1MHz carrier with 1kHz modulation at 100%. The input impedance of the detector is around 300kΩ, which doesn’t appreciably load the source. The diode is operated with a slight forward bias so a regular silicon signal diode (e.g., 1N914) can be used. The output impedance of the circuit is low and the load here has little effect on the distortion level.
Figure 2 Improved AM detector
The simulation input and output patterns are shown in Figure 3. The output audio waveform is shown with the correct DC level with respect to the center line which is 0 volts. Note that the output voltage does not go to zero even with 100% modulation. There is no visible distortion in the output and that is unusual with 100% modulation.
Figure 3 Simulated signals of the improved AM detector
This circuit was not only analyzed with SPICE, but it was actually used in a shortwave receiver that appeared in a magazine article and two books (see references). The receiver has been built by the author and numerous readers.
If you need AGC with the detector, things get more complicated. The DC level of the output is +4V with no signal and it increases as the input signal increases. One usually needs a negative voltage for AGC with zero volts output with zero carrier voltage input. To get this we use an op-amp to invert the voltage, shift the level to zero when there’s zero signal input, and provide some DC gain. The op-amp needs a positive and negative supply voltage. The schematic is shown in Figure 4.
Figure 4 Improved AM detector with AGC output
The simulated output of the AGC and the audio are shown in Figure 5. Both the audio and the AGC line are DC referenced to the center line. The AGC output is about -4V with a 5V signal input to the detector.
Figure 5 Audio and AGC signals
- Nuts and Volts Magazine, June 2002, page 44
- The New Radio Receiver Building Handbook by Lyle Russell Williams, page 137
- Fun With Electronics by Lyle Russell Williams, page 45
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—Lyle Williams has a BSEE and is retired from McDonnell Douglas in St. Louis. He is the author of numerous articles and Design Ideas, and of "The New Radio Receiver Building Handbook" which is available from Amazon.