Diode RF detector types, advantages, disadvantages

[neazoi]

Here are 2 types of RF to DC converters.
Circuit 1 is a classic envelope detector. Circuit 2 converts RF to DC by shunting the negative portion of RF to the ground.

Circuit 1 suffers from the diode dropout in the positive cycle.
Circuit 2 does not.

Circuit 1 provides RF isolation, so RF cannot be fed from the right to the left.
Circuit 2 does not provide this isolation.

What other advantages disadvantages do you see between the two circuits?

My goal is to achieve a good output current by rectifying a powerful RF generator. Can these simple circuits provide higher current to light a 300mA 6v bulb for example, provided that there will be a power RF generator as a source?

 

[betwixt]

Provided there is sufficient power in the signal source you can extract any amount of power, there is no theoretical limit.

Circuit 1 isn't necessarily an envelope detector, as shown its a peak detector until you draw current from it.
Circuit 2 rectifies by shorting out negative half cycles when the diode conducts. It therefore needs some impedance adding to the input to prevent damage to the source. The capacitor is also across the signal so unless it is part of a resonant circuit it might simply dump signal to ground.

To drive a load like a 2W bulb you could use the RF directly as it doesn't care about frequency or polarity. If you want to convert to DC first, use circuit 1 with the bulb across the capacitor or circuit 2 with the bulb in series with the source. Circuit 1 has the advantage that one side of the load is grounded.

Use a suitable diode, even Schottky power diodes will only work up to a few hundred KHz. Remember that if unloaded and driven with an AC coupled signal, the capacitor in circuit 1 will charge to peak positive voltage so on negative input half cycles the peak across the diode will be (2 * Sqrt(2)) * Vrms. Take care selecting a diode with sufficient PIV and forward current.

Brian.

 

[neazoi]

Provided there is sufficient power in the signal source you can extract any amount of power, there is no theoretical limit.

Circuit 1 isn't necessarily an envelope detector, as shown its a peak detector until you draw current from it.
Circuit 2 rectifies by shorting out negative half cycles when the diode conducts. It therefore needs some impedance adding to the input to prevent damage to the source. The capacitor is also across the signal so unless it is part of a resonant circuit it might simply dump signal to ground.

To drive a load like a 2W bulb you could use the RF directly as it doesn't care about frequency or polarity. If you want to convert to DC first, use circuit 1 with the bulb across the capacitor or circuit 2 with the bulb in series with the source. Circuit 1 has the advantage that one side of the load is grounded.

Use a suitable diode, even Schottky power diodes will only work up to a few hundred KHz. Remember that if unloaded and driven with an AC coupled signal, the capacitor in circuit 1 will charge to peak positive voltage so on negative input half cycles the peak across the diode will be (2 * Sqrt(2)) * Vrms. Take care selecting a diode with sufficient PIV and forward current.

Brian.

Thanks so much Brian for the valuable information!!

There is something else I need to figure out too.
In circuit 1, let's say I apply DC to the input of the diode, and assume that there is some kind of DC return path to the ground for the diode at it's output (eg a suitable inductor shunt to the ground).
Now I also apply RF at the output of the diode at the time it conducts.
Will then the diode let RF pass through it from the output to the input? (right to left)

 

[betwixt]

Yes, if the DC is positive with respect to ground. The diode will only work as a rectifier if the polarity across it reverses, given enough DC to keep it conducting it will look more like a resistor from the signals point of view.

That is basically how diode switches work, forward conducting they look like resistors, reverse biased they look like small capacitors. Diodes intended for RF switching have their construction optimized for low forward resistance and low capacitance. Look up 'PIN diode' to see more information on this.

Brian.