Ideal Diode.

[lord loh.]

I was just wondering, if the diode were ideal, could it be used as a demodulating device?

By ideal diode I mean that the diode would behave as a conductor in forward bias stage.

What do you think ?

 

[Sebi]

The ideal diode not existiert, but for demodulation the best choice is a germanium dione (only have 200mV fw bias).

 

[Russlk]

If you mean demodulate an AM signal, yes, you only need to rectify the RF and filter to demodulate.

 

[Miles Prower]

I was just wondering, if the diode were ideal, could it be used as a demodulating device?
By ideal diode I mean that the diode would behave as a conductor in forward bias stage.
What do you think ?

For modulation/demodulation, what you need to avoid is time invariant linearity. That means that any non-linear (i.e. plate detector) or time-varying linear (dual gate MOSFET, singly balanced differential product detector, pentagrid converter, OTA) or piece-wise linear (switching detector) will demodulate.

An ideal diode would give an ideal piece-wise linear transfer characteristic.

 

[lord loh.]

The ideal diode characteristics.

If you mean demodulate an AM signal, yes, you only need to rectify the RF and filter to demodulate.

You need a diode to rectify it, don't you ?

An ideal diode would give an ideal piece-wise linear transfer characteristic.

Attached is a the characteristic of an Ideal Diode. Will you call it "time invariant linearity" ? I think yes.

X axis : Voltage
Y axis : Current

 

[Dr.EM]

Just wondered. A vacuum tube diode, not being made of silicon or germanium, does that also have a voltage drop?

 

[Russlk]

This is the characteristic of a 6H6 dual diode:

 

[lord loh.]

Can anyone explain me the graph.

The X and Y axis is the same as diode characteristics.
What is a dual diode?
And why is the voltage negetive? and the sloping line that says RMS ?

I have never seen a characteristic like this.!! :shock:

 

[Nigel Goodwin]

Can anyone explain me the graph.

The X and Y axis is the same as diode characteristics.
What is a dual diode?
And why is the voltage negetive? and the sloping line that says RMS ?

I have never seen a characteristic like this.!! :shock:

Welcome to valves :lol:

But what exactly are you trying to do?.

 

[Russlk]

The intersection of the RMS input line and the resistance load line gives the output voltage and current. For example, 20 VRMS input and 50,000 ohms intersect at 22.5 volts output and 450 uA current. This particular dual diode has two plates and one cathode, which is typical of most tube (valve) rectifiers.

 

[lord loh.]

But what exactly are you trying to do?.

As in my first post, I was just wonderinf if an ideal diode shall demodulate AM signals? And the Ideal Diode chacteristics as I belive, is posted above.


And about the valve picture - I guess that X-axis is the DC voltage the Y-axis is the Current (DC ?) and the characteristics also shows AC characteristics (the sloping lines)

But the title says that it is about a half wave rectified output. So everything should be RMS and the input should be AC. In this case what is the DC voltage developed by the diode(X-Axis)?

 

[Russlk]

A diode, ideal or otherwise, passes current in only one direction, so if AC is applied, the output is DC. The current can be collected in a capacitor which will provide a peak voltage at no load. In order to demodulate an AM signal, the capacitor must have a load that will discharge the cap as fast as the highest frequency that is to be recovered.

 

[lord loh.]

okay... I think I got it...

I was thinking that there is no voltage change when the diode is forward biased there might be no signal....

I just realised that the graph(The one I posted.) showed the voltage drop across the diode and the signal would be developed as the voltage drop across the load resistor.