biasing a diode detector

[per_lube]

Hi,

I saw the following cct. diagram in Avago AN1088: Designing the Virtual Battery.



What I understand is to bias the detector diode to reduce the impedance of it, a negative (-ve) DC voltage should be supplied when there's no RF input.
However, when there's an RF input, a positive voltage getting created at the cathode of the diode.

Then it becomes a +ve voltage from RF input and -ve voltage of the DC bias supply at the same point which becomes a short circuit.

I know this shouldn't be what's happening there.
Could anyone help me to understand to apply proper bias for this circuit?

Then in voltage doubler circuit, if I add a -ve DC bias voltage as shown below, again when there's no RF input, the two diodes become forward bias.
Similar to the previous situation, when there's an RF input, the doubler output generates a +ve voltage and bias pulls down to a negative voltage.



I made the following doubler circuit.


When there's RF in, and no bias voltage, the doubler gives out a negative voltage output.
When I apply the positive bias and apply a 12 GHz RF input, the circuit does not work and diodes burns out.

Could you please tell me what's the problem here and how to properly bias the these circuits?

cheers,
per_lube

 

[jiripolivka]

All shown DC-biasing circuits can be used. The purpose of DC bias is to operate the P/N or Schottky junction at the best point where then open/closed" ratio offers the best rectification of RF input signal.
Some circuits leave the DC-bias voltage at the DC or video output. THe best solution is to use a pair of detector diodes connected to (+) and (-) inputs of an opamp, and the DC bias is introduced from a common source to both diodes. Only one diode detects the RF signal.
By such connection the opamp output reads zero when no RF signal is applied while temperature compensation is achieved.
Some good detectors use a Zero-Bias (ZBS) Schottky diodes for a high sensitivity and a good RF matching.

 

[per_lube]

Thanks jiripolivka.

My question is the polarity of the generated voltages from the detector and biasing voltage.

For example, the third cct. in my post generates a negative voltage output when there's no bias voltage applied.
However, to bias the two diodes, I should apply a positive voltage to the output of the detector circuit.

Is it someting like short circuiting the cct. (trying to pull up a negative voltage, into a positive voltage)?

Same happens in the second circuit... I'm really confused... please help me on this...

When I actually build this cct., without bias it works really well... got a minimum detectable power around -40 dBm. But when I apply bias, the diode burned out twice...
I don't know what's wrong there... When I remove the bias (had to replace the diode) , it works again...

could someone explain what's going on there...

 

[biff44]

The "diodes burn out"??? Just how much power are you applying?

 

[jiripolivka]

Thanks jiripolivka.

My question is the polarity of the generated voltages from the detector and biasing voltage.

For example, the third cct. in my post generates a negative voltage output when there's no bias voltage applied.
However, to bias the two diodes, I should apply a positive voltage to the output of the detector circuit.

Is it someting like short circuiting the cct. (trying to pull up a negative voltage, into a positive voltage)?

Same happens in the second circuit... I'm really confused... please help me on this...

When I actually build this cct., without bias it works really well... got a minimum detectable power around -40 dBm. But when I apply bias, the diode burned out twice...
I don't know what's wrong there... When I remove the bias (had to replace the diode) , it works again...

could someone explain what's going on there...

It looks to me that you do not understand your simple detector circuits.

The DC bias polarity should push a forward current through the diode(s) of one to five microamperes only. The DC voltage across the diode induced by DC bias circuit should move the operating point to the "knee" on the V/I curve. For Schottky diodes, typical vltage i8s 0.3 to 0.7 V. Zero-bias diodes have ~0.25 V or less.
Use a microammeter and use not more than 5V DC bias; adjust one microampere of FORWARD current through the diode. A typical series resistor is >100 kOhms.
Use the DC bias only to improve detector sensitivity for RF input of less than -20 dBm.
Also, use a good voltage regulator or a battery for DC bias. Prevent any discharges, do not touch the diode in circuit. Use a good screen to prevent stray signals in the detector circuit.

 

[per_lube]

Hi jiripolivka,

Thanks for the reply and explanation.
I isolated the problem. Now it works... now having about -40 dBm minimum detectable signal power in x-band.

cheers,
per_lube

 

[jiripolivka]

Congratulations! Sensitive detectors require attention to optimize their operation.