Varicap @ mV or even µV?

[Moritz]

Hi there,

I'm curius. Do varicaps exist, that can operate with mV or even µV?
And what should I search for?

Thank you and best regards,
Moritz

 

[JimB]

The quick answer is yes, they all do.
What are you trying to do with this varicap?

I assume that by varicap you mean a variable capacity diode.
They will all work with a low voltage signal, but they all need several volts of DC bias to adjust the capacitance.

JimB

 

[Moritz]

Yeah you're right. I meant a variable capacity diode.
And I obviously asked the wrong question.
What I meant is: are there any varicaps that can adjust theire capacity by milivolts of DC bias?

 

[Nigel Goodwin]

What I meant is: are there any varicaps that can adjust theire capacity by milivolts of DC bias?

They all do, but only by a tiny amount - you really need to explain EXACTLY what you're trying to do.

 

[Moritz]

Well I have a signal of milivolts and want to detune a LC-circuit with such a varicap without amplifying the signal before. That's it.
So what amount we're talking about? Several nF? Because the datasheets I previously looked at, hadn't contained any mentions of Voltage in this magnitude.

 

[Nigel Goodwin]

You're still been pretty vague

But varicaps need fairly large voltages to change appreciably, and you'll probably struggle finding one that changes nF's even then?.

Changes in mV are likely to only produce capacitance changes in small fractions of a pF.

I suspect you need to completely rethink what you're doing, hence my request for EXACT details.

 

[Tony Stewart]

Well I have a signal of milivolts and want to detune a LC-circuit with such a varicap without amplifying the signal before. That's it.
So what amount we're talking about? Several nF? Because the datasheets I previously looked at, hadn't contained any mentions of Voltage in this magnitude.

Let's give you some theory and my practical experience.

• ALL diodes behave like are "varicaps" where diode capacitance is reduced from 0V bias to negative bias range in a log scale.
  
• However the tolerance is huge.
  
• The special diodes with process controls to tighten tolerances are called Varicaps.
• Diode capacitance increases with power ratings and conductor gap dimensions of the device, while ESR is inversely related to Pd max as size affects all 3 parameters.
• Typically diode Capacitance is measured in pF (not nF)
• Tuning range of 0 to 30V is common, or 0 to 5V with reduced range. ( NOT in mV ! .... you may be thinking of carrier detection)

But to resolve your query, first define your requirements in tuning range and accuracy. ( i.e. a design spec)

A VCXO is much more accurate and VCO chips are also more accurate and stable than a "DIY " VCO using LC parts as temperature stability and tolerances are much bigger.

 

[audioguru]

He said he "has a (low level) signal" and he wants to detune (change the frequency) of an LC circuit that probably generates that signal.
But he did not say the signal frequency and did not say how much he wants to change its frequency.

 

[Diver300]

Varicaps are diodes made to change their capacitance more than other diodes when the voltage changes. Varicaps are usually quite poor diodes in most other ways.

The capacitance change depends on the reverse bias. The larger the reverse bias, the larger the depletion region in the diode, so the conductive parts are further apart and the capacitance is less. Often the quoted voltage range is 2 - 18 V, and on some hyperabrupt varicaps the range is up to about 10 V.

Significant capacitance changes are not possible for small voltage changes, so you will have to find a way to amplify the voltage to get the tuning to work.

 

[Tony Stewart]

Let's give you some theory and my practical experience.

• ALL diodes behave like are "varicaps" where diode capacitance is reduced from 0V bias to negative bias range in a log scale.
  
• However the tolerance is huge.
  
• The special diodes with process controls to tighten tolerances are called Varicaps.
• Diode capacitance increases with power ratings and conductor gap dimensions of the device, while ESR is inversely related to Pd max as size affects all 3 parameters.
• Typically diode Capacitance is measured in pF (not nF)
• Tuning range of 0 to 30V is common, or 0 to 5V with reduced range. ( NOT in mV ! .... you may be thinking of carrier detection)

But to resolve your query, first define your requirements in tuning range and accuracy. ( i.e. a design spec)

A VCXO is much more accurate and VCO chips are also more accurate and stable than a "DIY " VCO using LC parts as temperature stability and tolerances are much bigger.

Using Digikey's search engine one can see the wide range of varactor diodes rated @50MHz and many voltages and many voltage ratios.
e.g. C1/C10 is the capacitance ratio from 1~10V. or you can choose high C ratios and low voltage like 5V but there are limits according to the diode properties (many)

- to illustrate my point that ALL diodes have a C ratio, here is a power diode below but not included in the Varactor diode list with a C1/C25=10 . Although Schottky power diodes are leaky but you can get nF size values.