Do varactor / varicaps could have their use in clippers?

[Steben]

"The voltage-dependent device means the output of the diode depends on their input voltage. The varactor diode is used in a place where the variable capacitance is required, and that capacitance is controlled with the help of the voltage."

This means a larger signal could lead to lower capacitance, leading to more treble cut for example in feedback loops.

[amptramp]

Most varactors are designed for RF frequencies so the values will be quite low, only being useful in high-impedance circuits.  One circuit that could use a varactor is the Magic Monitor:



This circuit is the original noise reduction circuit from the 1950's and what we see is a voltage amplifier followed by a shunt diode rectifier that drives a 6BA6 reactance tube.  When the signal is low, there is little bias on the grid of the 6BA6, so there is a lot of amplification in the pentode.  With a lot of signal, the 6BA6 is closer to cutoff, at very low gain.  The key to the circuit is the 27 pF capacitor from plate to the high-impedance output.  When the gain is low, the plate voltage (which is driven in opposite phase to the signal at the output) doesn't vary much, so at cutoff, it looks like 27 pF.  With low signal, the 6BA6 gain is high and the extra drive in the opposite polarity makes the capacitance seem larger since the current through the cap is the same as a grounded cap of much higher value.

This cap and the 6BA6 could be replaced by a varicap although the signal modulation would add some harmonics.  This circuit offers noise reduction and would be preferable to a noise gate because there is no abrupt shutoff of the signal.  A varicap designed for AM radio tuning would be best as it would be several hundred pF at zero bias and only a few pF at full bias (which is often 30 volts), but there are ones for portable and car radios that operate at 6 or 12 volts.

This might not be a clipper circuit but similar design considerations apply.  Varicaps biased to both rails and driven by the signal at Vcc/2 average level would tend to lose high-frequency content and flatten the signal as the voltage approaches the rails.  Could be very effective!

[PRR]

"The voltage-dependent device means the output of the diode depends on their input voltage.
...
This means a larger signal could lead to lower capacitance,...

The first phrase is talking about a DC (one-way) voltage.

When you bring in audio "signal" you have ab AC (both-ways) voltage.

So what it does on one side of the signal swing, it does the other thing on the other side.

And as Ron says: they don't make VariCap dies big enough to have the pFd to do much in audio circuits.

[G. Hoffman]

And as Ron says: they don't make VariCap dies big enough to have the pFd to do much in audio circuits.

Run 100 of them in parallel?   


Gabriel

[amptramp]

The AM tuning varicaps may be 300 to 400 pF but there is another trick you can use: zener diodes have a high capacitance, in some cases thousands of pF and some people have made tuneable audio filters from them.