Diode and BJT VCR's?

[Ribbledox]

Hi everyone=)

That a JFET can be operated as a voltage controlled resistor is often mentioned, but is it also possible to operate a BJT or a diode as a VCR?

Shouldn't the fact that both have a p-n junction between source and collector make it difficult to apply an peak to peak audio signal across the device, when the diode (for example) only pass current/voltage in one direction? If used as a part of a voltage divider...

Cheers!

/R

 

[NewYorkDave]

Diodes and BJTs have been used as voltage-controlled attenuators, but the signal level must be kept very low or the level of distortion is hideous.

Two diodes connected cathode-to-cathode with a variable positive bias applied to the cathodes can be used as a simple switch or attenuator. This is more often seen at RF (with "pin diode" switches) than at audio frequencies.

One other problem is that the capacitance of the P-N junction varies with the reverse bias. This is exactly the property that is exploited in "varactor" devices, but it's a drawback in signal switches and attenuators.

So, in summary, it's do-able, but usually not worth it.

 

[Ribbledox]

Thank you Dave!

But when we are talking about diodes used as voltage controlled attenuators, are we then reffering to variable resistance?

Do BJTs and diodes have an ohmic region just like the JFET?

And about dc feedthrough, I guess that because BJTs and diodes require a bias to operate, it would be hard to deal with thump...or am I wrong?

Cheers!

 

[gyraf]

[quote author="Ribbledox"]

Do BJTs and diodes have an ohmic region just like the JFET?

[/quote]

No. BJT's are "one-way" devices, the trick with FET's is that they - for low levels - are two-way devices. This is also why drain and source are often interchangable for low signal levels.

There are lots of good books on semiconductor theory - you may want to closely read some of those to get the basic concepts right..

Jakob E.

 

[Ribbledox]

Cheers Gyraf!

But about the JFET, is it right to state that:

"Thanks to its junction-free channel it can be operated in a symmetrical manner and, thus, no bias is needed across the device, which makes it possible to operate without any net-bias...thus no risk for thump"?

Just want to confirm that I have got the whole concept right

Thanx!

/R

 

[jhaible]

[quote author="gyraf"]No. BJT's are "one-way" devices, the trick with FET's is that they - for low levels - are two-way devices. Jakob E.[/quote]

In theory you are right, and looking at "high quality" applications, you're right in practice as well.

But actually it has been done - using BJTs as voltage controlled resistors for AC signals - in Synthesizer circuits. They are not exactly the most linear things, they have high CV feedthru, and the nonlinearity isn't even symmetrical. And yet you can build an Audio VCA from just a resistor and a single BJT acting as voltage controlled resistor.

Korg has used (probably pioneered?) this method early, and later Roland followed.

I've built a synthesizer inspired by these Korg circuits last year. It has 50 VCAs, each built around a single BJT Voltage controlled resistors.

And as there was a question about diodes: It also has 50 even more simple VCAs based on a single diode in voltage controlled resistor mode.

As I said, this is only remotely related to studio stuff, but it gives a good overview over the "quality" of different ways to create a VCA (or VCR) function:

For Resonance control (where no rapid change of control voltage is needed), a single diode is used.

For VCA of each synthesizer voice (where rather distortion is tolerable, and where intermodulation between different tones won't occur), a single BJT is used.

But for the VCA and VC mix functions that work on all voices together (i.e. on complex music signals), LDRs / Vactrols are used, just like in these optoelectronic compressors.

It was great fun building this, but I still don't really understand *how* the BJT works as a two-way (AC) voltage controlled resistor.

http://www.oldcrows.net/~jhaible/polykorg/jh_polykorg_clone.html

JH.

 

[PRR]

> Do BJTs and diodes have an ohmic region just like the JFET?

Yes, but it is very small. Say a few mV, compared to a hundred mV for FET and a thousand mV for most vacuum tubes. (But also consider the noise voltage: far smaller for a BJT than a VT.)

In http://cache.national.com/ds/LM/LM389.pdf page 6, transistor 678 works as a variable resistor against the 22K resistor from the mike. But the signal level at the collector does not rise much above 0.3mV peak (1.2V AGC rectifier threshold, forward gain of about 200*20=4,000). And this is a very low quality system (note DC bias to the record head) equivalent to the old $13 cassette recorders.

> without any net-bias...thus no risk for thump"?

Any unbalance (things are never perfect) will thump.

BJTs are cheap, and available in both polarities, so it is not unreasonable to use four of them plus ano opamp or two as a variable amplifier that can be reasonably balanced for DC and low-thump. Working push-pull considerably extends the low-distortion range.

This sounds like a university paper. In which case I say try it. Any EE lab has the goodies. Get a BJT and a signal generator, set it up like the LM389 circuit, and get a good o'scope across the C-E terminals to see what happens. And write it up with plots and graphs.

If your professor is an expert in VCAtt/VCAmp designs, you need to sharpen your words. But the average EE professor who has not thought a lot about it will probably not know enough to argue.