BJT instead of fets as variable resistors in phaser?

[mac]

i was wondering if bjt can be used as variable resistors in phasers instead of fets. matched bjt are easier to get.
but i do not know if bjt resistance is easy to control, or if there are another issues that make them unusable for the job.
pros and cons?

mac

[PRR]

> wondering if bjt can be used as variable resistors in phasers

If you have to ask, then "no".

Circuits designed for FETs generally will not work with BJTs. BJTs need a DC bias current to have a "resistance", and that resistance is sharply curved.

> i do not know if bjt resistance is easy to control

It is actually "easier" to -predict- BJT resistance, IF the circuit is designed for BJTs. BJTs follow a universal rule (Shockley's Law) very well. FETs are all different.

I have not looked at an analog phazer since the last century. IIRC it is related to an adjustable filter, cascaded. The prime example of a BJT variable filter is Moog's low-pass. Works good, but works VERY differently than most FET-based designs.

FETs are cheap enough to buy by the bag, and easy enough to match by hand.

[R.G.]

i was wondering if bjt can be used as variable resistors in phasers instead of fets. matched bjt are easier to get.
but i do not know if bjt resistance is easy to control, or if there are another issues that make them unusable for the job.
pros and cons?

In general, BJTs have two problems when used as a variable resistor. (1) the signal voltage must be quite small to not be grossly distorted by the curvature of the Rce response; probably under 10-15mV. (2) there is a significant offset voltage on the collector that must be somehow removed from the signal to avoid feeding the LFO through with the signal.

BJTs work well enough for muting, where the signal is either unaffected (transistor at high resistance) or shorted (signal voltage is tiny because the resistance is so low.)

BJTs are used in one tremolo I know of (EH Pulsar), so there is some hope. But matching JFETs is not a big deal. One opamp, 2-3 resistors, a battery and a meter give you a JFET matcher, per geofex.com.

[Nasse]

I think I have seen a suggestion to use diodes as voltage controlled resistors in opamp phaser but maybe there is not much advantages

[R.G.]

I think I have seen a suggestion to use diodes as voltage controlled resistors in opamp phaser but maybe there is not much advantages

See http://www.diystompboxes.com/smfforum/index.php?topic=80228.0 for a discussion of diodes as variable resistors.

Note that using diodes and transistors as controlled resistors in a setup where the voltage change with signal across them is tiny does work just fine. This is the trick behind so-called ladder filters/filters/phasers used in the synth world. They are used in a way that the diode/transistor changes currents into the summing node of an opamp, not so much voltage.

[mac]

But matching JFETs is not a big deal. One opamp, 2-3 resistors, a battery and a meter give you a JFET matcher, per geofex.com.

my P90 is fantastic because of the jfet matcher!

BJTs are used in one tremolo I know of (EH Pulsar), so there is some hope.

the pulsar and dr quack made me think about this.
and when i was working on my fet phaser i thought about using bjt instead of ldr.
it is challenging for me because of my little experience in this area but if there is some hope i'd like to try something new, just for fun.

my main idea is to "replace" the ldrs in my fet phaser with bjts, think univibe. or work around the dod 595 ldr-based phaser.

mac

[R.G.]

it is challenging for me because of my little experience in this area but if there is some hope i'd like to try something new, just for fun.

my main idea is to "replace" the ldrs in my fet phaser with bjts, think univibe. or work around the dod 595 ldr-based phaser.

Go for it! It can't hurt.

[mac]

it hurts when i fail!!! ... big pride, you know 

mac

[frank_p]

Don't want to talk for somebody else Mac, but if it can give you persistence:

Is it a tested advice or I will screw up again.

I don't know about you, but I will *always* screw up again.