LDR transistor gain

[Kipper4]

Question.
Can I use a transistor gain stage for audio, but use an LDR as one of the two base biasing resistors.
LDR's resistance, obviously controlled by an evelope detector.

What I would really like is a transistor that ramps the gain up in a linear fashion without transistor turn on noise.

A bit like controlling the gain of an op amps nfb with an ldr.

Heres a light sensor circuit doing the same sort of thing, but i have a feeling it wont work well for audio and linear gain.

http://www.cyberphysics.co.uk/graphics/diagrams/electronics/transistorLDR.png



I'm looking for ideas and guidance please.

Thanks
Rich

[R.G.]

Yes, but...  (You knew there would be one of those, right?  )

The biasing resistors also have a huge effect on the static biasing point, and as such on the amount of signal both in and out that can be amplified without distortion. So the collector's voltage wanders full range from cutoff (i.e. zero current, and output = power supply) to saturation (i.e. collector and emitter at nearly the same voltage, and probably near ground).

Bipolar transistor gain is directly controlled by the collector current, but separating out the DC effect from the AC effects you want is tough.

That's exactly the chain of thought that once led to "Hey! I'll use TWO bipolars as a diffamp, control the emitter current, and then have a varying differential signal between the two collectors!".  The emitter biased diffamp pair is the basis of all bipolar multipliers, with additional complexity growing to further isolate and enhance the changing gain part from the wildly varying DC conditions.

It would be fairly simple to put the LDR in series with the base and modulate the "gain" that way, leaving the transistor gain fixed, and varying how much you attenuate the input signal. That is the basis of several effects, including the Vox Repeat Percussion, which uses a fairly pure form of attenuate-then-amplify-back.  Attenuation doesn't necessarily change DC conditions at all.

[Kipper4]

Thanks RG.
Just as I thought. I might as well use another op amp, It's not like I'm short of room.
I've got the attenuation about sorted. I just need to try and make the outputs amplitude rise and see if I still like it or opt for a fixed gain recovery.
Cheers

Phew I dodged another jellybean.

By the way thanks for that I havent come across the repeat percussion before.
I like how simple it looks.
To all intents and purposes it's an pso lfo letting the inupt signal down to Gnd through a transistor.
I'm away and go listen to one. For some reason I thought the repeat percussion was a sample and hold type thing.

[bluebunny]

I'm intrigued by your fascination with envelope-controlled stuff, Rich.  Can't wait to see what you come up with next.



Mrs. Kipper: why's the TV picture so dim?

Rich: what?

Mrs. Kipper: I said, WHY'S THE TV PICTURE SO...  Oh, that's better.   

[Kipper4]

Just rehashing some old stuff.

Sent from my SM-G920F using Tapatalk

[digi2t]

OK, I'll take a whack at it....

Transistor(s) based fuzz circuit, with an LDR "Expandora" style gain envelope?

I was wonder about that a few weeks back. It's like the Kipper is in my head when I saw this thread.

[bluebunny]

Just rehashing some old stuff.

You keep rehashing, mate.  I'm gonna grab a beer and some popcorn.      (Genuinely fascinated by this stuff you come up with and genuinely fearful for my backlog!)

[EBK]

I'm wondering if the first compressor/expander was born out of the thought, "Well, we've tried controlling everything else with an envelope follower, so why not an envelope-controlled envelope?" 

[Kipper4]

You guys are just too funny.

I don't know what it is about enveloped stuff and me.

Enveloped envelope sounds like just the project I need right now though Eric.

Rehashing the ldr Sweller.

My grey paper will be ready soon.
Looking for ideas on how best to host it.
Probably PDF format.

Thanks
Rich.

[bluebunny]

I don't know what it is about enveloped stuff and me.

You're a wannabe postman??      

[robthequiet]

From the peanut gallery:

Could the LDR have enough variability to modulate the emitter current but in series with the emitter bypass cap so that it affects the AC gain but leaves the DC biasing alone? Or would be like a warp control? I'm thinking about the configuration in which the emitter resistor is a pot as a voltage divider with the cap connecting wiper to ground. So make the wiper an LDR to cap to ground?

[Transmogrifox]

From the peanut gallery:

Could the LDR have enough variability to modulate the emitter current but in series with the emitter bypass cap so that it affects the AC gain but leaves the DC biasing alone?

This sounds like an EA tremolo with an LDR instead of a FET.

[samhay]

>Question.
>Can I use a transistor gain stage for audio, but use an LDR as one of the two base biasing resistors.

2 base resistors?

This is what I would do, if I had to:
>Could the LDR have enough variability to modulate the emitter current but in series with the emitter bypass cap so that it affects the AC gain but leaves the DC biasing alone?

But, you will likely need an LDR that can swing to quite low resistance - significantly lower than the value of the base resistor - think ready-made vactrol rather than roll-your-own.

[deadastronaut]

Keep pushing the envelope...



sorry, coats on...



1m ldr in place of the gain pot, on say a TS?...thinking aloud..

[Kipper4]

Always Rob.

This kinda thing?

[samhay]

^yes, but you may need to scale up the base and emitter resistor values by 10x or more.

[Kipper4]

Excellant thanks Sam.

[anotherjim]

Grey? Manilla paper, surely?

LDR providing negative feedback. Think BMP clipper stage where the diodes are.

[Kipper4]

You mean replace the feedback diodes in the triangle bmp with an ldr Jim?

Link
http://www.pisotones.com/BigMuffPi/psst/BMP_versions.htm

[anotherjim]

Yeh, replace, in series, in parallel...
Collector-base is really not much different than using the negative feedback loop of an opamp .

Keep it AC coupled so it don't change the DC bias. There's usually 390k-470k setting the DC bias already and LDR range compares better to that than 100R> emitter degeneration resistors.