Piggybacking effect using 1 diode?

[z-zero]

I'm sitting here at work rather bored and I was looking around and reading about the Miss Piggy. Thanks Brett. I was thinking you might be able to achieve the same thing (gain reduction) by putting a diode between the base and emitter of a single tranny. It seems your only using one of the P/N junctions of the tranny so I thought hey thats a diode! As long as the "extra" N material on the tranny's open collector really doesn't effect anything. So you just put the 10K resitor in series with the diode connecting the base to the emitter. Anode of the diode on the base and the cathode on the resistor going to the emitter. It might not work, I'm not sure how the P/N conjunctions differ in a diode compared to a trasistor. I might get a chance to try it tonight....

Any thoughts on this?

z-zero

EDIT: I think I had my Anode and cathodes confused...

[R.G.]

The idea behind piggybacking is to have a second, almost identical junction steal current from the first junction. That seems to work, but the match has to be quite close, as there are small differences in the forward voltage of silicon junctions at a given current, depending on the doping profiles of the junction. All that means - all silicon junctions are not alike, although they are closer to each other than to germanium or gallium arsenide.

So a diode of any kind might have too high a junction voltage and not decrease the effective gain of the transistor, or it  might have too low a junction voltage and decrease it too drastically.

When I came up with piggybacking, the idea was to use another identical transistor to make sure it was at the same voltage as the working junction.

[z-zero]

Cool, thanks for the reply R.G.

So by some slim chance there might be a transistor diode combo that could work, I'll have to try a few just to see. While were on the topic, can one take 1n34A diodes for example and pair them up to act like a transistor? I think I've seen it done at the end of some circuits. Some odd half Si/Ge tranny setups? Biasing them would be a pain and maybe in some cases not possible. Any thoughts?

z-zero

[R.G.]

While were on the topic, can one take 1n34A diodes for example and pair them up to act like a transistor?

Sadly, no. While a transistor looks in some respects like a base-collector diode reverse biased and a base-emitter diode forward biased, the thing that makes it work as a transistor is that the base is the same physical region in both cases. You are actually injecting charge carriers into the base region of the semiconductor in a transistor, and that is what lets charge carriers flow through the reverse biased collector-base diode. Two separate diodes don't do this, because the part that's conducting is not physically at the reverse biased junction.

[brett]

From what I've heard, you can mess with chunks of n and p doped material, and if you get a good configuration, the transistor might give a current gain of 3 or 4.  Not really a practical transistor.
I've tried piggybacking different transistors, and (from memory) the effects are quite variable.  I suspect that doping levels etc make big differences.

Also, a point of clarification: a few people seem to wrongly attribute the piggyback idea to me.  It was 100% RG Keen's idea.  I merely tried it out and found that it worked really well (especially with a resistor betwen the emitters).

cheers

[z-zero]

Bummer I thought it might be a long shot as it seemed so simple that someone would have used before if it worked. Thanks for explaining that RG. Man I love this forum.

Thanks to all,

z-zero

[bioroids]

I was thinking you might be able to achieve the same thing (gain reduction) by putting a diode between the base and emitter of a single tranny.
z-zero

Hey check out this page:

http://www.geocities.com/SunsetStrip/Studio/2987/britface.html

At the middle of the page there's a description on using a parallel diode to reduce gain of the transistor: ie piggybacking with a diode. I'm wondering what are the odds of this working right without selecting parts.

Might be usefull to put a resistor in series with the diode to regulate the amount of gain lost too.

Luck!

Miguel

[z-zero]

Woh, cool thanks bioroids! I wonder in the NPN vs PNP transistor matters? Similar to but not exactly how it effects Joe D's diode compression circuits. I can understand why he would use a GE diode with the lower break over voltage. I need to mess with some of this.

Thanks,

z-zero

[puretube]

http://www.diystompboxes.com/smfforum/index.php?topic=38046.0
http://www.diystompboxes.com/smfforum/index.php?topic=37944

[z-zero]

Cool thanks Puretube. 

z-zero