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  #1  
Old 01-14-2009, 01:00 PM
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Bill_Marsden Bill_Marsden is offline
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Default Transistor Buffer Design

OK, one another thread I came up with this...



to which eblc1388 wrote...
Quote:
Originally Posted by eblc1388
Quote:
This circuit has no gain, but it will change the output impedance from 600Ω to an ohm or less. I've never build it, but it should work.
I simulated the circuit and found that it cannot cope with output loading of less than 56Ω. Any loading less than 56Ω would result in lower output voltage.

The red trace is output with loading of 1Ω, yellow trace 3Ω...etc...

Just so you would want to know about it.

I like learning about this kind of stuff. It is said you learn more from failures than successes. So why would this circuit fail to feed 50Ω with unity gain?

The transistor power dissipation is a problem, but that would be a failure mode (as in letting the smoke out).

Gain is the possible answer. I figure a β would be around 100 or so, but is the software showing less, or am I missing something obvious? I wonder if they were Darlingtons and the bias adjusted accordingly whether it would make a difference? I think the output for 50Ω is pretty decent overall.

BTW, thanks EB.
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Old 01-14-2009, 01:10 PM
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Thinking about it, 3Ω would be ľ less signal, even with infinate gain, since the 1Ω resistors would take some of the signal. With a β gain of 100, which isn't actually worst case, and an output of 50Ω the loading transfer would be around 5KΩ at the base. Lower resistances at the emitter would start to significantly load the input.
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Old 01-14-2009, 01:56 PM
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Hi Bill,

The simple answer is that the transistors cannot provide sufficient current to drive the 1Ω load impedance.

Additional stages are needed.
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Old 01-14-2009, 02:25 PM
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Try something like the NJT4031N and itís complementary NJT4030PT1G. The NJT4031N specs a worst case 0.3V sat at 0.3A base drive for 3A collector current,the NJT4030PT1G is similar. Power disapation would likely be a problem though.

Zetex also makes the FMMT618 And other high gain low Vce sat bjtís they ere pricey though. The Onsemi ones I mentioned are about half the price of comparable zetex.

http://www.onsemi.com/pub_link/Collateral/NJT4031N.PDF

http://www.onsemi.com/pub_link/Collateral/NJT4031N.PDF

Models

http://www.onsemi.com/PowerSolutions...pn=NJT4031NT1G

http://www.onsemi.com/PowerSolutions...s&rpn=NJT4030P

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Old 01-14-2009, 03:41 PM
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I don't think it is any inherent current limiting. Transistors will burn themselves out if allowed. One way to establish it though, add another 2N2222 and 2N2907 to make a Darlington. I'll redesign the circuit for that. My preferred Darlingtons is a TIP102 / TIP107 complementary pair. Don't forget the 1Ω is going to enter into the calculations too.

I'll get back here when I have a redraw.
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Old 01-14-2009, 03:46 PM
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Quote:
Originally Posted by Bill_Marsden View Post
I'll get back here when I have a redraw.
Hi Bill,

Do you use any simulation package?
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Old 01-14-2009, 03:53 PM
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Nope, need to start evidently. All you see comes out of my head and calculator.

SgtWookie is pretty good with his. Yours is the second I've seen used.
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Old 01-14-2009, 04:12 PM
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Quote:
Originally Posted by Bill_Marsden View Post
SgtWookie is pretty good with his. Yours is the second I've seen used.
Nope, I'm just a novice. Been using it for a few months only.

Mostly used it to prove a simple circuit design actually works as it should before giving out the design to others.

SqtWookie & Ron_H (and possibly many more) are the experts on this.
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Old 01-15-2009, 08:26 AM
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OK, here is the revised schematic...



So where did you get your SPICE program (I assume its name is Booster)?
Attached Images
File Type: gif Transistor Buffer 2.GIF (4.2 KB, 96 views)
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"Good enough is enemy of the best." An old engineering saying, Author unknown.

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Old 01-15-2009, 09:06 AM
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Quote:
Originally Posted by Bill_Marsden View Post
OK, here is the revised schematic...
I corrected your transistor connections and adjust the bias to give 20mA through the 1Ω resistor in the darlington. Then I did another simulation.

Surprise, surprise!!!. The traces are identical to the results above so I don't bother to post them.



Quote:
Originally Posted by Bill_Marsden View Post
So where did you get your SPICE program (I assume its name is Booster)?
The SPICE program is the free LTSpiceIV. The circuit that I'm simulating is called "Booster", which is just any name one can use to identify it among other circuit files.
Attached Images
File Type: png boosterx.png (8.0 KB, 88 views)
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Last edited by eblc1388; 01-15-2009 at 01:37 PM.
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