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Old 15th January 2010, 09:56 PM   #11
sigpl is offline sigpl  Europe
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tiefbassuebertr,

This was my first thougt too, but the voltage drop is too small, only about 0.6V, in this case to have that function.

Neither do I think it is to limit the current to charge the capacitor. The current is already very small, due to the R1.

And no it has no impact on the voltage gain since the voltage is not used by T2.
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Old 15th January 2010, 11:03 PM   #12
Bigun is offline Bigun  Canada
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The 70's, that was right in the middle of the Cold War. It was a practice to introduce strange technical items that had no purpose but would tie-up the other guys scientists for years trying to figure it out.

Keep trying to figure it out guys !
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Old 16th January 2010, 12:00 AM   #13
wahab is offline wahab  Algeria
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Quote:
Originally Posted by Bigun View Post
The 70's, that was right in the middle of the Cold War. It was a practice to introduce strange technical items that had no purpose but would tie-up the other guys scientists for years trying to figure it out.

Keep trying to figure it out guys !
this resistor role is logic, as explained earlier in the thread..
without it, there would be circumstances where the fet could
stand high currents that would destroy it...
if the drain was connected directly to the bjt base, impulse current
would be limited only by the source resistor connected to the
capacitance that goes to ground..
since the value of this resistance is very low and as it
define the closed loop gain, the current must be limited in the
other side of the fet, in the drain circuit, using this
"mysterious" resistance...
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Old 16th January 2010, 12:44 AM   #14
sregor is offline sregor  United States
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A curiosity - no one mentioned the 120 ohm resistor between the base of t4 and the hot side of the load. Kind of a bootstrap circuit, but why send your voltage gain current through the load, unless some kind of bias for the load was desired? My guess is that this circuit was a drawn but not tested. Has anyone tried to simulate it?
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Old 16th January 2010, 01:01 AM   #15
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they used that kind of bootstrap in another circuit too and that circuit is deliberatly mentioned as "build and measured".
There are published specs for our mystery amp, so probably there was a prototype.
Regards
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Old 16th January 2010, 01:06 AM   #16
kevinkr is offline kevinkr  United States
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Quote:
<snip> My guess is that this circuit was a drawn but not tested. Has anyone tried to simulate it?
That's a pretty big swag. In my experience with the semiconductor industry (I designed semiconductor ATE in a past life) I am almost certain that TI would not have published an untested design - does not mean it is the ultimate performer perhaps, but I can almost guarantee it will work.

Consider another possibility, R1/R2 reduce open loop gain by at least 6dB in addition to the other possibilities discussed here..
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Old 16th January 2010, 02:51 AM   #17
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Everything about that circuit makes good sense from a minimum cost perspective where high input impedance was required in the 70s. R2 is probably used to limit RF response of T2.
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Old 16th January 2010, 07:01 AM   #18
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Gareth,
Quote:
The 70's, that was right in the middle of the Cold War. It was a practice to introduce strange technical items that had no purpose but would tie-up the other guys scientists for years trying to figure it out.
You just gave the most credible explanation for the magical mystery resistor: enemy deception. I really like it.
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Old 16th January 2010, 07:07 AM   #19
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Quote:
Originally Posted by Bigun View Post
The 70's, that was right in the middle of the Cold War. It was a practice to introduce strange technical items that had no purpose but would tie-up the other guys scientists for years trying to figure it out.

Keep trying to figure it out guys !
Well, it may keep us busy, but I doubt that it would keep 'the other guys scientists' busy for more that 100mS or so
I mean, it's not that this secret amp would revolutionize warfare....

jd
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Old 16th January 2010, 10:47 AM   #20
SY is offline SY  United States
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Quote:
Originally Posted by kevinkr View Post

Consider another possibility, R1/R2 reduce open loop gain by at least 6dB in addition to the other possibilities discussed here..
I don't see how. R1 is paralleled by the low Rbe of T2. It looks to me like it's just there to set DC conditions of T2, which is a current source. R2 looks like it limits the base current of T2.
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