F5 power amplifier

[KSTR]

Now, having read the article, I' like to get back to one of Papa's statements: "The feedback mechanism for this amplifier is R3 through R6, a dual pair of low impedance voltage dividers which feed the output to the Source pins of Q1 and Q2. Low impedance feedback has been (incorrectly) referred to as 'current feedback'"

Is this to be understood as a general critique on the term (which actually is a misnomer of sorts), or does it refer to that specific amp, meaning that this amp doesn't fall into that category, regardless of semantics?

My take would be the former, as I still tend to see the amp as a cleverly simplified and specialized current feedback operational amplifier.

Any comments?

- Klaus

[Stefanoo]

Sorry guys i am trying to simulate the circuit with PSpice but probing the current on the input jfets it says 10mA are flowing instead of the supposed 6mA.
Here is the schematic i am using (i also tried to use the 2sj74 and 2sk170 but i get the same results)
Do you see any mistake?
Could it be some error on the models? (in case may somebody post the spice model for the parts used?)


thanks a lot.

[sandstorm33]

Originally posted by Stefanoo
Sorry guys i am trying to simulate the circuit with PSpice but probing the current on the input jfets it says 10mA are flowing instead of the supposed 6mA.
Here is the schematic i am using (i also tried to use the 2sj74 and 2sk170 but i get the same results)
Do you see any mistake?
Could it be some error on the models? (in case may somebody post the spice model for the parts used?)


thanks a lot.

I think your assumed values for P1, P2 , R15,R16 are in error. All four are variable in an effort to maintain the 4+ volts across the 2.2k R3/R4.

[Stefanoo]

no, i set them in order to obtain 4.2V (even though the bias current of the jfets accordingly to the simulator is 10mA(!?!) instead of 6mA).
Once i have adjusted them i get 0.2A at the output stage!
Something is definitely wrong.

[jacco vermeulen]

4200/6=700 Ohm.

1/700=10/7000.
1/R3=1/2K2~ 3.2/7000
1/R15=1/7000

10-3.2-1=5.8
7000/5.8=3500/2.9=1000 +600/2.9=1200+20/2.9~1206.
P1 is around 1206 Ohm for 4.2V.
According to John Nash it's "all in the mind".

[Stefanoo]

Sorry Mr. Nash setting P1 to 1206ohm doesn't give me the 4.2V (how about P2? i guess that it should be the same, right?...at least on the ideal case.)
I have a 10mA current flowing on the jfets, is this a correct?

[twitchie]

Maybe I misunderstand here, but from what I understand from the article, you don't adjust P1=P2, but rather you adjust them independently to get minimal dc offset as well as maintaining 0.59 volts across R11 and R12

Quote from the article:

For each channel you will be adjusting P1 and P2 alternately in order to achieve 0 volts DC at the output and .59 volts across R11 and R12. Each time you adjust P1 you will probably have to go back and adjust P2 again, and so I recommend adjusting the pots in half-measures, alternately setting the pots half-way to their voltage goals and measuring the DC values.

Unless there is something very wrong, when the output is at 0 V DC, the values across R11 and R12 will be equal.

[jacco vermeulen]

Originally posted by Stefanoo
10mA current flowing on the jfets, is this a correct?

Question : what good would a Spice model for a JFET be if it could only be used at a fixed Idss value ?

[Stefanoo]

i guess not too much.
But, what has to do with the simulation? what do you mean?
Do you have any suggestion?

[Zen Mod]

Originally posted by Stefanoo
i guess not too much.
But, what has to do with the simulation? what do you mean?
Do you have any suggestion?

that means that simulation is here to help you to understand how thing works , and then you must learn from books, schematics , ppl , and real work ;

sims can't make you Papa .