Or add more output FETs
although the added open loop ampl. raises the closed loop freq.
just ignore that last post of mine
coffeeless brain is even emptier , than usual
. . . because they are in phase . .
Could you tell me what the intrinsic square-law is or give me a link where I can learn about it?
It sounds like only the active devices contribute to the distortion of the amp. Are there other jfet/mosfets that we might want to consider for the F5 that might not have perviously been considered?
A FET is called a square-law device because of the relationship of ID to the square of a term containing VGS.
So the transfer characteristic is not a straight line but curved.
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I now see a simpler and more general way to express the problem. The exact Vout(Vin) function for the JFET+MOSFET chain can be a Taylor series expansion of the full equations, resulting in polynomials:
P(x) = sum(i, c*x^i)
The output from pair of amplifier chains with feedback is then:
Vout = P1((1-a1*Gain)*Vin - Vt1) - P2((1-a2*Gain)*Vin - Vt2)
The harmonic analysis in now done in the Fourier domain. For a given input Vin(f) at frequency f, the output response at frequency i*f is c*Vin(f). This approach allows the use of complex coefficients, representing phase information (and capacitors and inductors).
I haven't completed the details, but I expect the response for i-th harmonic something like:
Vin^i * (c1*((1-a1*Gain)) + (-1)^i * c2((1-a2*Gain))
Another interesting discovery:
Source resistor degeneration is known to be a type of local negative feedback. I have equations that show that source degeneration introduces higher order harmonic distortions in proportion to the amount of local feedback. This suggests higher harmonics might be reduced by reducing source degeneration. Unfortunately, that increases the open-loop gain, which increases either the amplifier Gain or the global feedback.
Start an F5 theory thread?
All of you who are interested in circuit theory as applied to the F5 (and related amplifiers) I suggest we start a new thread. I am very interested in thoroughly understanding why the F5 can be adjusted to work so well.
All of you who are interested in circuit theory as applied to the F5 (and related amplifiers) I suggest we start a new thread. I am very interested in thoroughly understanding why the F5 can be adjusted to work so well.
I find equations helpful for understanding the phenomenon. In particular, how else can you really understand the distortion cancellations that take place in the F5 push-pull pair or in SUSY amplifiers?
In the F5-Turbo paper, Nelson suggested adding caps between the output
and the MOSFET gates, rather than across the feedback resistors. In Spice simulations I found that I could greatly reduce the 20KHz harmonic distortion by adding about 500pf from the output to gate of the P-channel MOSFET, an IRFP9240 in my case. 500pf is the right amount to equal the balance the gate-drain capacitances of the 2 MOSFETs.
The overall frequency response is still peaks by about .15dB at 360kHz. This was cured by adding 200pf from the JFET gates to ground.
This looks much more promising than caps across the feedback resistors.
The smaller one against the rectifier. This reduces the inrush current. It is the inrush that upsets even the biggest rectifier blocks (thermal/reverse) and transformers (saturation effects). I choose even smaller up front one that still can handle 10 amp ripple.
Thanks, I have it the other way around and I think it might be causing some small problems.
This is a god idea [I should do too]; it is because the input capacitances of tewo P-FETS and two N-FETS differ.
One extra trick: connect the 500 pF not to ground but to the source resistor of the P-FET. That way the static value is reduced just like the gate-source resistance dynamically on the N-side.
I am still curious as to how the .15 dB level shift occurs; in my implementation I remember it stays rather over a broad frequency range (that is why I call it a shift). So if this also occurs with the IRFP[9]240 . . it is not due to my Toshiba's and my air-wiring to my dislocated TO-3 devices.