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A class A amplifier
is defined as one which is biased to a point where plate or drain current
flows for the entire swing of the input signal between the cut off and
saturation. This is
done by biasing
the stage around the middle of its course between cut off and saturation
points. More exactly the stage should be biased to dissipate the max power
with no input signal, but sometime to centre the linearity of the transfer
curve the bias point is shifted from the max dissipation point. The JFA
has six single ended Jfet stages working in this way. The sixth stage has
no gain; it simply acts as phase inverter of the signal already available
at PIN 8. .
Why does JFA generate even harmonics?
It is known that
tubes and Jfets use the same working principle. In the tube a negative
voltage interferes with the flow of electrons running in the vacuum from
the cathode to the plate, thus modulating the current flow in the tube.
In the Jfet the same negative voltage interferes with the flow of electrons
running in the silicon from the source to the drain, thus modulating the
current flow in the Jfet. Tubes and Jfet exhibit the same smoothed shape
transfer characteristic curves. Transistors, on the other hand, have
an abrupt transfer characteristic curve with a threshold knee around 0.7
V. Tubes and Jfet produce even harmonics if used in a single ended
stage, while in a push pull stage the even harmonics are cancelled.
What do we mean for class A and pure class A?
Once a single
ended stage has to be biased two modes are available; self bias using a
cathode/source resistor, or fixed bias, using a negative bias voltage and
grounding the cathode or the source. The first way is the most widespread
while the second is especially used in guitar power amplifiers where a
negative feedback is often applied from the output to the input stage.
The JFA uses self biased stages using a particular Rdrain/Rsource ratio.
This guarantees a stable gain & bias and low distortion. This is what
we mean by class A and it should serve to satisfy all HI FI guys, but could
leave some music guy less happy, so:
the Sa, Sb pins
allow the user to bypass the source resistor with a high value capacitor
(100 mF suggested). This allows the stages to work purely in A, in other
words the output current / drain voltage is exactly the consequence of
the Jfet transfer characteristic curve. This frees all the even distortion
of the stages. Furthermore the first stage of the JFA can be fixed biased
applying a negative voltage at INPUT pin 2 (-1.7 Volt suggested) and grounding
the relative source Sa. This is what we mean by pure class A
Some notes
about operational amplifier.
Long time before
the birth of digital computers, operational amplifiers born as analogical
computational blocks for the first analogical computers, as the blocks
were connected in order to calculate
a mathematic function. Their duty at the beginning was to produce an output
voltage depending on several input analogical variables and they were intended
to be used with DC. However the OP developments lead to an increase of
the operative speed, so someone found it useful to use the OP to amplify
audio signal. Due to the internal circuit diagram an OP is a network
of multiple AB push pull transistor stages, which are in no way related
to a linear amplifier, but because of their very high gain, a heavy feedback
can be used to produce good results. In any case the gain of any OP decreases
with the increase of frequency, thus any OP based amplifier will exhibit
a higher distortion at high frequency. The JFA can amplify a signal
without using feedback because it is linear, therefore its response is
the same at any audio frequency. Considering that the JFA input noise is
spectacularly low (2.5 nV/Hz at 1000 Hz) this increases its ability to
amplify very small signals, resulting in a “fast”, clean and crystalline
sound.