In this section, main features of that special biasing will be discussed. However, since there is little interest in make pentodes or beam tetrodes operating in class A2, we will deal only with triodes. We have previously seen that class A1 is limited by apparition of grid current; if we relax this constraint (assuming that the driver stage has a low enough internal impedance so that dynamic grid resistance is high with respect to it), useful domain of operation is extended and, like in the case of pentodes, load resistance can be choosen to be :
with an associated efficiency :
This seems prodigiously interesting since most powerful triodes are designed to run continuously with grid current and promise high power even in single-ended mode. However, stress put on the driver is stringent, so that few designs will reach the top quality.
Main trouble with class A2 is that for a grid current 
and a
driver internal resistance 
, the product 
must remain well below the maximum grid excursion
. A rule of the thumb is to compute dynamic impedance of grid at
maximum positive modulation. Since at positive biasing the
grid-cathode system behaves essentially like a conducting diode, we
have approximately 
with G the perveance of that
equivalent diode. Dynamic impedance at 
can be computed as :
We see that it is lower than the expected value 
. A driver
for a tube running in class A2 must have an internal impedance ten
times lower than this value. Let us take the example of the 833A, a
directly heated triode with 300W plate dissipation and an
amplification factor 
of 35. Since is high, the useful
operating rang in class A1 will be very narrow unless high voltage
plate are applied (2500V to 3000V will be required). Class A2 biasing
is much more appealing : with 1250V and 
, optimal load
resistance is 5 kOmega and output power 
. Maximum
positive grid voltage will be 
, with an associated grid
current 
as indicated on the datasheet. The dynamic
resistance computes as 1.6 k 
. The driver tube must have
an internal impedance in the 100 range or lower. Very few triodes can
have such a low value; some tubes used in serie regulation will match
this requirement (for example the well known 6C33C). Other options are
possible, like using a step down interstage transformer so that
grid impedance appears higher. In the previous example a 2:1
transformer will give 4.4 k resistance and a driver
near 400 . An no-compromise approach will use
a higher ratio, like 4:1 or 6:1 in order to allow directly heated
triodes as drivers (300B for the first ratio and 845 for the
second). It worth note that interstage transformers are more difficult
to build when ratio is above 10:1.
In many cases, feedback can be used in order to reduce both distorsion
and output impedance : those quantities are roughly reduced by the
feedback ratio. A 6C33C based driver will be able to reach 20
, yielding a nearly perfect grid control. When an interstage
transformer is present, it worth including it in the feedback loop
(it's an inner feedback loop in case of global feedback) since this
results in minimizing phase shift effect from that component, thus
increasing phase margin of the outer feedback loop (most class A2
designs must use global feedback). Note that feedback can be obtained
by wiring the driver in cathode follower too !
Here are the key points for class A2 :