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Home>
Audio-design
Design Center > How
To Article

Distortion in power amplifiers, Part VIII: Class A amplifiers

Douglas Self -February 20, 2008

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Quiescent control
systems
Unlike class B, precise control of quiescent current is not
required to optimise distortion. For good linearity there just has to be enough
of it. However, Iq must be under some control to prevent
thermal runaway, particularly if the emitter follower output is used, and an ill
conceived controller can ruin the THD. There is also the point that a precisely
held standing current is considered the mark of a well bred class A amplifier; a
quiescent that lurches around like a drunken sailor does not inspire confidence.

Thermal feedback from the output stage to a standard Vbe
multiplier bias generator will work,8 and should be sufficient to
prevent run-away. However, unlike class B, class A gives the opportunity of
tightly controlling Iq by negative feedback. This is
profoundly ironic because now that we can precisely control
Iq, it is no longer critical. Nevertheless it seems churlish
to ignore the opportunity.

There are two basic methods of feedback current control. In the first, the
current in one output device is monitored, either by measuring the voltage
across one emitter resistor, (Rs in Figure 6(a)), or by a
collector sensing resistor. The second method monitors the sum of the device
currents, which as described above, is constant in class A.

The first method as implemented in Figure 6(a)7 compares the
Vbe of Tr4 with the voltage across Rs, with
filtering by RF, CF. If quiescent is
excessive, then Tr4 conducts more, turning on
Tr5 and reducing the bias voltage between points A and B.

In Figure 6(b), which uses the voltage controlled current source approach,
the voltage across collector sensing resistor Rs is compared
with Vref by Tr4, the value of
Vref being chosen to allow for Tr4
Vbe.9 Filtering is once more by
RF, CF.

For either Figure 6(a) or 6(b), the current being monitored contains large
amounts of signal, and must be low pass filtered before being used for control
purposes. This is awkward as it adds one more time constant to worry about if
the amplifier is driven into asymmetrical clipping, and implies the desirability
of large electrolytic capacitors to minimise the a.c. voltage drop across the
sense resistors. In the case of collector sensing there are unavoidable losses
in the extra sense resistor. It is my experience that imperfect filtering can
produce a serious rise in distortion.

The better way is to monitor current in both emitter resistors. As
explained above, the voltage across both is very nearly constant, and in
practice filtering is unnecessary. An example of this approach is shown in
Figure 6(c), based on a concept originated by Nelson
Pass.10 Here Tr4 compares its own
Vbe with the voltage between X and B; excessive quiescent
turns on Tr4 and reduces the bias directly. Diode D is
not essential to the concept, but usefully increases the current feedback loop
gain; omitting it more than doubles Iq variation with
Tr7 temperature in the Pass circuit.

The trouble with this method is that Tr3
Vbe directly affects the bias setting, but is outside the
current control loop. A multiple of Vbe is established between
X and B, when what we really want to control is the voltage between X and Y. The
temperature variations of Tr4 and Tr3
Vbe partly

Figure 6: Quiescent current-control systems. Only that at
Figure 6(c) avoids the need to low pass filter the control signal; C simply
provides feed forward to speed up signal transfer to
Tr2.

cancel, but only partly. This method is best used with a CFP or quasi output
so that the difference between Y and B depends only on the driver temperature,
which can be kept low. The 'reference' is Tr4
Vbe, which is itself temperature dependent. Even if it is kept
away from the hot bits it will react to ambient temperature changes, and this
explains the poor performance of the Pass method for global temperature changes
(Table 2).

Table 2: Iq change per °C change in
temperature (assuming 0.22 Ω emitter resistors and 1.6A
Iq.)

To solve this problem, I would like to introduce the novel control method in
Figure 7. We need to compare the floating voltage between X and Y with a fixed
reference, which sounds like a requirement for two differential amplifiers. This
can be reduced to one by sitting the reference Vref on point
Y. This is a very low impedance point and can easily swallow a reference current
of 1 mA or so. A simple differential pair Tr15,16 then
compares the reference voltage with that at point X: excess quiescent turns on
Tr16, causing Tr13 to conduct more and
reducing the bias voltage.

The circuitry looks enigmatic because of the high impedance of
Tr13 collector would seem to prevent signal from reaching the
upper half of the output stage; this is in essence true, but the vital point is
that Tr13 is part of a NFB loop that establishes a voltage at
A that will keep the bias voltage between A and B constant. This comes to the
same thing as maintaining a constant Vbias across
Tr13. As might be imagined, this loop does not shine at
transferring signals quickly, and this duty is done by feedforward capacitor
C4.

Without it, the loop (rather surprisingly) works correctly, but HF
oscillation at some part of the cycle is almost certain. With
C4 in place the current loop does not need to move quickly,
since it is not required to transfer signal but rather to maintain a DC level.

The experimental study of Iq stability is not easy because
of the inaccessibility of junction temperatures. Professional Spice
implementations like PSpice allow both the global circuit temperature and
the temperature of individual devices to be manipulated; this is another aspect
where simulators shine. The exact relationships of component temperatures in an
amplifier is hard to predict: I show here just the results of changing the
global temperature of all devices, and changing the junction temp of
Tr7 alone (Figure 7) with different current controllers.
Tr7 will be one of the hottest transistors and unlike
Tr9 it is not in a local NFB loop, which would greatly reduce
its thermal effects.

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