12.4. STABILIZATION OF THE QUIESCENT VOLTAGE

As explained in chapter 9.1. the transistor is a component, which is very sensitive to increasing temperatures.

It is a semiconducting component, and therefore with growing temperature its resistance is decreasing.

In case of our transistor in an amplifier-stage, the relatively high collector-current causes a rise in temperature. This increase of temperature causes a bigger conductivity in the transistor. Bigger conductivity lets rise the collector-current (even though the base-current is still the same). Even if this process is a slow one, the increase of collector-current, causes a decrease of output-voltage at quiescence, and therefore a drifting quiescence-voltage.

fig. 149

There are several methods, how to diminish this effect:

EXAMPLE A:

By tapping the biasing path from the collector-current.

Function:

If the collector-current increases because of increased transistor temperature the collector current increases and the voltage across the voltage divider will tend drop as well. So the base-emitter-voltage will drop too - which lets decrease the base-current and therefore the original increase of the collector-current will be cancelled.

fig. 150 - A

EXAMPLE B:

By a so called EMITTER-RESISTOR. This resistor is dimensioned in order to drop about 10% of the supply-voltage ant quiescence.

Function:

If the collector-current at quiescence tends to increase the voltage at the emitter-resistor tends to increase as well. So base-emitter-voltage is the difference of the voltage across the lower biasing resistor and the voltage across the emitter resistor, the base-emitter-voltage tends to drop then as well. Dropping base-emitter-voltage causes a decreasing base-current and this causes the collector-current to decrease as well.

fig. 150 - B

So the original increase of collector-current will be cancelled.

WHAT FOR THE EMITTER-CAPACITOR THEN?

The emitter-resistor has - beside its stabilizing effect - a big disadvantage as. The input-currents and the output-currents have to pass it. And this causes a voltage drop for input and output-values.

So, by an emitter-resistor exclusively the amplifier-stage would loose a lot of its efficiency. The solution can be found by the following thoughts:

fig. 151a

Input- and output-currents are ac-currents while the distorting effect of the increasing collector-current is a dc-component. So we have to bypass the ac-components. As we know a capacitor lets pass again ac- but not dc-values. So the currents - bypassed - are rather high here, the capacitor must be rather big (very often it is an electrolyte-type).

fig. 151b

NOW WE KNOW HOW A NORMAL SO CALLED CLASS A AMPLIFIER IS MADE UP.

WHERE ARE CLASS A AMPLIFIERS USED?

Always if the outputpower of the amplifierstage is low. This means, we will find this amplifier class for example in IF-stages, RF-stages and audio-preamplifiers.

HOMEWORK:

1. Label all the components in the circuit.

2. State which voltages you should measure (roughly) at this amplifierstage at quiescence.

fig. 152

A:...........................................
B:...........................................
C:...........................................
D:...........................................
E:...........................................
F:...........................................
G:...........................................
H:...........................................
V₁:..........................................
V₂:..........................................
V₃:..........................................
V₄:..........................................
V₅:..........................................