21. MATCHING OF AMPLIFIERSTAGES

A single amplifierstage is only able to amplify within certain limits. If these limits are not observed thoroughly there will arise heavy distortions.

In most electronic devices the overall-amplification (amplification of the whole device) is far beyond the limits of a single stage. Therefore it is necessary to “couple” amplifierstages together.

If we amplify a signal via several stages the connection of such stages looks like a chain of circuits as shown in fig. 209.

fig. 209

Each part of that chain consists of an

- “output-side” (which plays a part like an energy source here) and an
- “input-side” (which stands here for the consumer of the energy delivered by the output).

The aim is to produce at last a signal of a satisfying power as necessary and a shape as similar in shape as possible to the input signal of that chain.

The output power of an amplifierstage is depending on:

- the amplification factor of the stage, and
- the input-power of the signal.

So the amplification-factor for one is given for a certain stage, the factor which has to be kept in mind during coupling two stages, is to make sure to get as much as possible energy from the output of stage “n” to the input of stage “n+1”. The method of achieving this is called MATCHING.

It is done on the basis of the following thoughts:

If you consider an amplifier as a “black box” you can easily see that the input draws a certain current at a certain voltage (so consuming a certain power).

All in all the input behaves like a resistor. As we talk about ac-voltage and currents we call it an impedance. Therefore we call this behaviour of the amplifier its: INPUT-IMPEDANCE

fig. 210a

The energy consumed by the input impedance is delivered by the output of the last stage. So the output acts here like a voltage-source. But as we know, each voltage-source has its internal resistance. Again we should keep in mind, that we deal with ac-values and therefore we call this internal resistance of the output the: OUTPUT IMPEDANCE.

figure

As we learnt earlier:

A voltage source is transmitting its maximum of power to a consumer if the load resistor is equal to the internal resistor of the source as it is explained once more in fig. 212.

fig. 212

We can use this knowledge accordingly to our matching problem and therefore we can state:

AMPLIFIERS ARE MATCHED EXACTLY IF THE OUTPUT - IMPEDANCE OF THE LAST STAGE IS EQUAL TO THE INPUT-IMPEDANCE OF THE FOLLOWING STAGE.

There are several possibilities to calculate the input- and output-impedance theoretically. But these methods would be beyond the limits of this course. Here we will mention only a possibility to measure the impedances.

If a radiotechnician intends to couple amplifiers together which have not been coupled before, he has to know how to measure these two values.

MEASUREMENT OF THE INPUT-IMPEDANCE

As Ohm's Law shows, the input resistance can be determined by measuring the input voltage and the input-current. But especially to measure the current is very difficult. So another method has been introduced:

A signalgenerator with a preferable low output-impedance is needed. Most of the signalgenerators are stabilized ones nowadays and therefore we can expect the output-impedance to be almost cero.

At first we set the signalgenerator to a voltage reasonable for the amplifier which should be researched.

The amplifier is connected to the signal-generator via a potentiometer whereby the potentiometer should be set to the highest values possible as shown in fig. 213.

fig. 213

Now the potentiometer is slowly set to lower resistances, whereby we observe the voltage measured at the input terminals of the amplifier.

As you might have concluded already:

THE INPUT IMPEDANCE OF THE AMPLIFIERSTAGE IS EQUAL TO THE RESISTANCE OF THE POTENTIOMETER WHEN THE INPUT-VOLTAGE HAS REACHED EXACTLY HALF OF THE ORIGINAL VALUE (Before reducing the resistance).

MEASUREMENT OF THE OUTPUT-IMPEDANCE

The output resistance is - as explained above - like the internal resistance of the voltage-source which the amplifier-output represents. In order to determine it, we have to measure the voltage-drop at the output terminals caused by a current increase. But again it would be too difficult to measure the current. Therefore we use again another method. The output-voltage is measured at first without any load. Now we connect a potentiometer to the output of the amplifier whereby we set first the potentiometer to the highest resistance. Then we start to decrease slowly the resistance whereby we observe the voltmeter.

fig. 214

As you might have concluded already:

THE OUTPUT RESISTANCE OF THE AMPLIFIERSTAGE IS EQUAL TO THE RESISTANCE WHICH THE POTENTIOMETER IS SET TO WHEN THE OUTPUT-VOLTAGE HAS REACHED EXACTLY HALF OF THE ORIGINAL OUTPUT VOLTAGE.

EXCEPTION: Very often it is not practicable to decrease the output voltage to 50%. In this case there might occur a heavy distortion. So it is often better to reduce the output voltage only to 90% of the original value. The output resistance is then 1/9 times as high as the resistance to which the potentiometer is set too.