I found a circuit of an audio preamplifier that uses a differential amplifier with two transistors that, according to the text, should be matched. What does that mean?
Mateched transistors are transistors that have exactly the same characteristics, especially the gain. As we know that in a batch of transistors they can assume gains in a very wide range of values, these transistors need to be chosen. For example, if you are the BC548, we know that the gains can be between 125 and 800. For the indicated circuit we should take about 20 units and select two that are with the next gains, for example one with 210 and one with 215. These two transistors can then be used in the circuit. For this purpose we will need a transistor tester that measures the gain, or we can even mount a circuit that measures the gain using a common multimeter.
