Multistage LF Amplifiers

Abstract

RC sections or transformers are used to couple LF amplifier stages. The aim of connecting several transistors in series is to reach the greatest possible multiple of amplification at the same amplitude for all the frequencies to be transformed. All frequency-dependent sections must therefore be so designed that they play no part in the frequency range to be transformed. Apart from the coupling sections which are frequency dependent, the transistor is also a factor to be considered under certain circumstances, with its input capacitance which in transistor T₁, for example, is ≈ 5000 pF. The RC coupling is simple but has the drawback that the high source resistance R _i of the pre-amplifier transistor is loaded by the low resistance input capacitance of the following stage. The fact that transistor stages which are virtually operated in a short-circuit condition yield a voltage amplification is due entirely to the large slope of the transistors. Transformer coupling is more costly as regards material, but it has the advantage of a higher stage amplification because the input resistance r _i of the succeeding stage can be matched to the source resistance R _i *.