![]() ![]() Q5 served a double purpose being simultaneously the audio power output stage and voltage regulator for the radio circuitry. The transistor would be mounted on a heat sink of some kind maybe the metal chassis of the radio. The output transformer would likely be a 5 k ohm to 4 ohm unit not unlike the ones used in tube radios. This circuit was made possible by the fact that high voltage power transistors had come down in price sufficiently to make the circuit practical. ![]() It came close to being the transistor equivalent of the all American five tube radio but never really caught on.įigure 7.2 Line Operated Transformer Coupled Amplifier. There was already so much distortion in these vintage circuits the designers probably felt that a little more wouldn't be noticed.Ī variation of the transformer coupled amplifier was seen in line operated table radios from the mid 1970s onward. This omission likely slightly reduces power output and increases distortion. In most vintage circuits the capacitor across this resistor has been omitted. In vintage equipment using germanium transistors there will be a thermistor in parallel with the resistor that is equivalent to the 270 ohm resistor. The upper base biasing resistor is effectively returned to the collector for DC but the effect of this will be small for a failure mode analysis. For the single ended amplifier of Q1 the 1.8 k ohm resistor will be the collector load for DC and the failure modes will be almost the same. That case will be identical to that for an open collector resistor. The main difference is that in the push-pull pair consisting of Q2 and Q3 the collector voltage will mostly be at Vcc for all failure modes except an open transformer winding. If you were to construct this circuit using 2N3904s or any other small silicon NPN transistor it has a high probability of working and delivering approximately 250 mW, a little more on 12 volts.Īlthough the transistors are transformer coupled the failure analysis given in section 6.1 for the common emitter configuration mostly applies here. The values given in the circuit are NOT from a vintage radio but have been designed to work with modern silicon transistors. Those transistors needed all the help they could get and the transformers provided it. Note that both transformers are impedance step-down which translates to current step-up. Transistors are current devices unlike tubes which are voltage devices. Transformer coupling was used because early transistors had low current gains. Figure 7.1 is an example.įigure 7.1 Transformer Coupled Audio Amplifier.Īlthough this circuit is very old dating back to the dawn of the transistor age it still exists in vintage radios and phonographs and is occasionally used in new equipment. The very first audio amplifiers employing transistors were designed as the audio section in transistor radios. There are thousands of variations on the basic audio amplifier so only a few representative samples can be given here. The term "audio amplifier" can cover a multitude of sins and in this section we shall cover the most important of them. Therefore this chapter is about consumer grade electronics rather than exotic devices that may be found in a scientific research laboratory. Transistorized Consumer Equipment.Īs you read in the foreword of this book it has been repurposed from a university textbook to a web publication whose target audience is more likely to be home experimenters and hobbyist. Transistorized Consumer Equipment.Ĭhapter 7 Transistorized Consumer Equipment. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |