SE amplifier for reception, suitable as a modulator
This amplifier, which was originally specified by Telefunken, is well suited for use in DIY radios with transistors or possibly also with tubes (operation via the heating circuit). Running as a single-ended amplifier, the slightly larger power consumption is disadvantageous. In motor vehicle operation, however, this is less important. Without the use of an IC, this disadvantage is opposed by a very simple replica. Sufficient cooling is required for the end transistor. Despite the germanium transistors used, the temperature stability of the quiescent current, which is a about 500 mA, is very good. This is due to the fact that it is stabilized by a negative feedback over all three stages.
The transformer should have about 280 turns, tapped at about 150 turns, seen from minus. It is used copper enamel wire with 0.5mm diameter for winding. The core used is an EI-42 type with air gap, which can be obtained, for example, by unwinding a audio output transformer from a tube radio. In AM transceivers, the amplifier can also be used in the transmit mode as a modulation amplifier, if the output transformer has a suitable winding. This option is used in my 10m AM transmitter presented elsewhere. The amplifier used there is based on this circuit, but it has been redimensioned for silicon-NPN transistors.
If a tube transmitter is to be modulated, it is also possible to use an inversely connected output transformer from a tube radio or tape recorder, that means the actual primary winding must be inserted into the anode lead of the transmitter stage. The original output winding is then between the negative pole and the collector of the transistor AD155. Best results are achieved when the quiescent current of this amplifier reversely magnetizes the transformer, like the current of the unmodulated transmitter. In the case of unsatisfactory results, thus one first should try to reverse one of the two windings.