|Radio and Electronics (DED Philippinen, 66 p.)|
Amplifierstages have a HIGH INPUT IMPEDANCE while they have on the other hand A RATHER LOW OUTPUT IMPEDANCE. As explained during the chapter MATCHING this causes problems if we want to secure an efficient coupling.
COUPLING BY TRANSFORMERS
The most effective way to couple two amplifier stages for ac-signals is to couple them by a transformer.
In this case it is easily possible to use as a collector resistance of the first amplifier-stage which will deliver the signal the primary coil of the transformer. The secondary coil of the transformer is connected within the path from the biasing circuit to the base of the second.
An example is shown in fig. 206. By choosing the fitting number of turns it is possible to adjust the impedances of the two coils exactly to their complimentary impedances within the amplifierstages.
This method was used especially in the early days of transistor technology, when it was necessary to have as few transistors as possible (because of their high prices at that time).
Now you know all the different blocks of a simple AM-radio and you should be able to read the circuit diagram below.
1. What does FEEDBACK mean?
2. What advantage has:
a) positive feedback
b) negative feedback?
3. What does matching mean?
4. Which type of coupling of amplifier-stages is the most accurate one?
5. Can you differentiate the blocks of the radio?
6. Explain what the components in the blocks are for!