26-10-2020, 09:10 AM
(This post was last modified: 26-10-2020, 09:11 AM by Murphyv310.)
Hi.
The method of obtaining bias in some older circuits with germanium transistors as in the Philips kits was quite common. We are aware of the shortfalls of not using a potential divider but in fact this simple method has two advantages.
Number 1 it introduces a little NFB which can be useful in fact the negative feedback leads to number 2 which gives a small degree of safety from thermal runaway, if this occurs the collector volts reduce which is reflected to the base thus reducing its voltage and then the transistors collector current. As can be seen on the output transistor it's only 27k, and in this case the AC126 ran slightly warm or quite hot if you forgot the heat sink, an emitter resistor though and a potential divider bias arrangement would have been the best route of course giving a better overall result. No doubt the main reason would be to make the kits as simple as possible.
The method of obtaining bias in some older circuits with germanium transistors as in the Philips kits was quite common. We are aware of the shortfalls of not using a potential divider but in fact this simple method has two advantages.
Number 1 it introduces a little NFB which can be useful in fact the negative feedback leads to number 2 which gives a small degree of safety from thermal runaway, if this occurs the collector volts reduce which is reflected to the base thus reducing its voltage and then the transistors collector current. As can be seen on the output transistor it's only 27k, and in this case the AC126 ran slightly warm or quite hot if you forgot the heat sink, an emitter resistor though and a potential divider bias arrangement would have been the best route of course giving a better overall result. No doubt the main reason would be to make the kits as simple as possible.
