[Amps] CB Amps or full spectrum???

[Ian White GM3SEK]

Peter Chadwick wrote:
>>BTW, a 1N5408 on top of an output transistor makes a fair temperature
>regulator. Not perfect but its cheap and simple as a retrofit of an 
>existing brick.< If the diode is used just as a shunt regulator, that's 
>a not very good bodge. If it's used as asensor in a regulator, that's 
>better, although the thermal coupling is nowhwere near as good as one 
>would like.

I've never understood why people drape a diode over the top of a 
transistor package. Everybody does it... but why?

The diode cannot possibly sense short-term temperature changes inside 
the RF transistor. Inside the transistor, there is often an air gap 
between the silicon die and the underside of the ceramic top cap. To 
reach the diode chip, the heat has to jump that gap, flow through the 
thickness of the top cap, then through some random quantity of thermal 
grease, and finally through the glass or plastic encapsulation of the 
diode.  If there is a sudden, severe overheating problem (eg because the 
antenna fell down), the transistor will be long dead before the diode 
could possibly sense any change.

(Oh, and another thing: the diode is making a large pickup loop, over 
the top of an RF power transistor, with a rectifier right there in the 
loop. That doesn't seem like the best route to a predictable bias 
circuit.)

The best practical location for a temperature sensor is on the top side 
of the mounting flange, which is very closely bonded to the transistor 
die. Also, a small tab-mounted power transistor makes a faster 
temperature sensor than a wire-ended diode (use the base-emitter 
junction). A small tab-ended SMD power transistor would be faster still.

A simple active bias circuit can give better performance than a passive 
diode, and also save on bleed current. There is a classic two-transistor 
bias circuit (with design notes) at:
http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm
That circuit gives a reasonable compromise between performance and 
simplicity, and is a reasonable retrofit to existing bipolar amplifiers.

Even then, the bias circuit can only track the relatively slow 
temperature changes that you'd expect in normal operation. That's the 
best you can possibly expect. For sudden problems, you'd better have 
other forms of protection.


-- 

73 from Ian GM3SEK